International Journal of Modern Physics B

We report on Fabrication of two-dimensional electrons in AlAs quantum wells with mobilities up to 31 m²/Vs. Magnetoresistance measurements reveal fractional quantum Hall states at high-order filling factors, and up to ν = 11/3. Shubnikov-de Haas oscillations of high-density samples suggest that electrons occupy two X-point valleys. We also study properties of hysteretic resistance spikes occurring at transitions between quantum Hall ferromagnets in AlAs quantum wells, and show that the spike hysteresis depends sensitively on the number of occupied energy levels involved in the transition.

Recent experiments on the tunneling conductance between parallel 2D electron gases at total Landau level filing ν_tot = 1 are described. When the two layers are close enough together the ground state of the system may be viewed as a Bose condensate of excitons consisting of electrons in one layer paired with (conduction band) holes in the other. The measured tunneling conductance exhibits a spectacular resonance around zero bias which resembles the dc Josephson effect. This resonance is a signature of long wavelength Goldstone collective modes in the phase coherent ground state. Experiments performed with an added in-plane magnetic field have demonstrated the expected linear dispersion of this mode.

The stripe phase of a two-dimensional electron system in a weak magnetic field bears a close analogy to liquid crystals. However, reduced dimensionality and unusual dynamics give rise to important differences. At finite temperature they cause divergent fluctuations and nonperturbative renormalization of hydrodynamic parameters. Such effects can be verified in microwave experiments. At low temperatures the physics is dominated by quantum fluctuations. When they are large, the transition to a novel quantum nematic phase may occur, driven by quantum proliferation of dislocations. It will be signaled by an additional low-frequency resonance in the microwave response.

A recently developed technique is reviewed with the potential for all-optical coherent control over electron spins in semiconductors. In these experiments, ultrafast laser pulses "tip" electron spins by generating effective magnetic fields via the optical Stark effect. Measurements of Stark shifts have provided estimates of the net tipping angle as a function of tipping pulse energy, intensity, and polarization. Background contributions to the measured tipping angle arise from the undesirable excitation of additional carriers by the tipping pulse.

Bilayer quantum Hall system at total filling factor ν=1 shows a rich variety of broken symmetry ground states because of the competition between the interlayer and intralayer Coulomb interactions. When the layers are sufficiently close, a bilayer system develops spontaneous interlayer phase-coherence that manifests itself through a spectacular enhancement of the zero-bias interlayer tunneling conductance. We present a theory of this tunneling conductance anomaly, and show that the zero-bias conductance is proportional to the square of the quasiparticle tunneling amplitude.

We report observation of new fractions: 4/11, 5/13, and 6/17 between ν=2/5 and ν=1/3, 4/3 and 5/17 between ν=1/3 and ν=2/7, and 4/19 between ν=2/9 and ν=1/5. The ν=4/11 state was studied in detail. The tilting magnetic field data show that its ground state is spin-polarized.

When the effective filling factor of composite fermions is an integer, the residual interaction between them can often be neglected because the ground state of the non-interacting model is unique and incompressible. However, at non-integer composite fermion (CF) filling factors the ground state of composite fermions is enormously degenerate if the interaction between them is neglected, and consideration of the inter composite fermion interaction is necessary for determining the true ground state. In this article, we summarize certain results regarding what new states the inter composite fermion interaction can possibly produce. More details can be found in Refs. [11] and [12].

We report on magnetotransport measurements of GaAs/GaAlAs quantum cascade structures in magnetic fields up to 62 T parallel to the current. We observe novel quantum oscillations series in tunneling current that are periodic in reciprocal magnetic field and have field positions independent of the applied bias. These oscillations are explained as intersubband magnetophonon resonance due to electron relaxation by emission of optical or acoustic phonons.

In this talk I will discuss quantum interference phenomena of composite fermions in quantum Hall systems. In particular, I will focus on huge mesoscopic fluctuations of non-linear response and of Coulomb drag in single and bi-layer systems. Finally, I will give a simple physical picture of dephasing (de-coherence) of composite fermions, with simple expressions for their coherence length.

Most of the talk will be based on work done in collaboration with I. Aleiner and B. Narozhny (Stony Brook).

Spin waves, or magnons, are intrinsic excitations in magnetic materials and have attracted considerable attention since introduced by Bloch in 1930.¹ Recently ideas of spintronics have impacted this field dramatically. Berger² and Sloczewski³ introduced completely new aspects of the physics of metallic ferromagnets in the presence of an electric current of high density. Excitation of magnons by an electric current has been predicted and seen in magnetic multilayers,^4-6 where the driving force for the excitations is the spin-momentum transfer associated with an electric current flow. We have developed a microcontact technique (magnetic microcontact spectroscopy) to investigate the current-driven magnon generation.^4,5 In our experiments we inject current densities as high as 10⁹ A/cm² into a Co/Cu multi-layer through a point contact made using a sharpened Ag wire carefully brought into contact with a multi-layer file. We observe current-driven magnons in multi-layers with both ferromagnetic^4,5 and antiferromagnetic⁷ configuration of the adjacent layer magnetizations.

Some further interesting applications of the technique, particularly, for studying fluctuation phenomena, resonant magnon generation, acoustic wave generation, etc., will be presented.

There is compelling theoretical evidence¹ that the ν = 5/2 fractional quantum Hall state is a Moore-Read state² - a state which can be viewed as a spin-polarized p-wave superconductor of composite fermions. The question remains, how can one test this hypothesis experimentally? To address this we have developed a semi-phenomenological description of this state in which the Halperin-Lee-Read³ theory of the half-filled Landau level is modified by adding a p-wave pairing interaction between composite fermions by hand. The electromagnetic response functions for the resulting mean-field p-wave superconducting state are then calculated and used in an RPA calculation of the physical electronic response. In particular, we predict the wave-vector and frequency dependence of the longitudinal conductivity σ_xx(q,ω) which can be measured in surface-acoustic-wave propagation experiments.⁴

Low temperature diamagnetic shifts and FWHM linewidths of excitonic transitions in Al_xGa_1-xAs alloys are reported as a function of magnetic field (50T maximum) at 4K using photoluminescence spectroscopy. Two samples with aluminum compositions of 5 and 20%, are discussed. The slope of the diamagnetic shift at high fields allows a direct measure of the excitonic reduced mass μ.

The interaction between strong THz radiation from a free electron laser and an electron sheet in a high-mobility, low-density, GaAs/AlGaAs structure has been investigated in magnetotransport experiments over a wide range of wavelengths, intensities, magnetic fields, and temperatures. Photovoltage and photocurrent effects are evident in both longitudinal and transverse potential differences. Broad cyclotron resonance is observed to high temperature and connected with a decrease in electron density. The change in electron temperature under THz radiation is estimated from changes in the magnitude of the magnetoresistivity oscillations at low lattice temperature. The effect of the magnetic field is to suppress the rise in electron temperature relative to the zero field case.

Effects of the electron-electron interaction on tunneling into a metal in a very strong magnetic field (ultra-quantum limit) are considered. It is shown that for short-range repulsive interactions, tunneling is dominated by the renormalization of the coupling constant leading eventually to the CDW instability. Near the gap, the conductance g_T assumes a universal form. For the long-range interaction, there exists an intermediate energy E_PL such that for Δ ≪ E ≤ E_PL the conductance obeys a power-law scaling form, similar to that of a 1D Luttinger liquid. For smaller energies, scaling breaks down and g_T is again dominated by the CDW-instability. It is also shown that the energy dependence of g_T may be non-monotonic: whereas g_T is always suppressed by the interaction at lower energies, at higher energies it may also be enhanced above its non-interacting value. Such an unusual behavior gets a natural explanation in terms of multiple scattering from the Friedel oscillation near the surface.

Photoluminescence (PL) and reflectivity spectroscopy in magnetic fields up to 60 Tesla are used to investigate the excitonic optical transitions within CdTe/(Cd,Mg)Te 2D electron gases. The samples are 100 A and 200 A wide quantum well (QW) wherein the electron density is varied smoothly from 1 × 10¹¹ to 4.5 × 10¹¹ cm^-3 across the wafer. This allows us to follow the energy diagram of the neutral exciton (X₀) and the charged exciton singlet and triplet spin states (X^-_s, X^-_t) versus magnetic field and the electron density. We observe a weak PL peak above 12 T (15 T) for the 200 A QW (100 A) at the low energy side of the spectra in addition to the X^-₀ and X^-_s PL. This peak could be the signature of a fundamental dark triplet state recently predicted (Phys. Rev. B62, 4630). Moreover, for high density samples a balance of PL intensity between the X^-_s and an other X^-_t occurs in the 25T-30T range. These results are interpreted on the basis of the calculations by Wojs et al. (Phys. Rev. B62 4630) taking account the Zeeman energy of the charged exciton.

We calculate the conductivities for a model system of the integer quantum Hall effect including electron-phonon interactions, with a substrate potential chosen such that microscopic processes can be understood in detail. We find that in general the macroscopic Hall current is not only composed of velocities arising from the Schrödinger time evolution but also of velocities generated by electron-phonon interactions and that the latter can contribute to the formation of the quantized Hall plateaus. We show for typical quantum Hall systems the phonon induced contributions lead to Hall plateaus which are somewhat larger than the plateaus of the dissipative conductivity. Experimentally this difference has been known for a long time, but it seemed unexplained so far. In the case where the substrate potential has no spatial fluctuations in the direction of the macroscopic electric field, all states are conducting and the dissipative conductivity vanishes only near integer filling factors. Nevertheless, the calculated Hall conductivity shows broad quantized plateaus, which here are entirely generated by velocities arising from phonon induced relaxation processes. These results show that absence of dissipation is not indispensable for the existence of quantized Hall plateaus.

We investigate the possibility of superluminal propagation of massive particles in high magnetic fields. Formal arguments suggest that this could occur in the quantum Hall regime near zero temperature, since here a few conducting states in the center of a broadened Landau band carry a macroscopic Hall current. We investigate the microscopic processes which lead to the high velocities of conducting states in the quantum Hall effest and ask what are the highest possible velocities which can be generated in that way. Our analysis shows that in typical quantum Hall systems at the most a tiny fraction of a carrier wave function can describe superluminal behaviour but that the center of mass velocity is far below the velocity of light. On the other hand, on pure theoretical grounds we can construct special systems (with ≪exotic≫ parameters) where the Schrödinger equation does admit solutions with superluminal center of mass velocities. The described propagation has no analogy in classical physics. Einstein's causality is not violated. The time evolution of the corresponding wave function is similar to that of a recently observed superluminal light pulse.¹ We discuss analogies and differences between the two phenomena.

We study the effect of dilute impurities on a system of interacting electrons subject to a quantizing magnetic field. For the case of point impurities, the calculation of the scattering cross section can be mapped onto a 1D problem of tunnelinng conductance through a barrier for interacting electrons. We find that the electron-electron interaction produces temperature-dependent corrections to the cross-section, and thus to the tensor of conductivities. Upon summation of the most diverging corrections in all orders of perturbation theory, a scaling behavior of the conductivities emerges. This behavior is similar to that of 1D Luttinger liquid. The scaling exponents depend on the magnetic field and are calculated explicitly in the weak-coupling limit. The limitations on the scaling behavior due to the formation of a charge-density wave are also discussed.

Edge reconstruction has been a well-known effect for integer quantum Hall liquids in the presence of both electron interactions and a confining potential generated by charged background. At more generic fractional fillings, we point out that confined two-dimensional interacting electrons can exhibit the similar reconstruction effect. Our exact diagonalization results show that, in a fractional quantum Hall system with a sharp cleaved edge potential, the electron density oscillation near the edge increases with the distance between the electron gas and the background charge layer. As a results, the outermost hump can detach from the bulk beyond certain point. We suggest that the edge reconstruction effect is relevant to the recent edge tunneling experiments,¹ as well as the microwave absorption experiment on two-dimensional electrons in an antidot array.² Calculating the finite-temperature density profiles, we estimate the temperature above which the edge reconstruction disappears to further discuss the relevance to the microwave absorption experiment.

A rotator, made from polycarbonate and operated hydraulically using liquid helium, has been developed at the High B/T Facility of the National High Magnetic Field Laboratory (NHMFL) in Gainesville, It can be tilted more than 90 degrees by applying a pressure of several bars. The frictional heating was found to be negligible down to 8.0 mK. The sintered silver powder heat exchangers, specially designed for the cooling of the two-dimensional electron gas (2DEG) system in the fractional quantum Hall effect (FQHE) experiment,^1,2 are also attached to the rotator. We have conducted an angular-dependent study of the even-denominator FQHE state at ν = 5/2. Our preliminary data showed that the rotator performed well at ultra-low temperatures and high magnetic fields. This technique should be applicable for other transport measurements at such extreme experimental conditions.

This talk will discuss non-Fermi liquid and quantum critical behavior in heavy fermion materials, focussing on the mechanism by which the electron mass appears to diverge at the quantum critical point. Is the quantum critical point merely a case of electron diffraction off a quantum critical spin density wave, or does it involve a fundamental break-down in the composite nature of the heavy electron place at the quantum critical point? We discuss the nature of the critical langrangian and show that the Hall constant changes continuously in the first scenario, but may "jump" discontinuously at a quantum critical point where the composite character of the electron quasiparticles changes.

YbAl₃ is an intermediate valent compound with a Kondo temperature T_K in excess of 500 K and a rather low conduction electron density of ~0.5/atom. Recent measurements are suggestive of a second energy scale T_coh of order 50 K that dominates the low temperature (T ≪ T_coh) thermodynamic properties. Previous de Haas-van Alphen (dHvA) measurements on YbAl₃ in magnetic fields to 17 T reveal a fairly simple Fermi surface with 6 branches having effective masses m* ranging from 8 to 24 m₀ (see Refs. [2-3]). We report magnetization and dHvA results on YbAl₃ in pulsed magnetic fields up to 60 T. For T ≪ T_coh we indeed find that the magnetization 'crosses' over from the zero field energy scale T_coh to the high temperature energy scale T_K at a magnetic field B* ≈ 40 T(≈K_BT_coh/μ_B). For B > B*, we find dHvA oscillations when magnetic field is applied along the <100>, <110> and <111> directions. For magnetic field applied along <111>, the Fermi surface changes very little for B > B*, and the effective masses are all reduced by a factor of 2-3 relative to their low field values. This is due to the large change in the characteristic temperature, which goes from T_coh below B* to T_K above B*. We believe this is the first work to directly observe the two energy scales and to observe the crossover in the dominant energy scale as a function of magnetic field.

The strongly correlated metals U₂Pt₂In and U₃Ni₃Sn₄ show pronounced non-Fermi liquid (NFL) phenomena at ambient pressure. Here we review single-crystal electrical resistivity measurements under pressure (p ≤ 1.8 GPa) conducted to investigate the stability of the NFL phase. For tetragonal U₂Pt₂In(I ‖ a) we observe a rapid recovery of the Fermi-liquid T²-term with pressure. The Fermi-liquid temperature varies as T_FL ~ p-p_c, where p_c=0 is a critical pressure. The analysis within the magnetotransport theory of Rosch provides evidence for the location of U₂Pt₂In at a p=0 antiferromagnetic critical point (QCP). In the case of cubic U₃Ni₃Sn₄ we find T_FL ~ (p-p_c)^1/2. The analysis provides evidence for an antiferromagnetic QCP in U₃Ni₃Sn₄ at a negative pressure p_c= -0.04±0.04 GPa

To understand the electronic structure of the heavy fermion superconductors CeMIn₅, we have performed a comprehensive magnetic study of these materials.^1-4 Our quantum oscillation studies reveal that the Fermi surface becomes systematically more 2-d (and displays heavier effective masses) as one progresses from M=Rh to M=Ir to M=Co, consistent with the observed increase in superconducting T_c. Furthermore, dilution studies show that the f-electrons in CeRhIn₅ are substantially localized whereas in CeIrIn₅ and CeCoIn₅ a more itinerant character is observed.

The filled skutterudite compound PrOs₄Sb₁₂ displays superconductivity with a critical temperature T_c ≈ 1.85K which appears to involve heavy fermion quasiparticles with an effective mass m* ~ 50 - 100 m_e as inferred from the slope of the upper critical field curve near T_c and the specific heat. New measurements of electrical resistivity ρ(T) in applied magnetic fields, thermoelectric power, and X-ray diffraction are presented. A peak appears in the dρ/dT data for magnetic fields above 4 T that suggests the occurrence of a phase transition in the normal state of PrOs₄Sb₁₂ below ~ 1K.

Recently Takeuchi reported a weak, 'metamagnetic-like' increase in the magnetization around 42 T in single crystals of CeIrIn₅ for field in the c-direction. We report specific heat measurements on single crystal CeIrIn₅, H parallel c-axis, measured in the dc hybrid magnet at NHMFL in Tallahassee between 1.4 and 10 K. A clear anomaly in C/T in 35 T is observed to peak at 1.8 K, with an entropy of 6% of RIn2. This anomaly grows in size and shifts upwards in temperature (both monotonically) with increasing H until at 45 T T_peak=4.1 K, with the entropy associated with the transition equal to 14% of RIn2. C/T data at 32 T show³ only the high temperature side of the peak occurring above 1.4 K, while C/T data at 28.5 T show no anomaly down to 1.4 K. This is consistent with our T_peak vs H data, which imply T_peak → 0 at 26 T.

The quasi-one-dimensional S = 1/2 antiferromagnet Yb₄As₃ is studied by using low-temperature measurements of the specific heat C(T,B), thermal expansion α(T,B) and longitudinal elastic mode c₁₁(T,B). As has been previously shown [M. Köppen et al., Phys. Rev. Lett. 82, 4548 (1999)], finite magnetic fields perpendicular to the spin chains induce a gap in the spin-excitation spectrum (reminiscent of massive, soliton-like excitations) which manifests itself in distinct anomalies in the specific heat and thermal expansion. In this paper, we present an extension of the above work placing special emphasis on the lattice response and the evolution of the gap at higher fields. The main observations are: (i) the field-induced gap causes a minimum in the c₁₁ elastic constant both as a function of temperature and field. Applying a simple two-level model allows for a determination of the gap value Δ(B) as well as the constant G(B) = ∂Δ/∂ε introduced to account for the spin-lattice coupling. (ii) At B ≤ 9 T, the Δ(B) values derived from the various quantities are consistent with Δ(B) ∝ B^2/3 as predicted by the quantum sine-Gordon model. (iii) Measurements of C(T,B = const) in dc-fields up to 18 T and c₁₁(T = const, B) in pulsed fields up to 50 T,however, reveal deviations from this behavior at higher at field. (iv) Isothermal measurements of c₁₁(B) show a sharp increase above 35 T which is almost T-independent for T ≤ 10 K and whose origin is unknown.

The theory of the Abrikosov lattice structures in the unconventional superconductor UPt₃ under magnetic field parallel to the hexagonal crystal axis is presented. Only the two-component E₂ superconducting state among the other states of different symmetry is proved to be compatible with the recent observations¹ of the flux lattice in the A phase misaligned with crystallographic directions. Unlike to the one-dimensional superconductivity where anisotropic interactions caused by non-local electrodynamic corrections are essential for the vortex ordering the formation of slightly distorted triangular flux lattice in UPt₃ due to the two-dimensional nature of its superconducting state can be described already in local electrodynamics.²

We review the properties of the recently discovered CeMIn₅(M=Co, Ir, Rh) heavy fermion superconductors and discuss the present state of our understanding of these materials. A particular focus is the role that magnetic fields have played in elucidating the properties of these materials. Specifically, we discuss quantum oscillation measurements on CeMIn, the influence of applied field on the linear coefficient of specific heat, γ, and the nature of the HT phase diagrams in both the normal and superconducting states of these materials.

The electronic properties of heavy fermion alloys are dominated by spin fluctuations which are expected to become critical when tuned by pressure to a quantum critical point (QCP), entering a magnetic ordered state. Apart from the onset of exotic superconductivity, unexpected "normal conducting" behavior is found close to the QCP, which does not seem only to escape the conventional view of metals (Fermi liquids) but also the "conventional view" of an antiferromagnetic quantum phase transition in these f-metals. So far only few compounds have been investigated by neutron scattering to directly reveal the critical fluctuations spectrum. In CeCu₅₉Au₀₁ the fluctuations develop an unusual energy dependence, characterized by an exponent α = 0.75, which persist over the entire Brillouin zone, provoking an unexpected local non Fermi liquid behavior. The same unusual exponent derived from E/T scaling determines the H/T scaling of the uniform magnetization. Recent neutron scattering data in magnetic fields further confirm this picture of nearly free local magnetic moments (modified by α) emerging at the antiferromagnetic QCP in this strongly correlated electron system.

Ultrasonic velocity measurements were performed on a single crystal of URu₂Si₂ at various temperatures in the 20 ms 50T pulsed magnets at the NHMFL, Los Alamos, NM. At 4 K a three-fold splitting of the metamagnetic transition is seen; corresponding features were observed earlier in magnetization studies. As the temperature is raised, the three-fold splitting evolves first into two features, then into a single broad dip, after which it disappears entirely.

UIrGe crystallizes in the orthorhombic TiNiSi structure and undergoes an antiferromagnetic transition around 14.1 K. The low-temperature longitudinal magnetoresistance (I // B) exhibits a pronounced field-induced step at about 13 T (14 T) and a much weaker step at about 17 T (out of field range) for B // b axis (c axis). No transition was seen for B // a. Here, we report on the magnetization results in fields up to 38 T applied along the principle directions. In addition, we present new magnetoresistance results taken in the transverse (I ⊥ B)configuration in fields up to 18 T applied along the c axis. The data show intriguing differences in comparison to those taken in the longitudinal configuration. The results are discussed in terms of field-induced magnetic transitions and/or Fermi-surface changes.

The interplay of magnetism and superconductivity has been a recurring theme in heavy fermion compounds. Recently, alloys of CeRhIn₅ (antiferromagnet at 3.8 K) and CeIrIn₅ (superconductor at 0.4 K) have been grown, i.e. CeRh_1-xIr_xIn₅. For x = 0.5, specific heat and magnetic susceptibility measurements indicate at ~3.6 K an antiferromagnetic transition occurs. Decreasing temperature still further, a superconducting transition is observed at 0.8 K. Thus this alloy series offers the possibility of exploring the coexistence of superconductivity and magnetism in a Ce-based heavy fermion system. We have initiated a magnetotransport study from 1.4 to 300 K with applied fields to 18 T, to probe the behavior of these systems at temperatures above the possible coexistence region. In this paper we will focus on comparing the high field magnetotransport properties of CeRh_0.9Ir_0.1In₅ to the end compounds CeRhIn₅ and CeIrIn₅.

Measurements of the de Haas-van Alphen effect in Ce_xLa_1-xMIn₅ show that x dependence for the three different value of M behaves quite differently. The Ce 4f electrons remain localized and have little interaction for all x in the Rh alloys, but strongly interact with the conduction electrons in both the Ir and Co alloys. Both the dependence on x of the observed frequencies and the effective masses are presented. Preliminary measurements on CeRh_1-xIr_xIn₅ will also be given.

The anisotropic properties of CeNiGe₂ with a layered crystal structure have been studied by measurements of electrical resistivity, magnetic susceptibility and magnetization. It is confirmed that CeNiGe₂ undergoes two-step antiferromagnetic transition at T_N^I = 4 K and T_N^II = 3 K as reported earlier on polycrystalline samples. CeNiGe₂ is found to exhibit highly anisotropic properties with an easy magnetization axis along the longest crystallographic b direction. The magnetization ratio M(H//b)/M(H⊥b) is estimated to be about 15 at 5 T. The in-plane resistivity ρ_⊥^b(T) shows double maxima typical of that expected when an interplay of crystal-field and Kondo effects plays a role. The low-T maximum at 4-K in ρ_⊥^b(T) is strongly suppressed with increasing magnetic field. This negative magnetoresistance is a result of the strong reduction of magnetic scattering by the ferromagnetic alignment of Ce magnetic moments.

The special quantum oscillations of the kinetic coefficients in magnetic field ("high-temperature" oscillations - HTO) discovered by the author in 1974 have been considered. The oscillations are periodic in the reciprocal magnetic field and are characterized by a frequency higher then that of Shubnikov-de Haas (SdH) oscillations. The results of joint studies of SdH oscillations and HTO of the magnetoresistance for pure Bi and alloy BiSb in magnetic field up to 33 T are presented. The oscillations are measured for different magnetic field directions at temperature 4 – 30 K. It was found that SdH holes oscillations and holes HTO reached its quantum limit at the same value of magnetic field. The analysis of the experimental data verified one of two alternative models of HTO.

We measured the magnetization of Y_1-xCa_xVO₃ compounds with 0 Ω × Ω, 0.6 in pulsed magnetic fields up to 56 T. YVO₃ exhibits a slightly S-shaped magnetization curve with an inflection point at B_c = 42 T. the critical field B_c was found to decrease with increasing calcium content, and no sign of an S-shape is seen for the compound with x = 0.6, which is at the boundary of the metal-to-insulator transition. Our data provide further evidence that the metal-to-insulator transition in this system coincides with the transition from a paramagnetic to a magnetic ground state.

The magnetic fluctuation spectra are calculated for various phases of anisotropic superconductors. Both singlet and triplet superconducting phases can support antiferromagnetic fluctuations for small values of the Coulomb repulsion, the spectra show power law frequency dependencies due to the nodes in the superconducting order parameter. For larger strengths of the Coulomb interaction, low energy critically damped collective excitations develop from the quasi-particle density of states in the gap. The q and ω dependence of the spectra can be used to deduce the character of the superconducting order parameter.

The velocity and attenuation of longitudinal ultrasound were measured on two single crystals of UPt₃ in magnetic fields up to 33 T and temperatures down to 40 mK. For fields applied in the basal plane, a strong dip in the velocity is seen at a field of 20 T in both samples and is accompanied by a simultaneous increase in the dc magnetization. At lower temperatures the dip gets more pronounced and sharper. At a temperature around 0.6 K the velocity dip develops a second feature at a slightly higher field of ~21.6 T.

The synthetic organic compound λ(BETS)₂FeCl₄ undergoes successive transitions from an antiferromagnetic insulator to a metal and then to a superconductor as a magnetic field is increased. We use a Hubbard-Kondo model to clarify the role of the Fe³⁺ magnetic ions in these phase transition. In the high-field regime, the magnetic field acting on the electron spins is compensated by the exchange field H_e due to the magnetic ions. This suggests that the field-induced superconducting state is the same as the zero-field superconducting state which occurs under pressure or when the Fe³⁺ ions are replaced by non-magnetic Ga³⁺ ions. We show how H_c can be extracted from the observed splitting of the Shybnikov-de Haas frequencies. Furthermore, we use this method of extracting H_e to predict the field range for field-induced superconductivity in other materials. We also show that at high fields the spin fluctuations of the localized spins are not important.

Experimental studies made on organic conducting salts of the composition α-(BEDT-TTF)₂MHg(SCN)₄ (where M = K, Tl and Rb) indicate that they exhibit persistent currents at magnetic fields exceeding 23T. The presence of currents cannot be explained by the quantum Hall effect, while superconductivity seems unlikely. All indications point towards a new type of dissipationless current flow involving relative gradients in the pinning of a CDW and quantized orbital magnetism.

The interlayer transfer integrals of various organic superconductors have been deduced using high-magnetic-field techniques. The measurements demonstrate the inappropriateness of criteria used to denote incoherent interlayer transport.

Electronic states have been investigated under high magnetic fields for quasi-two-dimensional organic conductors λ-(BETS)₂Fe_xGa_1-xCl₄ (x = 1.0, 0.47, 0.45, 0). Under magnetic field parallel to the c-axis, various electronic states appear, depending on x, field, and temperature. The global phase diagram for this alloy is presented and the evidence of the strong internal field due to the Fe moments is shown.

Theoretical overview of the quantum Hall effect in quasi-one-dimensional (Q1D) organic conductors (TMTSF)₂X will be presented.¹ The emphasis will be on the recent developments, such as the temperature evolution of the Hall effect^2,3 and the chiral edge states.⁴ Time permitting, a theory of the nonchiral electron edge states in a Q1D triplet superconductor⁵ and the holon edge states in the charge-gap regime⁶ will be presented too.

The pressure-tuned quantum phase transition from spin-density wave (SDW) to Fermi Liquid (FL) ground states is studied in (TMTSF)₂PF₆ by NMR relaxation and spectroscopy. As for recent transport measurements, we find evidence in the distribution of local hyperfine fields for phase segregation, from which we conclude the transition between normal and SDW phases is discontinuous. A tricritical point (TCP) is identified. A crossover to FL behavior identified by spin-lattice relaxation measurements intersects the normal to SDW phase boundary very close to the TCP.

The effects of hydrostatic pressure and magnetic field on the conductivity of oriented polyacetylene films doped with FeCl₄- up to metallic state have been studied at T = 0.35-300 K. It was found that application of pressure up to 10 kBar increases the conductivity at 300 K by a factor of 1.3 and suppresses the resistivity minimum in ρ(T) at 270 - 280 K. At T < 2 K the temperature dependence of resistivity ρ(T) ~ ln T (at ambient pressure and at 10 kBar) which remains unaltered by a magnetic field up to 14 Tesla. Transverse magnetoresistance (MR) was found to be negative, linear and temperature independent at T < 2 K (at ambient and high pressure). We attribute the observed ρ(T) and MR behavior to weak localization accompanied by effect of electron dephasing time saturation due to scattering by two-level systems of a special type. More strong behavior of ρ(T) and MR at T > 2 K related to further suppression of weak localization due to scattering by low-energy excitations of a "glassy" type.

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor λ-(BETS)₂FeCl₄. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T, and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field compensates the exchange field of aligned Fe³⁺ ions. We further argue that the Fe³⁺ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state. Here we provide experimental evidence indicating that this state, as well as the insulating antiferromagnetic ground state, is extremely sensitive to hydrostatic pressure.

Pulsed magnetic fields to 53 tesla reveal a new high field phase transition in τ-[P-(S,S)-DMEDT-TTF]₂ (AuBr₂)(AuBr₂)_y (where y~.75). This transition appears as an abrupt increase in the magnetoresistance at a threshold field (B_Th~44 tesla for T = 0.55 K). B_Th shifts toward higher magnetic fields with increasing temperature, and there is evidence of higher field sub-phases. Measurements of Shubnikov de Haas oscillations below 33 tesla have provided compelling evidence as to the two-dimensional (2D), metallic nature of this compound. In the following report we discuss this new unexpected state, reminiscent of the field-induced spin density wave (FISDW) behavior in the quasi one-dimensional (Q1D) Bechgaard salts.

We report detailed angle dependent studies of the metallic state microwave (40 to 200 GHz) magneto-conductivity of single crystal samples of the α-(BEDT-TTF)₂KHg(SCN)₄ organic charge density wave conductor, and the perovskite superconductor Sr₂RuO₄. We observe series' of resonant absorptions which we attribute to periodic orbit resonances - a phenomenon closely related to cyclotron resonance. By performing measurements on several samples, and in different electromagnetic field configurations, we are able to couple to different orbital modes (+ harmonics), which derive from deformations (warpings) of the quasi-one and quasi-two-dimensional Fermi surfaces of these compounds. These studies provide vital information concerning interlayer dispersion which, in turn, affects the Fermi surface nesting characteristics which are believed to play a crucial role in the low temperature physics of these exotic materials.

The effect of interfibrillar interaction on the charge transport of doped polyacetylene is investigated by studying the high field magnetoconductivity of iodine doped helical polyacetylene. The zero-field resistivity ratio, ρ_r = ρ(1.2 K)/ρ(300 K), is comparable to that of stretch-oriented high-density polyacetylene, which indicates the partial alignment of chains inside a polymer fiber. At low magnetic fields, the small negative component of magnetoconductivity was observed and its magnitude increases as the ρ_r value increases. In the high field region, the magnetoconductivity is positive and it clearly shows the linear dependence on the magnetic field up to H = 30 T. The linear field dependence of magnetoconductivity is different from what is expected in the three-dimensional weak localization picture. For the same ρ_r value samples, the magnitude of negative magnetoconductivity of S-polyacetylene is much bigger than that of R-polyacetylene, which could be attributed to the difference in the degree of helicity determining the strength of interfibrillar interaction.

We present vibrating - wire viscosity measurements on a very dilute ³He-⁴He mixture (χ₃ = 150 ppm) in fields up to 14.8 T and temperatures down to 3 mK. The ³He spin polarization is greater than 99% for the highest field and lowest temperature used. In these conditions, the s-wave scattering rate decreases due to a lack of quasiparticles with the minority spin state. This enhances all transport coefficients including the viscosity. At the lowest temperature, the hydrodynamic damping of the viscometer in a 14.8 T field was more than 10 times larger than in low fields. This indicates a many-fold increase in the mixture viscosity due to spin polarization.

Results of NMR studies of the nuclear spin-spin relaxation times at low temperatures are reported for commensurate monolayers of ³He physisorbed on hexagonal boron nitride. Temperature independent relaxation rates are observed for 0.11<T<0.56 K, and are interpreted in terms of the quantum exchange motions of the ³He atoms. The relaxation times and the inferred effective exchange rates were observed to change significantly on replacing the ³He atoms with relatively immobile Ne atoms. This result is understood in terms of a large 3-spin exchange in addition to 2-spin exchange.

Experimental studies of the low temperature behavior of mono-layers of ortho-H₂ - para-H₂ mixtures using low temperature NMR techniques reveal an unusual phase diagram. An extension of the mean field approach is proposed to explain the qualitative features of the observed phase diagram. The competition between the highly frustrated short-range quadrupolar fields and the local crystal fields plays a key role in determining the nature of the ordered structures.

By extending the BCS theory to the strong-coupling regime with small bipolarons we explain the common origin of the anomalous upper critical field in high-Tc cuprates, in some organic superconductors, spin ladders and cubic oxides. The bipolaron theory accounts well for the normal state c-axis negative magnetoresistance in high magnetic fields,¹ normal state diamagnetism,² c-axis tunneling and Andreev reflection,³ angle-resolved photoemission,⁴ normal gaps and stripes⁵ observed in cuprates.

Spinless point defects in the CuO₂ planes of the cuprates have been shown for long to induce a magnetic response on the neighboring copper sites which behaves as a nearly free local moment in the under-doped case.^1,2 In the case of Li⁺ substitution on the Cu site, accurate ⁷Li NMR shift data allow to evidence that the local moment susceptibility displays a (T + T_K) ^-1 dependence, with a Kondo like temperature T_K which increases abruptly near optimal doping.³ While a Curie paramagnetic response can be understood on general theoretical grounds in the case of undoped quantum spin systems, the Kondo screening of the moment might directly reflect the influence of charge carriers. This Kondo like behavior is also corroborated by measurements of the fluctuation time of the local moment deduced from ⁷Li NMR spin lattice relaxation time.⁴ The local moment is found to survive in the superconducting state although a marked reduction of the screening is observed when T_K > T_c (see Ref. [5]), as might be expected from theoretical considerations.

Correlatively it will be shown that the scattering of charge carriers by spinless defects can be studied accurately in single crystals in which such defects are created by electron irradiation. The low T upturns of the resistivity can be associated with a Kondo-like T dependence of the scattering in the under-doped case. In the over-doped case, T_K being very high, the scattering becomes T-independent and 2D weak localization effects dominate.⁶

Both impurities and magnetic field can be used as a tool to investigate the properties of superconducting state of cuprates. Recent STM tunneling experiments on impurity states in superconducting state provide a new set of results that can be compared with the expectations for d-wave pairing in cuprates. We will argue that impurities and magnetic field could be used to test the nature of the pseudogap state (PG).

i) We will present the extension of the theory of impurity on the pseudogap regime. The impurity such as Zn or Ni, will generate resonance state in PG state on very general grounds. Single vs double impurity peak will appear depending on the superconducting vs non-superconducting origin of the PG.

ii) For the particular proposal of the d- density wave PG (DDW) we found that out of plane magnetic field will induce complex, proportional to the field, id_xy-wave density wave component. Physical properties of this, previously not considered, state will be discussed.

We consider the pairing state due to the usual Bardeen-Cooper-Schrieffer mechanism in substances where the Fermi surface forms pockets around several points of high symmetry. The symmetry imposed on the multiple pocket positions can give rise to a multidimensional nontrivial superconducting order parameter. A complicated mechanism is not required for unusual superconductivity to exist, just the usual phonon attraction and Coulomb repulsion. Time reversal symmetry is broken, so the superconducting state also has orbital magnetism. In some sense, these superconductors are similar to superfluid³ He, which is known to have an 18-dimensional order parameter and a complicated phase diagram. We suggest several candidate substances where ordering of this kind may appear and discuss how these phases may be identified. Superconductivity in these new materials is expected to be much less sensitive to impurities than in other non s-wave superconductors.¹

The pseudogap phase of the cuprate superconductors is argued to be characterized by a hidden broken symmetry of d-wave character in the particle-hole channel that leads to staggered orbital magnetism. This proposal has many striking phenomenological consequences, but the most direct signature of this order should be visible in the neutron scattering experiments. The theoretical underpinning of these experiments is discussed.

Perhaps the most puzzling attribute of the high-temperature cuprate superconductors is that for underdoped materials the energy gap normally associated with superconductivity is observed well above the superconducting transition temperature T_c. Neutron scattering measurements of YBa₂Cu₃O_6.6 have identified small moments that increase in strength as the temperature is reduced below the pseudogap temperature T*. Polarized beam neutron scattering measurements demonstrate that the signal is magnetic in origin. Furthermore, the pattern of neutron intensities found in the experiment is unusual as it is not readily understandable in terms of conventional spin magnetism. Although we have considered the origin of the moment in terms of the orbital bond currents, new experimental data suggest that only part of the signal is consistent with this interpretation. We present detail analysis of polarized neutron data and systematic studies of oxygen chain ordering. Finally, we discuss our most recent results on spin-glass phase of Ca-doped YBa₂Cu₃O₆.

Cuprate superconductors in the normal state exhibit the "anomalous Hall effect",¹ie., the longitudinal components of the current scattters linearly with T whereas the Hall angle obeys a T² scattering, where T is temperature. To help resolve this anomaly, we have developed a technique to measure the Hall angle at far-infrared frequencies. Measuring the magnetotransmission spectrum of polarized light through thin film of YBCO, we are able to independently measure both the real and imaginary parts of the complex Hall response. With a spectral range from dc to 250 cm^-1, a lineshape analysis of the Hall response reveals that the Hall angle spectrum has a non-Lorentzian from thereby ruling out Drude-like transport models of the Hall effect. Instead it shows a dominant Loretnzian-squared response, implying a square-scattering behavior with a scattering rate that is linear in T and which quantitatively fits the linear scattering rate of the longitudinal current. The experimental results strongly constrain the number of theories that can accurately describe transport in normal state cuprates.

We map the field-temperature (H-T) diagram of the pseudogap state in Bi₂Sr₂CaCu₂O_8+y crystals in a wide range of hole doping. By probing the quasiparticle density-of-states with the c-axis interlayer resistivity measurements in fields up to 60 T we make a systematic evaluation of the pseudogap closing field H_pg that restores the superconductor to its ungapped state. In contrast to the characteristic fields of the superconducting state, H_pg(T) at low temperatures is temperature independent and scales with the pseudogap temperature T* by a simple Zeeman energy scale, indicating a predominant role of spins over the orbital effects in the formation of the pseudogap.

There is strong evidence that magnetic interactions play a crucial role in the mechanism driving high-temperature superconductivity in cuprate superconductors. To investigate this further we have done a series of neutron scattering measurements on La_2-xSr_xCuO₄ (LSCO) in an applied magnetic field. Below T_c the field penetrates the superconductor via an array of normal state metallic inclusions or vortices. Phase coherent superconductivity characterized by zero resistance sets in at the lower field-dependent irreversibility temperature (T_irr). We have measured optimally doped LSCO (x = 0.16, T_c = 38.5 K) and underdoped LSCO (x = 0.10, T_c = 29 K); both have an enhanced antiferromagnetic response in a field. Measurements of the optimally doped system at H = 7.5 T show that sub-gap spin fluctuations first disappear with the loss of finite resistivity at T_irr, but then reappear at a lower temperature with increased lifetime and correlation length compared to the normal state. In the underdoped system elastic antiferromagnetism develops below T_c in zero field, and is significantly enhanced by application of a magnetic field. Phase coherent superconductivity is then established within the antiferromagnetic phase at T_irr; thus, the situation in underdoped LSCO is the reverse of that for the optimally doped LSCO where the zero-resistance state develops first before the onset of antiferromagnetism.

Recent neutron scattering, nuclear magnetic resonance, and scanning tunneling microscopy experiments have yielded valuable new information on the interplay between charge and spin density wave order and superconductivity in the cuprate superconductors, by using a perpendicular magnetic field to tune the ground state properties. We compare the results of these experiments with the predictions of a theory which assumed that the ordinary superconductor was proximate to a quantum transition to a superconductor with co-existing spin/charge density wave order.

Nuclear spin-lattice relaxation rate 1/T₁ at the planar O(2,3) and bridging O(4) oxygen sites in NdBa₂Cu₃O₇ have been measured as a function of temperature in fields up to 42T. Data is characterized by extremely fast relaxation due to Nd. A well-defined peak is observed, indicative of spin freezing, occurring well below T_c, and higher than T_N, and whose physical origin is unknown. Extracted T₁ data for Cu(2) shows enhanced Cu-Cu correlations in the superconducting state, suggesting a short-ranged order at lower temperatures.

We report low-temperature Hall coefficient in the normal state of the high-T_c superconductor Bi₂Sr_2-xLa_xCuO_6+δ. The Hall coefficient was measured down to 0.5 K by suppressing superconductivity with a 60 T pulsed magnetic field. The carrier concentration was varied from overdoped to underdoped regimes by partially substituting Sr with La in a set of five samples. The observed saturation of the Hall coefficient at low temperatures suggests the ability to extract the carrier concentration of each sample. The most underdoped sample exhibits a diverging Hall coefficient at low temperatures, consistent with a depletion of carriers in the insulating ground state. The Hall number exhibits a sharp peak providing additional support for the existence of a phase boundary at the optimal doping.

The properties of electron(n)-doped cuprate superconductors show significant deviations from those of their hold(p)-doped counterparts. Experiments prior to 2000 suggested an s-wave pairing symmetry as opposed to d-wave pairing symmetry in hole-doped cuprates. Recent experiments have suggested that n-doped cuprates have a d-wave pairing symmetry. However tunneling spectroscopy of these materials have not revealed a zero bias conductance peak (ZBCP), which is a classic signature of d-wave symmetry. We present the first tunneling spectroscopy data on n-doped Pr_2-xCe_xCuO₄ (PCCO) using point contact junctions which show a systematic evolution of the ZBCP. This method of junction fabrication is important as it allows the barrier strength between the normal and the superconducting electrodes to be varied. We show that this is essential to observing the ZBCP. The n-doped cuprates have a low T_c (~25 K) and H_c2 (~10 T). The low H_c2 enables us to obtain the normal state in PCCO at low temperatures. We have used this to probe the density of states in the normal state of PCCO. We observe an anomalous gap even in the normal state.¹ This normal state gap (NSG) becomes smaller on the over-doped side. We discuss the behavior of this NSG in the context of the pseudogap which has been observed in hole-doped cuprates.

We study the sensitivity of T_c and the pseudogap onset temperature, T*, to low fields, H, for cuprate superconductors, using a BCS-based approach extended to arbitrary coupling. We find that T* and T_c, which are of the same superconducting origin, have very different H dependences. The small coherence length makes T* rather insensitive to the field. However, the presence of the pseudogap at T_c makes T_c more sensitive to H. Our results for the coherence length ξ fit well with existing experiments. We predict that very near the insulator ξ will rapidly increase.

We present specific-heat measurements on Mg¹¹B₂ in magnetic fields to 9 T. The anomaly at T_c is rapidly broadened and attenuated in fields, as expected for an anisotropic, randomly oriented superconductor. At low temperature there is a strongly field-dependent feature that shows the existence of a second energy gap. The Sommerfeld constant, γ, increases rapidly and non-linearly with magnetic field, which cannot be accounted for by anisotropy. It approaches γ_n = 2.6 mJ K^-2mol^-1, the coefficient of the normal-state electron contribution, asymptotically for fields greater than 5 T. In zero magnetic field the data can be fitted with a phenomenological two-gap model, a generalization of a semi-empirical model for single-gap superconductors. Both of the gaps close at the same T_c; one is larger and one smaller than the BCS weak coupling limit, in the ratio ~ 4:1, and each accounts for ~ 50% of the normal-state electron density of states. The parameters characterizing the fit agree well with those from theory and are in approximate agreement with some spectroscopic measurements.

We consider structure of the Cooper wave function for superconductivity in a surface layer. Broken space inversion at the surface results in lifted spin degeneracy and in two branches of the gapped energy spectrum as caused by the spin-orbit interaction. The pair wave function consists of a mixture of both singlet and triplet components. Anisotropy of the Knight shift measurable in the NMR experiments is calculated in the whole temperature regime. Implications for a few known experimental situations is briefly discussed.

Magnetotransport properties of pure Mg metal and MgB₂ samples with varying amounts of excess Mg were systematically studied in magnetic fields up to 18 T. It is found that the both the Mg and inhomogeneous MgB₂ samples show large low temperature conductivity, residual resistance ratio (RRR) and magnetoresistance (MR) under high fields. Calculations of the generalized effective medium theory show that the large RRR and MR of the inhomogeneous MgB₂ samples can be explained by unusual magnetotransport properties of pure Mg metal.

Anomalous re-entrant superconducting-normal resistive transition was observed in Sr_0.4K_0.6BiO₃ superconductor i.e., normal – supernormal behavior as temperature is increased. Contrary to previously reported re-entrant resistive behaviors in other compounds, the re-entrant resistivity appearing at zero magnetic field in Sr_0.4K_0.6BiO₃ is suppressed to zero by applying an external magnetic field (H) or increasing the electrical transport current (I): an observation of a zero resistive superconducting state induced by H or I. Comparisons of the normal-state resistivity data in different samples indicate an important role that disorder in the junction barriers between superconducting grains might play on the observed re-entrant resistivity behavior. Possible physical origins of this anomalous phenomenon are discussed.

The NMR and ESR analysis revealed the dynamical coexistence of superconductivity and the antiferromagnetic correlations in the entire superconductivity region of La_2-xSr_xCuO₄ with the spin stiffness enhancement near x = 0.12.

There is strong evidence that magnetic interactions play a crucial role in the mechanism driving high-temperature superconductivity in cuprate superconductors. To investigate this we have done a series of neutron scattering measurements on La_2-xSr_xCuO₄ (LSCO) in an applied magnetic field. Below T_c the field penetrates the superconductor via an array of normal state metallic inclusions or vortices. Phase coherent superconductivity characterized by zero resistance sets in at the lower field-dependent irreversibility temperature (T_irr). We have measured optimally doped LSCO (x = 0.16, T_c = 38.5 K) and under-doped LSCO (x = 0.10, T_c = 29 K); both have an enhanced antiferromagnetic response in a field. Measurements of the optimally doped system at H = 7.5 T show that sub-gap spin fluctuations first disappear with the loss of finite resistivity at T_irr, but then reappear at a lower temperature with increased lifetime and correlation length compared to the normal state. In the under-doped system elastic antiferromagnetism develops below T_c in zero field, and is significantly enchanced by application of a magnetic field. Phase coherent superconductivity is then established within the antiferromagnetic phase at T_irr; thus, the situation in under-doped LSCO is the reverse of that for the optimally doped LSCO where the zero-resistance state develops first before the onset of antiferromagnetism.

We propose methods to reveal the existence of a two-component triplet order parameter (which is recently suggested to break a time reversal symmetry in Sr₂RuO₄) and to test a possible two-component singlet d-wave order parameter in high-T_c superconductors (which may also break the time reversal symmetry). We have also determined a spin part (i.e., d(k) = [d_x(k) ≠ 0, d_y(k) = 0, d_z(k) = 0)] of a triplet order parameter^1,2 in (TMTSF)₂PF₆ under pressure by means of an analysis of the experimental H_c2 measurements (I. J. Lee et al., Phys. Rev. Lett. 78, 3555 (1997)).

The U(1)×SU(2) Chern-Simons gauge field theory, proposed by the authors to explain in a unified fashion the metal-insulator crossover of the in-plane resistivity upon temperature decrease in heavily underdoped cuprates without magnetic field and a similar phenomenon, observed in several classes of superconducting cuprates, when a strong magnetic field suppresses the superconductivity, is briefly outlined and confronted with recent experiments.

Measuring the electrical resistivity of layered superconductors yields interesting information regarding the mixed and normal state of the system. Because of the high degree of anisotropy of many systems it becomes impossible to distinguish between the relatively small resistivity associated with ρ_xy and the much larger ρ_zz using traditional 4 lead techniques. Moreover, dissipation associated with vortex pining masks the crossover from superconducting to normal state. Recent development of radio frequency (rf) techniques in milli-second pulsed high magnetic fields have resulted in a powerful tool to investigate metals and superconductors at extremes in H-T space. The rf technique is sensitive to changes in the most conducting regions of the sample, hence the in-plane resistivity is measured. By measuring the change in skin depth as a function of applied magnetic field and temperature, a reliable method for mapping the upper critical field is realized. Direct comparison with thermodynamic measurements such as magnetization and thermal conductivity show compelling agreement.

We use electron irradiation at low temperature to introduce defects in a controlled manner in the cuprates. We have shown in YBCO₇ that the main influence on transport properties is due to stable defects in the CuO₂ plane. They are found analogous to Zn substitution on the copper planar site.¹ Compared to impurity substitutions, the great advantage of electron irradiation is to allow to study the physical properties of a single sample with an increasing defect content. T_c and electrical resistivity measurements have been performed on electron irradiated single crystals of different cuprates (YBCO, Bi-2212, Hg-1223, T1-2201) for a wide range of hole dopings n_h. A simple correlation between the decrease of T_c and the increase of residual resistivity ΔR_2D is found, as expected for d ware superconductivity. The ratio ΔT_c/ΔR_2D, which is expected to be proportional to n_h/m* is found to reproduce the variation of n_h from the under-doped to the over-doped regime.² This demonstrates that the hole content is the relevant parameter to describe the transport properties all over the phase diagram. For large defect contents, low T upturns of the resistivity are observed in under-doped YBCO_6.6 and over-doped T1-2201 (see Ref. [3]). In the highly over-doped T1 compound the decrease of conductivity scales as expected from weak localization theory. For YBCO_6.6 the large low T contribution to the resistivity is shown to be initially proportional to the defect content. It might therefore be associated to Kondo like spin flip scattering term. This would be consistent with the results on the magnetic properties induced by spinless defects such as Li (see Ref. [4]).

In-plane ρ_ab and out-of-plane ρ_c resistivity measurements were performed on the same antiferromagnetic YBa₂Cu₃O_6.25 single crystal over a wide range of temperatures T. ρ_ab(T) exhibits two crossovers with decreasing T: a crossover from metallic to weak localization behavior at 175 K and a second crossover to two-dimensional chiral variable-range hopping VRH behavior at 115 K. The latter reflects the topologic excitations induced in the spin system. ρ_c(T) displays an dependence at high T and a VRH type dependence below 115 K. The T derivative of ρ_c(T) shows a kink at 32.65 K which we attribute to the antiferromagnetic ordering of the Cu(1) spins.

We report magnetotransport measurements made on the anisotropic magnetic superconductor ErNi₂B₂C. We observe a minimum in the magnetoresistance with fields along the a-axis, a behavior consistent with the high field quenching of magnetic scattering. When the field is applied along the c-axis the magnetoresistance is positive. The temperature of the antiferromagnetic phase transition is depressed by magnetic fields applied along the c-axis (as it is along the a-axis) and is suppressed near 17 T. A feature in the first derivative of the temperature dependent resistivity may be related to the weak ferromagnetic phase. We find the temperature at which this feature occurs to be independent of magnetic fields to 18 T.

We report low-temperature magnetotransport in the normal state of the electron-doped superconductors, Nd_2-xCe_xCuO₄, Pr_2-xCe_xCuO₄, and La_2-xCe_xCuO₄, by suppressing the superconductivity with high magnetic fields. The normal state ρ-T curve shows an up-turn at low temperatures, which has a log T dependence with saturation at lowest temperatures. The up-turn is gradually suppressed with increasing magnetic field, resulting in negative magnetoresistance. We discuss these findings on the basis of the Kondo scattering originating from the magnetic moments of Cu²⁺ ions.

Using various synthesis techniques, we have obtained a number of interesting results regarding the magnesium diboride system. First, we have observed a maximum T_c of 42 K in Mg_1-xB₂ films on crystalline boron substrates, which is significantly higher than the reported value of 39 K. Furthermore, our study of Mg-vacant Mg_1-xB₂ specimens, fabricated by using high-pressure (35 MPa) sintering, provides convincing evidence for electronic phase separation, probably induced by Mg-vacancy clustering.

Simultaneous magnetotransport and torque magnetization measurements have been performed on superconducting (B, Ka)BiO₃ single crystals. The measurements have been carried out at very low temperatures (0.03 k < T < 2 K) and up to high magnetic fields: (30Tesla ~ H_c2(0)). The resistivity drops below the experimental resolution for a magnetic field H_R(T) which is very close to the irreversibility line deduced from torque magnetometry. Close to H_R(H) the resistive transition can be well by the vortex-glass scaling formalism¹ suggesting that the vortex solid melts into a liquid for H > H_R. Our measurements show that the irreversibility line in (K, Ba)BiO₃ is closely related to this vortex glass transition line but, surprisingly, a large reversible (i.e. liquid) phase can still be observed at very low temperatures (30 mK).

Magnetotransport measurements are presented on polycrystalline MgB₂ samples. The 'resistive' upper critical magnetic field reveals a temperature dependence with positive curvature from T_c = 39.3 K down to about 20 K, then changes to slightly negative curvature reaching 26 T at 1.5 K. The 26-Tesla upper critical field is much higher than what is known so far on polycrystals of MgB₂ but it is in agreement with the recent data obtained on epitaxial MgB₂ films.¹ The deviation of H_c2(T) from standard BCS might be due to the proposed two-gap superconductivity in this compound.² Recently we have found evidence for the existence of two superconducting energy gaps in this system using Andreev reflection spectroscopy.³ The temperature dependence of the resistivity and the normal state magnetoresistance are analyzed.

A large positive magnetoresistivity (up to ten percent or more) is observed for both under-doped and over-doped superconducting La_2-xSr_xCuO₄ epitaxial thin films at temperatures far above the superconducting critical temperature T_c. This magnetoresistivity can be related to the pairs eventually forming the superconducting phase at low temperatures. Our observations support the idea of a close relation between the pseudogap and the superconducting gap and provide new indications for the presence of pre-pairs above T_c.

We have made measurements of resistivity and penetration depth in dc-and pulsed-magnetic fields of the organic superconductors α-(ET)₂NH₄Hg(SCN)₄, and κ-(ET)₂Cu(NCS)₂ using a resonant rf circuit powered by a tunnel diode oscillator (TDO). We compare the critical fields as measured by resistivity, and the TDO. All the superconductors we have studied have anisotropic critical fields with a ratio H_c2// to H_c2⊥ of 5-17. In κ-(ET)₂Cu(NCS)₂ we observed an almost monotonic increase in the parallel critical fields up to 24 T at 390 mK, and a sharp cusp in the critical field as a function of angle when the sample is parallel to the magnetic field. This suggests that the superconducting layers are only Josephson-coupled and the superconductivity is two-dimensional(2D). Another salt, α-(ET)₂NH₄Hg(SCN)₄, shows similar cusp behavior, but this material shows evidence of extreme Pauli limiting when the field is turned parallel to the conducting layers.

An overview of the phenomenological and microscopic theory of the superconducting state in Sr₂RuO₄ is presented. Existing experimental evidence will be used to demonstrate that the superconducting state is a 2D analogue of the Anderson Brinkman Morel superfluid ³He state. A discussion of the microscopic theory will be presented that focuses on the recent experimental observations that line-nodes exist in superconducting Sr₂RuO₄. These nodes come as a surprise since theory would predict a spin-triplet pairing state to be nodeless for this quasi-2D material. Finally, the consequences of magnetic fields on this pairing state will be described.

We report a study of the angle-dependent millimetre-wave magnetoconductivity of the p-wave triplet-paired perovskite superconductor Sr₂RuO₄. We find two harmonic series. We assign the first to interlayer cyclotron resonance of the β-Fermi surface and its harmonics, yielding a cyclotron effective mass of . We assign the second series, which contains only odd harmonics, to cyclotron resonance of the γ-Fermi surface, yielding a cyclotron effective mass of . In addition, we find a very strong absorption mode in the presence of a magnetic field component parallel to the quasi-two-dimensional (Q2D) planes of the sample. Its dependence on the orientation of the magnetic field cannot be described in the context of conventional Q2D cyclotron resonance, and the origin of this mode is not yet clear.

Some recent neutron scattering works on CeRhIn₅ and Ce₂RhIn₈, together with related resistivity and specific heat measurements, are summarized. In spite of its layered crystal structure, CeRhIn₅ is shown to be 3 dimensional both magnetically and in transport. We also find that the Fisher-Langer behavior is closely followed in CeRhIn₅. This may circumvent the Kondo lattice model and support applying established Fermi-liquid superconductivity theory to heavy fermion superconductors.

Neutron scattering experiments have been carried out in high magnetic fields to understand the magnetized state of the uniform spin-1 antiferromagnetic chain. In zero field this system has a cooperative singlet ground state with a gap to a propagating triplet excitation. The quasi-one-dimensional uniform spin-1 antiferromagnets NENP (Ni(C₂D₈N₂)₂NO₂CIO4) and NDMAP (Ni(C₅D₁₄N₂)₂N₃PF₆) were examined. NENP has an alternating g-tensor such that staggered magnetization is induced for arbitrarily small fields and there is no finite field phase transition. In the high field phase the lowest energy mode has a field dependent gap and an unusually small effective spin wave velocity. NDMAP has only one spin per unit cell and hence a uniform g-tensor. This system has a critical transition at a finite field. Surprisingly the high field phase has quasi-two-dimensional or three-dimensional long-range order depending on the direction of the applied field. In the paramagnetic high field phase immediately above the ordering transition of NDMAP, there are gapless magnetic excitations that are broader in Q than the higher energy gap mode. The data is compared to theoretical predictions of a gapless incommensurate phase above the critical field.

Ca₃Ru₂O₇ has a double Ru-O layered orthorhombic structure. It is a "bad" metal characterized by various abrupt magnetic and electronic transitions that are highly anisotropic. Ca₃Ru₂O₇ undergoes antiferromagnetic ordering at T_N = 56 K and a metal to poorer metal transition at T_M = 48 K with a unique antiferromagnetic metallic phase intermediate between the paramagnetic and antiferromagnetic phases. Our recent study reveals a few more striking features: (1) Quantum oscillations, i.e., the Shubnikov-de Haas effect, of which the fast Fourier transform shows two frequencies F₁ = 23.4 and F₂ = 41 tesla, respectively; (2) Inter-plane tunneling magnetoresistance, whose ratio is more than 90% [ρ(0) - ρ(H)/ρ(0)]; (3) Anomalous angular dependence of magnetization, which leads to similar angular dependence of resistivity due to a strong spin-charge coupling. Ca₃Ru₂O₇ is a rare system that possesses a wide range of interesting yet complex physical properties that very often do not coexist in other materials. The results and discussions will be presented along with comparisons with other related systems.

We report extremely high field and frequency NMR measurements of the proton spin-lattice relaxation rate (1/T₁) in (TMTSF)₂PF₆. They probe the spin density wave (SDW) fluctuations in the critical regime near the SDW transition and in the ordered phase at lower temperatures. The highest field measurements were made using the NHMFL hybrid magnet at 44.7 T and a proton NMR frequency of 1.9 GHz. A quadratic increase in the transition temperature is observed that agrees with transport measurements made at lower fields. As the field is increased above 40 T, a change in the SDW critical dynamics is seen. Also, the behavior of 1/T₁ vs. the alignment of the magnetic field suggests a magnetic transition at moderate to high fields with the characteristics of a sharp change in the orientation of the SDW polarization, but with parameters that differ from the spin-flop transition seen at low field.

In this paper, we discuss the concept of a metamagnetic quantum critical end-point, consequence of the depression to zero temperature of a critical end-point terminating a line of first order first transitions. This new type of quantum critical point (QCP) is interesting both from a fundamental point of view: a study of a symmetry conserving QCP, and because it opens the possibility of the use of symmetry breaking tuning parameters, notably the magnetic field. In addition, we discuss the experimental evidence for the existence of such a QCP in the bilayer ruthenate Sr₃Ru₂O₇.

We review some recent NMR results obtained in Grenoble High Magnetic Field Laboratory on magnetic field induced phenomena in strongly correlated low-dimensional fermionic systems: i) magnetic field dependence of the soliton lattice in the IC phase of the spin-Peierls system CuGeO₃, ii) NMR study of the complete H-T phase diagram of the organo-metallic spin ladder Cu₂(C₅H₁₂N₂)₂Cl₄, and iii) the first "standard" NMR measurements (i.e., without optical pumping) on 2D electrons in Quantum wells, providing a detailed description of the fractional quantum Hall effect state at ν = 1/2 with the first determination of the corresponding effective polarization mass of composite fermions. Latest study of the ν = 2/3 state revealed, among other features, an unexpected phase transition.

We argue that an organic superconductor (TMTSF)₂PF₆ under pressure P = 6 kbar is a triplet superconductor^1,2 with a spin part of the triplet order parameter being d(k) = (d_x (k) ≠ 0, d_y (k) = 0, d_z (k) = 0). We obtain these results by means of an analysis of the anisotropic experimental critical fields H_c2 (see I. J. Lee et al., Phys. Rev. Lett.78, 3555 (1997)). By using symmetry arguments, we also discuss the most probably orbital part of the triplet order parameter in (TMTSF)₂PF₆. Our analysis is in accordance with the very recent Knight shift data obtained on (TMTSF)₂PF₆.

In this talk I will survey our recent experimental results in the detection and manipulation of magnetism at the spatial limit. Our experiments rely on atom manipulation techniques and scanning tunneling microscopy at low temperatures to enable atomic-scale imaging and control.

We have observed "quantum mirages" in focusing devices of order 10 nanometers in size, built by assembling structures out of individual atoms. We have directly imaged the spin perturbations due to isolated magnetic moments on a metal surface. The detection of this localized magnetism can then be utilized in a type of teleportation experiment, in which the spectroscopic signature of an atom is sampled and projected to a remote location by means of a surrounding sea of electrons confined in an engineered nanostructure. The quantum mirage thus cast by a single magnetic atom can be coherently refocused at a distinct point where it is detected as a phantom atom around which the electronic structure mimics that at the real atom. Once materialized, this phantom can interact with real matter in intriguing ways.

We have constructed other nanoscale magnetic structures which either elucidate the coupling between isolated moments or provide a mechanism for controlling and exploiting spin coupling over long distances. We have also been developing novel communication methods based on the fundamental effects we have discovered.

Ca_{2 - x}Sr_xRuO₄ is a unique quasi-two-dimensional multi-band system that connects the Mott insulator Ca₂RuO₄ with the spin-triplet superconductor Sr₂RuO₄. The isovalent Sr substitution changes the crystal distortions and thus modifies the hybridization and degeneracy of t_2g orbitals. Such effective change of electron correlation induces the Mott transition and rich evolution of itinerant magnetism. One of the striking features in this system is the appearance of ferromagnetic critical behavior in the metallic phase near a zero-temperature structural instability (x_c ≈ 0.5). The low temperature susceptibility shows Curie-like diverging behavior, and the electronic specific heat (C_e) divided by temperature, C_e/T, enormously increases up to 250 mJ/mol-Ru at x = 0.5. Moreover, critical enhancement of Wilson ratio is attributable to a ferromagnetic instability developed near the structural instability.

The fate of the metallic phase in parallel magnetic fields represents one of the major open issues in the studies of dilute, strongly interacting systems in two dimensions (2D). In this experiment, the temperature dependence of conductivity σ(T) of a 2D electron system in silicon has been studied in parallel magnetic fields B. At B = 0, the system displays a new and unexpected kind of metallic behavior with dσ/dT > 0, and a novel type of a metal-insulator transition,¹ which occurs at a critical electron density n_c(0). At low fields (B ≤ 2 T), n_c increases as n_c(B) - n_c(0) ∝ B^β(β ≈ 0.9), and the zero-temperature conductivity scales as σ(n_s, B, 0)/σ (n_s, 0, 0) = f(B^β/δ_n) (where δ_n = (n_s - n_c(0))/n_c(0), and n_s is electron density) as expected for a quantum phase transition. The metallic phase persists in fields up to 18 T, consistent with the saturation of n_c at high fields. These results strongly suggest that the 2D metal may exist even for spinless electrons.

Geometrical Frustration arises when the symmetry of a magnetic interaction is incompatible with the spatial symmetry of the crystalline lattice. Magnetic field can break this symmetry condition and we will illustrate different classes of behavior on the pyrochlore lattice – spin ice for Ising spins and spatial symmetry projection for Heisenberg spins.

The effects of the in-plane strain on the magnetic properties of LaMnO₃ thin films are calculated using an elastic energy expression and a tight binding Hamiltonian with electron-lattice coupling. Tensile uniaxial strain of the order of 2%, which is the order of the magnitude of those induced in thin films by lattice mismatch with substrates, is found to change the magnetic ground state from A-type antiferromagnetic state to purely antiferromagnetic state.

Well characterized single crystals of the four layer crystallographic form of BaRuO₃ were studied at low temperatures (20 < T < 850 mK) and high magnetic fields (0 < H < 30 T). Quantum oscillations have been observed via the Shubnikov - de Haas and de Haas - van Alphen effects. In marked contrast, BaRuO₃ with the nine-layer structure shows no similar behavior, suggesting a substantial change in the Fermi surface or quasi-particle relaxation times. The quantum oscillations provide insight into the ground state which, like those of other layered ruthenates, is characteristically sensitive to electron-lattice coupling. This observation is significant in that it represents the first example of quantum oscillations in a non-Ruddlesden–Popper type ruthenate and provides some insight into the Fermi surface even though a psuedo-gap is evident in other studies.

We studied the electronic conductivity of the quasi-one dimensional Ca₃Co₂O₆ single crystal. The results evidence a VRH conductivity with temperature-induced crossover between 1D (intra-chain) and 3D transport and the opening of a Coulomb gap in the d bands. At low temperatures, an applied magnetic field induces a large negative magneto-resistance (MR) independent from the 3D magnetic ordering. Both spin-dependent hopping and field-induced suppression of the Coulomb gap are discussed.

The general aspects of the influence of quenched disorder on the competition between ordered states separated by a first-order transition are investigated. A phase diagram with features resembling quantum-critical behavior is observed. The paramagnetic phase at low temperature consists of coexisting ordered clusters, with a random orientation of the order parameter. Specializing to manganites, this state is shown to have a colossal magnetoresistance effect. A new scale T* for cluster formation is predicted. It is argued that cuprates have similar features, compatible with the large proximity effect of the very underdoped regime.¹

The ⁵⁵Mn NMR has been measured in La_xSr_{1 - x}MnO₃ for x = 0.2 and 0.3 as a function of temperature, in zero field and in field. The samples had a room temperature resistivity of 10^-4 ohm-cm, and the magnetizarion showed a sharp FM transition 312 K. The zero field FM frequency follows T^3/2 power law at both x values consistent with the simple magnon model. A second feature at lower frequency of uncertain origin is observed.

²³Na NMR studies have been carried out to map the uniform to dimerized phase boundary up to 44.7 T in the NHMFL hybrid. The transition is observed by the splitting of the NMR line (H//a axis) by the lattice distortion. The field dependence of T_c up to 44.7 T, in agreement with previous magnetization data to 30 T, is very small (α = 0.092) compared to 0.4 for conventional SP systems and shows no deviation within experimental error from the h² dependence, while optical data show a decrease of the slope in the H-T phase diagram beginning around 40 T. The region of an anomaly near T 33 T and 32 K observed in optical data taken in the long pulse magnet at LANL¹ was also explored. The only evidence in the NMR of the observed optical anomaly near 33 T and 32 K was line broadening over a 2 T interval.

S = 1/2 TlCuCl₃ is a magnetic insulator with a singlet ground state and a finite spin gap to triplet excited states. At finite fields, the degeneracy of the triplet states is lifted according to the Zeeman interaction term. Field-induced magnetic ordering occurs in TlCuCl₃ at the critical field H_c ~ 6T, when the Zeeman interaction overcomes the spin gap at the antiferromagnetic zone center. A detailed characterization of the elementary excitations realized at H > H_c is obtained by means of inelastic neutron scattering on single crystals. The general framework of field-induced quantum criticality in a three-dimensional spin liquid is illustrated on the complete dynamic range for the first time.

As it was shown earlier [Dzero, Gor'kov and Zvezdin, J. Phys.: Condens. Matt.12, L711 (1000)], the properties of the first-order valence phase transition in YbInCu₄ in the wide range of magnetic fields and temperatures are perfectly described in terms of a simple entropy transition for free Yb ions. Within this approach, the crystal field effects have been taken into account and we show that the phase diagram in the B - T plane acquires some anisotropy with respect to the direction of an external magnetic field.

We address some of the results of the recent experiments by Izumi et al. where the properties of heterostructures composed of La_{1 - x}Sr_xMnO₃ (x = 0.4, 0.55) layers have been investigated. Anomalies in conductivity and large magnetoresistance measurements for several heterostructures with thin constituent ferromagnetic (x = 0.4) and antiferromagnetic (x = 0.55) layers are explained in terms of existence of the localized (Tamm-type) of states at the interface.

The magneto-optical absorption of the quantum Ising ferromagnet LiHoF₄ has been measured in the frequency range 38 to 140 GHz, the temperature range 0.5 to 20 K and magnetic fields of up to 15 T. The sample was studied in two orientations, with the external magnetic field aligned perpendicular or parallel to the uniaxial direction, i.e., along the quantum and classical directions. In each case, several families of resonances are observed, including linear EPR branches and a series of non-linear fast moving resonances. We present this data and propose an interpretation for the origin of these resonances.

Ferromagnetism being known to have a detrimental effect on superconductivity, we consider the Josephson current amplitude for junctions built up of antiferromagnetic metallic weak links. It is assumed that the latter consist of ferromagnetic layers with magnetizations aligned alternatively along perpendicular-to-the-layers direction. Currents between two superconducting electrodes flow along the layers. Such antiferromagnetic structure realizes itself in mixed valence manganites (the so-called A-phase), in an array of parallel ferromagnetic domains, or even in artificial GMR heterostructures. It is shown that even minor canting of magnetic moments in the presence of magnetic fields causes remarkable oscillations in the value of the Josephson current amplitude.

The antiferromagnetic ordering temperatures of Sm₂RuO₅, Tb₂RuO₅ and the ferromagnetic ordering temperature of Nd₃RuO₇ are reduced by the application of high hydrostatic pressure, whereas that of antiferromagnetic Gd₂RuO₅ remains unchanged. The results are discussed in terms of the enhancement of the crystalline electric field interaction on the Ln cation constituents. A versatile method of measuring magnetization in high fields and high pressure is presented.

We calculate the dynamical structure factors of the magnetization-plateau state in the S = 1/2 bond-alternating spin chain with a next-nearest-neighbor interaction. The results show characteristic behaviors depending on the next-nearest-neighbor interaction and the bond-alternation. We discuss the lower excited states in comparison with the exact excitation spectrums of an effective Hamiltonian.

We investigate the dynamical structure factors of the S = 1/2 spin ladder systems with a diagonal interaction in the magnetization-plateau state. The results show characteristic behaviors depending on the chain interaction α, the ladder interaction 1 + δ and the diagonal interaction 1 - δ. The large weights are located at q = π, when δ is small. With increasing δ, the wavenumber of the large weights shifts towards q = π/2 taking the incommensurate value. The almost dispersionless behavior comes out at several combinations of α and δ, indicating that the elementary excitation is localized spatially. We discuss our results in comparison with the exact excitation spectrums of the S = 1/2 XXZ model which is an effective Hamiltonian of the present system in α ~ 0 and δ ~ 1.

Dynamical structure factors of some S = 1/2 one-dimensional quantum spin systems in the magnetic fields were measured by inelastic neutron-scattering experiments^1,2 and it has been shown the theoretical results from the numerical diagonalization of finite systems can explain them.^3–5

We report high sensitivity, high field/frequency (up to 9 tesla/210 GHz) EPR measurements on the Fe₈ and Mn₁₂ single molecule magnets. Oriented single crystal studies, carried out in well defined experimental geometries, enable very accurate determinations of EPR lineshapes; we note that this is not possible when studying aligned powders or polycrystals. Analysis of individual resonances, which we can assign to known transitions, reveal complex nonlinearities in the linewidths as functions of energy eigenstate, frequency, and temperature. Our findings compare favorably with recent calculations which introduce distributions in the uniaxial anisotropy parameter D, the Landé g-factor, and dipolar interactions.

We have designed a high pressure apparatus for measuring electrical-transport properties at low temperatures, high magnetic field and hydrostatic pressure up to 10 kbar. Details of the high-pressure cell and an exemplary study on UNiAl are described and discussed briefly.

The Hall effect of (100)- and (110)-oriented films of the half-metallic ferromagnetic oxide CrO₂, fabricated by both chemical vapor deposition and high pressure, thermal decomposition methods, has been examined in large magnetic fields up to 60 T. In all cases the Hall effect exhibits a sign reversal from positive to negative with increasing field, which we take as evidence for multi-band behavior. (110) films fabricated by both methods exhibit this sign reversal at relatively low fields. The data may be fit with a simple two-band model, which indicates the existence of highly mobile holes of p-like parentage, along with a much larger number of heavy, d-like electrons. In the (100) film the sign reversal is at much higher field. The parameters obtained from the fits allow us (with help from band structure calculations) to infer the band structure near the Fermi level and how it depends on sample strain and other structural characteristics. These details will be important for understanding carrier transport through interfaces such as for spin injection or other "spintronics" applications.

The magnetic phase transition from the paramagnetic to antiferromagnetic state in insulating V₂O₃ shows some very interesting features. Not only is the magnetic order ng pattern anomalous, but the magnetic transition shows other surprises: I) the magnetic and orbital ordering occur at the same transition temperature; II) the transition is strongly first order; III) above the transition temperature, neutron scattering observes features in wave vectors different from the ordering wave vector. We study this system using the recently proposed S = 2 bond model (Phys. Rev. Lett.85, 1714 (2000)) for insulating V₂O₃. We show that this model not only explains the anomalous magnetic ordering, but also resolves the other questions associated with the magnetic phase transition (Phys. Rev. Lett.86, 5743 (2001)). We further point out the possibility of changes in the phase transition phenomenology in presence of a magnetic field, and another orbital phase transition at lower temperatures.

We study the magnetization process of a quasi-one-dimensional S=1 antiferromagnet with bond alternation by using the density matrix renormalization group method combined with interchain mean-field approximation. Particularly, we discuss how the interchain couplings affect the magnetization curve around a plateau structure in both of the Haldane phase and the dimer phase. It is shown that the antiferromagnetic correlation induced by the interchain couplings reduces the region of the plateau significantly, which will play an important role when the theoretical results are compared with actual measurements of the magnetization.

We have observed the negative magnetoresistance effect in the narrow-gap PbTe(Mn,Cr) semiconductor, in which the Fermi level is pinned within the gap nearby the conduction band edge. Previously the giant negative magentoresistance effect has been reported in PbTe(Mn,Yb), in which the Fermi level is pinned in the gap nearby the valence band edge. It is known that in the case of Yb doping the Fermi level pinning results from the 2+ - 3+ valence instability of an impurity. The same sort of the valence instability provides the Fermi level pinning in PbTe(Mn,Cr), but the conductivity is of the n-type, not of the p-type as in PbTe(Mn,Yb). Introduction of magnetic field leads to substantial drop of the PbTe(Mn,Cr) resistivity of about 30% at T = 4.2 K. This is however much lower than in PbTe(Mn,Yb), where the effect amplitude reached 3 orders of magnitude. The effect disappears at T = 15 K. Possible mechanisms of the effect are discussed.

The differential magnetic susceptibility of Gd_{0 01}Y_2.99Fe₅O₁₂ polycrystalline compounds is studied at the temperature range from 0.6 to 4.2 K in pulsed magnetic fields up to 50 T. It was observed that the magnetization of the compound with x = 0.01 shows a quantum (stepwise) behavior at T = 0.6 K in magnetic fields around 33 T. The Heisenberg model of a ferrimagnet with weakly coupled antiferromagnetic impurities is used to explain the obtained results. Following this model, the energy levels of the impurity ions become not equally spaced due to the magnon-magnon interaction between the iron spins and the rare earth spins. Increase of the external field leads to the crossover of the excited states with the ground one, which takes place in different fields. In result, several peaks of the magnetic susceptibility appear under a variation of magnetic field in the vicinity of a field, which is equal to the effective field of the Gd-Fe exchange interaction.

We report on magnetic resonances observed in the doped lanthanum manganite La_0.8Li_0.2MnO₃ (LLMO) below the Curie temperature T_c ~ 235 K. Fields up to 30 T were employed. The data are compared with that from the archetypal colossal magnetoresistance perovskite La_{0 7}Ca_{0 3}MnO₃ (LCMO) taken under similar conditions. In contrast to LCMO, LLMO exhibits two resonances, one above and one below the LCMO resonant field. These are attributed to a ferromagnetic interaction and a canted ferromagnetic interaction.

The magnetic breakdown effect can be seen by the growth of new frequencies in the quantum oscillations in clean metals as a function of magnetic field. We have studied the variation of the amplitudes in the quantum oscillations in the resistance (the Shubnikov-de Haas effect) as a function of angle in the quasi-two dimensional-organic conductor κ-(BEDT-TTF)₂Cu(NCS)₂. The measurements were made by means of a radio frequency (rf) tank circuit (~ 50 MHz) at very high magnetic fields(50T-60T) and low temperature(500 mK). The geometry of the rf excitation we used excited in-plane currents, and therefore we measured the in-plane resistivity. In contrast to conventional transport measurements that measure the inter-plane resistivity, the in-plane resistivity is dominated by the magnetic breakdown frequencies. As a result we measured much higher breakdown frequency amplitudes than conventional transport experiments. As is expected, the angular dependence of the Shubnikov-de Haas frequencies have a 1/cosθ behavior. This is due to the change of the cross sectional area of the tubular Fermi surface as the angle with respect to the magnetic field is changed. The amplitude of the oscillations changes due to the spin splitting factor which takes into account the ratio between the spin splitting and the energy spacing of the Landau levels which also has 1/cosθ behavior. We show that our data agree with the semi-classical theory (Lifshitz-Kosevich formula).

We determine the collective spin excitations and their contribution to the intrinsic resistivity in 3d-ferromagnets by high field magneto-transport experiments. In the paraprocess regime, the longitudinal magneto resistance has a non saturating negative slope of about 0.02-0.05μΩ.cm.T¹ at 300 K for Fe, Co and Ni. We demonstrate its magnetic origin and assign the effect to a decrease of the electron-magnon scattering due to the damping of spin waves in high field. We provide a unique estimate of the pure magnetic resistivity in 3d-ferromagnets. Our analysis also gives an insight into the low energy spin waves, i.e. the theoretical magnon saturation field and the magnon mass renormalization.

We use Raman scattering to investigate magnetic polarons and spin-carrier interaction effects in the (Eu,Gd)O system. Raman scattering studies reveal great richness in the phase behavior of this system: a spin-fluctuation-dominated paramagnetic (PM) phase regime for T > T* > T_C, a two-phase regime for T < T* in which magnetic polarons develop below T* and coexist with a remnant of the PM phase, and an inhomogeneous ferromagnetic phase regime for T < T_C. We will discuss how these phase regimes evolve as functions of magnetic field, temperature, and Gd-substitution, and will compare the results with less disordered systems such as EuB₆.

Simultaneous measurements of magnetic susceptibility from 0.5 to 10 mK and pressure from 2.88 to 3.54 MPa have been made in ³He nanoclusters embedded in a ⁴He matrix, following phase separation. The susceptibility of the 3.54 MPa, all –solid sample behaves similarly to that of bulk ³He for v = 21.3 cm³/mole, with a Weiss constant θ = -250μK. For the 2.88 MPa, liquid-droplet sample, θ = 140μK, indicating a ferromagnetic tendency, similar to 2-D films at some coverages. At intermediate pressures, χ has a peak near 1.05 mK, but without a discontinuity. For all samples, χ had a solid-like contribution to the lowest temperatures. Magnetic ordering in nano-clusters appears to be different than the U2D2 phase of bulk ³He.

Low-dimensional spin systems reveal new and unexpected physical phenomena such as distinct plateaus in the magnetization as a function of magnetic field. In this paper we present ultrasonic measurements for the quasi-two-dimensional spin system SrCu₂(BO₃)₂ in magnetic fields up to 50 T. From this technique we obtained detailed information about the spin state, the magnetic excitations and their interaction with phonons. The dimerized quantum-spin system SrCu₂(BO₃)₂ exhibits plateaus in the magnetization and shows surprisingly strong magneto-elastic effects as a function of temperature and magnetic field. The pronounced elastic anomalies indicate a resonant interaction between the sound wave and the magnetic excitations.

Results of the high-frequency/field electron spin resonance investigation of the quantum S = 1/2 chain compound LiCu₂O₂ are presented. Spin-singlet ground state (which occurs in LiCu₂O₂ as a result of the spin-dimerization) is revealed at a temperature above T ~ 23 K. The size of the gap between the spin-singlet ground state and the excited triplet is found to be Δ = 72 K. Electron spin resonance study confirms a magnetic phase transition in LiCu₂O₂, which results in the collapse of the spin-singlet ground state at T < 23 K.

Allosteric (conformation changing) proteins with transition metal atoms are at the heart of much important biological function (e.g., myoglobin hemoglobin used for storing and transporting oxygen in the bloodstream). In the case of myoglobin and hemoglobin, oxygen ligation to the iron center induces a spin crossover (high to low) coupled to a structural change; apart from the role of Hunds' exchange in the spin crossover, electron interaction effects have been ignored. We argue that the spin crossover/structure change observed in the similarly structured but far simpler cobalt valence tautomer molecules¹ necessitates an inclusion of underscreened Kondo like correlations for a complete description of the energetics of the transition and dynamics, e.g., for x-ray absorption data. We carry this study out with Varma-Yafet-Gunnarsson-Schonhammer wave functions, which, in chemistry language, are basis set restricted configuration interaction in character. We briefly review the applicability of such wave functions to the description of the putative Kondo molecules cerocene (Ce[(CH)₅]₂) and ytterbocene bipyridine (Yb[(CH)₅]₂(bipy)) and to the problem of electron transfer in biological molecules and organic conductors, where anomalous long range tunneling may occur.

Research supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Research.

Magnetic Resonance Force Microscopy (MRFM) is a novel scanned probe technique that combines the three-dimensional imaging capabilities of magnetic resonance imaging (MRI) with the high sensitivity and resolution of atomic force microscopy (AFM). This emerging technology holds clear potential for resolution at the atomic scale. When fully realized, MRFM will provide a unique method for non-destructive, chemically specifc, subsurface imaging with applicability to a wide variety of materials. I will review results to date spanning applications of MRFM to nuclear spin, electron spin, and ferromagnetic resonance. I will outline the MRFM technique, discuss its present status and indicate future directions of our effort.

We report recent progress in the generation and application of megagauss fields at the new Megagauss Laboratory of ISSP in Kashiwa. New facilities for studying physics in ultra-high magnetic fields were built at the new campus of the University of Tokyo at Kashiwa, where pulsed high magnetic fields are produced by three different techniques: electromagnetic flux compression, the single-turn coil technique and non-destructive long pulse magnets. Considerable progress has been made in each of these techniques, as compared with the old facilities at Roppongi. For electromagnetic flux compression, the rise time of the primary current was reduced by decreasing the residual inductance of the capacitor bank. The cylindrical symmetry of the liner implosion was remarkably improved by inserting a "feed gap compensator" between the primary coil and the liner. So far, a maximum field of 622T has been achieved. By the single-turn coil technique, very high fields exceeding 300T have been obtained depending on the coil bore, in two systems with horizontal and vertical coil axes. These fields are successfully applied in many different kinds of experiments. Non-destructive long pulse fields up to 60T are available from wire-wound coils.

With an aim towards measuring the high-frequency complex conductivity of correlated electron materials in the regime of low temperatures and high magnetic fields, we introduce a method for performing time-domain terahertz spectroscopy directly in the cryogenic bore of existing dc and pulsed-field magnets. Miniature, fiber-coupled THz emitters and receivers are constructed and are demonstrated to work down to 5 K and up to 6 Tesla, for eventual use in higher-field magnets. Maintaining the sub-micron alignment between fiber and antenna during thermal cycling, and obtaining ultrafast (<200fs) optical gating pulses at the end of long optical fibers constitute the major technical challenges of this project. Preliminary data on YBCO superconducting thin films is shown.

The accurate determination of the temperature of an experiment at low temperatures in high magnetic fields is difficult. We present the results of measurements made using a number of new techniques developed over the last few years. In particular we discuss the results of measurements made using a unique capacitor made with Kapton and copper in a cylindrical geometry.¹ This capacitance thermometer, dubbed the "Kapacitor", is different from other low temperature thermometers in that the minimum in capacitance vs. temperature can be moved to lower temperatures (to below 20 mK) by changing the construction technique. In addition, we discuss measurements on Coulomb blockade thermometers (CBT's) that offer the possibility of true primary thermomemtry at low temperatures without any magnetic field dependence. Both of these new techniques will be compared to the standard technique of resistance thermometry using RuO chip resistors. The crucial issues of accuracy and precision, usefulness for control, and noise sensitivity will be discussed for each of these technologies. In addition, recent measurements on the magnetic behavior of RuO thermometers at low temperatures and its relationship to anomalous low field peaks in the resistance that develop at temperatures below 50 mK are also presented.

Neutron scattering in magnetic fields can provide important imformation regarding the magnetism of a system under study on microcsopic scale. Although pulse magnets are capable to reach very high fields, for neutron scattering mostly superconducting magnets are used. At present, a split-pair 14.5 T superconducting magnet used for user-program experiments at HMI offers the highest magnetic field for neutron scattering. With Dy-pole tips placed symmetrically around the sample position a 2.5 T enhancement is achieved. The temperature range accessible with such an insert is between 1.8 and 100 K. Temperatures down to 30 mK can be achieved with a dilution stick, which can be inserted into the 14.5 T cryomagnet. In the contribution we report on the latest neutron scattering achievements in magnetic fields obtained at HMI.

Three ultrasonic spectrometers have been constructed for use with the continuous and pulsed magnetic field facilities of the NHMFL at the Tallahassee and Los Alamos sites. The first of these, designed for use with the dc magnets, is a heterodyne spectrometer with the capabilities of high velocity resolution and near-simultaneous measurements at several frequencies (typically near transducer harmonics). The other two spectrometers are designed for the short-pulsed magnets, with the capability of fast data acquisition. These spectrometers have been used in the study of metamagnetism in the heavy fermion materials.

Pressure can be used to expand the parameter space available in almost any experiment and allows for the continuous tuning of the electrical and orbital properties of a material. When combined with low temperatures and high magnetic fields, it becomes a powerful tool for the exploration of the band structure and defect levels in semiconductors, exotic transport mechanisms in molecular conductors, and the coexistence of magnetism and superconductivity. We have developed a variety of miniature pressure cells to allow the user to take full advantage of these opportunities. Metallic diamond anvil cells as small as 6 mm in diameter and 8 mm in height allow the sample to be rotated in field at millikelvin temperatures. Miniature plastic DACs and sapphire ball cells, rotators, and specialized He-4 and He-3 systems have also been developed to provide similar experimental capabilities in pulsed magnetic fields. Methods and designs to generate hydrostatic pressure and techniques to perform optical and electrical measurements in DC and pulsed fields will be presented.

We would like to acknowledge the technical assistance of Richard Desilets, Howard Kolb, John Farrell, and Mike Pacheco. A portion of this work was performed at the National High Magnetic Field Laboratory, which is sponsored by NSF Cooperative Agreement No. DMR-9527035 and by the State of Florida.

We have proposed to build a 100 T/10 ms, 70 T/100 ms, 60 T/1 s pulsed field user facility with a 50 MJ capacitor bank at the Forschungszentrum Rossendorf near Dresden. This would provide the appealing possibility to have access to Zeeman energies in the energy range of the infrared free-electron-lasers (5 μm to 150 μm; 2 ps; cw; > 10 W) now under construction at the radiation source ELBE (superconducting electron linear accelerator; 40 MeV; 1 mA; 2 ps; cw) in Rossendorf. The work is accompanied by computer simulations of the planned coil systems, of the power supply, and by the development of high-strength conductors aiming at a tensile strength of about 1.5 GPa at σ ≈ σ _Cu/2 (microcomposite CuAg alloys and Cu-steel macro compounds).

With a view of gaining experience in the construction and operation of pulsed magnets, a pilot pulsed field laboratory was established at the Institute of Solid State and Materials Research Dresden (IFW Dresden). The laboratory includes short pulse magnets with peak field up to 60 T in a 24 mm bore and a rise time of about 10 ms (coil from NHMFL, Tallahassee), and a 40 T long pulse magnet with 24 mm bore and rise time of about 80 ms (coil from METIS, Leuven). The repetition rate of 20 min between pulses is limited by the cooling time of the coils. The coils are energized by a 1 MJ, 10 kV capacitor bank with some special features. With this set-up measurements of magnetization and magnetotransport on 4f-electron systems, for example RECu₂, have been out in the temperature range of 1.5 to 300 K and at fields up to 52 T using high precision pick-up coils.

The DC and pulsed magnets now available at the NHMFL provide routine access to high magnetic fields in cryogenic environments (down to even dilution refrigerator levels), that are world-record unique. This uniqueness comes with a price that reflects constraints of the magnets and the low temperatures, including limited volume and time at peak magnetic field, cryogenic power limits on electronics, and, particularly for pulsed magnets, increased noise. In effect, the instrumentation constraints are similar for NHMFL superconducting, resistive and pulsed magnets. An NHMFL experimentalist therefore has a simple goal: acquisition of all the information produced by a measurement in the shortest time permitted by information theory, with minimum sensitivity to noise and interference. To assist with this, we propose here to eliminate commercial general-purpose lock-in amplifiers, preamplifiers and digitizers and replace them with commercial-quality custom building blocks optimized for NHMFL measurements, that are faster, quieter, more versatile, and cheaper. We will use these new instruments to support users by improving present measurements as well as adding new capabilities, including specific heat for materials that suffer adiabatic effects in pulsed fields, and thermal conductivity in both dc and pulsed magnets based on 3rd harmonic methods. We will use these techniques to measures the thermal conductivity of high T_c superconductors at high field in the normal state, and to test the Weideman-Franz relationship between electronic thermal conductivity and electrical conductivity in the extreme high-field limit.

We report the latest results of cyclotron resonance experiments on semiconductor nanostructures and diluted magnetic semiconductors (DMS) in very high magnetic fields up to 600 T produced by magnetic flux compression and the single turn coiled technique. Many new features were observed in the very high field range, such as characteristic behavior of low dimensional electrons, carrier dynamics or electron-electron interaction effects in quantum wells and quantum dot samples. In PbSe/PdEuTe quantum dots, which were regularly arranged to form an fcc superlattice, we observed an absorption peak with a splitting and a wavelength dependence of the absorption intensity. In DMS, such as CdMnTe and InMnAs, change of the carrier effective mass with Mn doping was studied in detail. We found anomalous mass increase with doping of magnetic ions. The amount of the observed mass increase cannot be explained by the k·p theory and suggests the importance of d-s or d-p hybridization.

As an exploratory study for NMR experiments in the future 20 MW Nijmegen high field magnet laboratory, the possibilities of field stabilization and field gradient compensation in a 25 T hybrid magnet in the present installation were evaluated. High frequency field fluctuations from the power supply can be compensated to better than 10^-3 ppm in the 10 Hz-10 kHz range using a computer controlled feedback system. Field mapping by ²H magnetic resonance using a homebuilt device not only showed that there are substantial axial but also strong radial field gradients. It can be shown that for any cylindrical multicoil Bitter magnet the main components of these gradients can be compensated with simple ferromagnetic inserts. In this way we achieved a linewidth under 5 ppm in 1 mm³ without further shimming or optimization. The low frequency drift of the field due to instabilities of the present power supply and the effects of temperature fluctuations of the coil are determined by simultaneous acquisition of an in-situ deuterium reference signal together with the signal of interest. This allows for a full compensation of the field fluctuations by deconvolution techniques. We will report preliminary NMR results on solid²⁷ Al samples in fields up to 25 T.