International Journal of Modern Physics B

What is the microscopic interaction responsible for high temperature superconductivity (HTSC)? Here data on temporal relaxation of T_c and the room temperature conductivity in YBa₂Cu₃O_6+x after abrupt alteration by light pulses or pressure changes are analyzed. The analysis proves, independently of microscopic details, that only electron–phonon interactions can cause HTSC in the cuprates; all other dynamical interactions are excluded by experiment.

Thermodynamic critical fields, H_c, have been measured for the La_2-xSr_xCuO₄ family of superconductors in order to determine the changes in free energy of the system as the number of carriers is reduced. Magnetization vs. magnetic field curves are thermodynamically reversible over large portions of the H-T plane, so the free energy is well defined in these regions. Magnetization vs. field data are then fit to theoretical models to determine the thermodynamic critical fields. As the Sr concentration is changed from x=0.10 to 0.23, the values of H_c(T=0) goes through a maximum at optimum doping in a manner similar to the T_c vs x curve. The ratio of H_c(T = 0)/T_c also peaks in the region of x somewhat larger than optimum doping. As the value of x increases from underdoped to optimum doping, the cross-over temperature in the M_sc vs. T curve, T*, approaches the transition temperature. In the overdoped regime there is no crossing in these M_sc vs. T curves.

We calculate the critical current density for Josephson tunneling between identical high temperature superconductors twisted an angle (ϕ₀) about the c-axis. Regardless of the shape of the two-dimensional Fermi surface and for very general tunneling matrix elements, an order parameter (OP) with general d-wave symmetry leads to . This general result is inconsistent with the data of Li et al. on Bi₂Sr₂CaCu₂O_8+δ (Bi2212), which showed to be independent of ϕ₀. If the momentum parallel to the barrier is conserved in the tunneling process, should vary substantially with the twist angle (ϕ₀) when the tight-binding Fermi surface appropriate for Bi2212 is taken into account, even if the OP is completely isotropic. We quantify the degree of momentum non-conservation necessary to render constant within experimental error for a variety of pair states by interpolating between the coherent and incoherent limits using five specific models to describe the momentum dependence of the tunneling matrix element squared. From the data of Li et al., we conclude that the c-axis tunneling in Bi2212 must be very nearly incoherent, and that the OP must have a non-vanishing Fermi surface average for T≤T_c.

We investigated the physical mechanism of dynamical bond formation, light-created defect centers and light induced electron spin resonance for Se chains and a realistic model of amorphous selenium by ab-inito molecular dynamic simulation. It is found that the Coloumb interaction between light-created charge centers plays the key role in the bond breaking, switching and reconstruction in Se chain and a-Se under the light irradiation.

We disprove the widely held notion that a surface with nontrivial roughness exponents fluctuates at all scales ("structure within structure") and has nontrivial fractal dimension. Strong counterexamples are Cantor staircases, which have nontrivial roughness exponents, do not fluctuate at all, and have trivial fractal dimension. Weak counterexamples fluctuate intermittently and have nontrivial fractal dimension. Characteristic of all counterexamples is: (i) they consist of terraces of all sizes and exhibit scaling over the entire range of terrace sizes. (ii) they have roughness exponents H_q that vary strongly with order q; (iii) they are self affine, but not all affinities are invertible. The strong variation of H_q drives a strongly varying surface response to different external interactions (different interactions are governed by different orders q) and abrupt changes similar to a phase transition, with H_q playing the role of temperature. A summary of this extraordinary functional tunability and its applications is given.

Amongst tools for fabricating periodic and aperiodic nanostructures and nanodevices, electron beam-induced organometallic chemical vapor deposition (E-OMCVD) offers a highly flexible and controllable one-step deposition process. E-OMCVD enables maskless fabrication of nanoscale research and custom structures that have least dimensions near or below 10nm – a scale at which other methods prove difficult or costly. Using the focused electron beam in a modified HB501 field-emission scanning transmission electron microscope (STEM), pads and wires with uniform thickness and well-defined shapes have been defined and deposited. Although conditions for fabricating the smallest deposits have not yet been optimized, the edge acuity (sharpness) of the deposits is consistently as low as 4nm or less and the corresponding smallest wire width is 8nm. Under different deposition conditions, three-dimensional open structured nanonetworks have been fabricated. Results of an investigation of E-OMCVD parameters are presented for the metallocene compound, nickelocene (Ni(C₅H₅)₂), as source organometallic.

We present measurements of some of the principal response functions of a thin film of nanodimensional Fe particles embedded in Al₂O₃, specifically the temperature dependence of the moment measured under field cooled and zero field cooled conditions at temperatures between 5 K ≤ T ≤ 300 K in a low applied field h_a=10 Oe, and the major hysteresis loop measured at T=5 K in applied fields between -2000 Oe≤ h_a ≤ + 2000 Oe. The data were analyzed within the framework of a generalized version of the Preisach model of hysteresis, which treats all magnetic systems as ensembles of interacting bistable Barkhausen elements, and which includes both thermal barrier relaxation effects and critical effects. Numerical simulations based on the model are able to replicate the principal structural features of the experimental data, and allow us to extract the characteristics of the bistable elements, specifically their moments μ, their coercive fields h_c and their interaction fields h_i.

We use a continuous-time path integral to obtain the semiclassical propagator for minimal-spread spin coherent states. We pay attention to the extra phase. This extra correction of phase is related to an anomaly in the fluctuation determinant. We show that if this extra factor is included, the semiclassical propagator has the correct short time behaviour to O(T²), and demonstrate its consistency under dissection of the path.

The magnetic structure of the transition metal sublattice in induction-melted SmMn_6-xFe_xSn₆ intermetallic alloys (x = 0, 0.5, 1.0, 1.5, and 2.0) K have been studied by x-ray and neutron diffraction techniques and SQUID measurements. These intermetallics crystallize in the HoFe₆Sn₆-type structure with the space group I mmm while the SmMn_6-xFe_xSn₆ intermetallic alloys (x = 0.5, 1.0, 1.5, and 2.0) have the TbFe₆Sn₆-type structure with the space group C mcm. Lattice constant, a, b, c, and unit cell volume contract with increasing iron content The net magnetization exhibits a small magnitude variation within the iron content range investigated. The magnetic moment of the iron sublattice couples positively with ferromagnetically coupled manganese and samarium moments. The Curie temperature (T_c) first increases, goes through a maximum at about x = 1.5, and then decrease again with the increasing iron content. The easy direction of magnetization for all of the samples is parallel to the bc plane of the unit cell at 300 K.

A theory of positive and negative colossal magnetoresistance (CMR) in dilute magnetic semiconductors (DMS) is presented. The phenomenon is described in the framework of bound magnetic polaron (BMP) hopping, driven by thermodynamic fluctuations of the local magnetization. The latter is caused by fluctuating spins of the magnetic atoms which strongly interact with the spin of a trapped electron or a hole within the localization volume. The authors' previous approach to the BMP hopping problem^1,2, based on the Ginzburg-Landau effective Hamiltonian method, has been generalized in order to take into account the anisotropy of the BMP states. This effect can be attributed to the anisotropy of the local exchange field and/or to the field-induced anisotropy of the magnetic susceptibility. In DMS the latter is shown to affect the position and the shape of the maximum in the magnetic-field dependence of CMR.

Inhomogeneous magnetization generated by point defects situated at the surface of a ferromagnet is studied. We analyzed two cases with different boundary conditions for the magnetization at the crystal surface. The first case is when the magnetic moment is rigidly fixed at the surface and the second is when the magnetic moment is determined by the interactions in the bulk of the crystal.

We propose a model for equilibrium magnetic configuration of CsDy(MoO₄)₂ which is based on the new space group established in recent neutron scattering experiments. These results are complimented by measurements of magnetization below magnetic critical temperature.

We have prepared (Sm-Co)/Co multilayers with two different Co thicknes to study the hard magnetic properties (coercivity H_ci and energy product BH_max) of nanostructured SmCo magnets consisting of hard SmCo and softer higher moment Co phases. Samples are annealed at a temperature in the range 450 – 700°C, for 20 minutes in vacuum (standard anneal) or rapidly annealed for 30 seconds under flowing nitrogen (rapid anneal) to form the hard Sm-Co phase. We find that samples subjected to a standard anneal at 500°C or a rapid anneal at 650°C show the largest energy products in these thin films, about 21 MG.Oe. This is closely related to the fact that the high field magnetization is highest for these anneal temperatures and decreases at higher anneal temperatures.

Several techniques were proposed to achieve solid state spin filtering such as magnetic tunnel junctions comprised of half-metallic compounds or solid state Stern-Gerlach apparatus. Another alternative consists in using spin-dependent resonant tunneling through magnetically active quantum wells. Recent advances in molecular beam epitaxial growth made it possible to fabricate exotic heterostructures comprised of magnetic films or buried layers (ErAs, Ga_xMn_1-xAs) integrated with conventional semiconductors (GaAs) and to explore quantum transport in these heterostructures. It is particularly interesting to study spin-dependent resonant tunneling in double-barrier resonant tunneling diodes (RTD) with magnetic elements such as GaAs/AlAs/ErAs/AlAs/ErAs/AlAs/GaAs, Ga_xMn_1-xAs/AlAs/GaAs/AlAs/GaAs, and GaAs/AlAs/Ga_xMn_1-xAs/AlAs/GaAs. We present the results of our theoretical studies and computer simulations of transmission coefficients and current-voltage characteristics of resonant tunneling diodes based on these double-barrier structures. Resonant tunneling of holes (Ga_xMn_1-xAs-based RTDs) is considered. Our approach is based on k·p perturbation theory with exchange splitting effects taken into account. We analyze exchange splitting of different resonant channels as a function of magnetization as well as spin polarization of the transmitted current as a function of bias. We found that resonant tunneling I – V characteristics of the double-barrier magnetic hererostructures strongly depend on the doping level in the emitter as well as on the orientation of the magnetization.

Recently there have been attempts to create physically realistic models for para/poly-crystalline silicon (containing randomly oriented c-Si grains embedded in a disordered matrix) by means of empirical molecular dynamics. These models demonstrate acceptable geometrical and vibrational properties but fail to reproduce the correct electronic bandgap due to the presence of numerous "frozen-in" coordination defects. We propose a new procedure for the preparation of more realistic models of paracrystalline silicon based on a modification of the bond-switching method of Wooten, Winer and Weaire. Our new method allows us to create interfaces between the crystalline and disordered phases of Si with no coordination defects. Models with 400, 1000, and 4000 atoms were constructed. All the models have ~10 atomic % of the crystalline phase. The two smaller models contain a single crystalline grain and the largest model contains 4 randomly oriented grains. Our models show good geometrical and vibrational properties compared to good continuous random networks models of a-Si and also display excellent optical properties, correctly reproducing the electronic bandgap of amorphous silicon.

Recent interest has been generated in composite materials for which both the dielectric constant and the magnetic permeability are negative. These composites behave as if they possess a negative index of refraction, although the broader, less specific adjective "left-handed" has also been applied to them. Such composites possess two sets of structures which are separately responsible for the negative ∊ and μ. However, materials having a negative μ are common in the microwave frequency range and such a material can replace one set of structures in the composite. An example of a simple composite is a YIG medium (μ<0) penetrated by sets of metallic strips or wires which give rise a negative dielectric constant in the appropriate frequency range. The index of refraction of the composite can be modulated by adjusting the applied magnetic field.

Ferromagnetic resonance measurements have been performed on several samples of Terfenol-D (Dy_0.73Tb_0.27Fe_1.95) at 16.95 GHz and over the temperature range 293 to 305 K. We find that the first magnetocrystalline anisotropy constant, obtained from one sample under nearly zero stress, is K₁ = (-1.4±1.0)× l0⁶erg/cm³ at 294 K. Our measurement is distinct from quasistatic torque measurements in that the lattice does not deform during the measurement and, hence, the anisotropy contribution due to magnetoelastic strain does not enter. The bare anisotropy constant, unmodified by static elastic strain, is and . The samples exhibited hysteresis; the position of FMR shifted by 4.0 kOe between measurements made with the magnetic field increasing and those made with the field decreasing.

We present a parallel algorithm for numerical calculations for electronic lattice models with interactions and disorder. In this scheme, diagonal and off-diagonal disorder are treated exactly per disorder configuration and interaction effects are evaluated using conserving approximations based on self-consistent perturbation theory. We demonstrate this method by calculating second-order corrections to the local pairing amplitude and specific heat for the attractive Hubbard model.

A Cremona transformation X=f(x, y), Y=g(x, y) is a rational mapping (meaning that f and g are ratios of polynomials) with rational inverse x=F(X, Y), y=G(X, Y). Discrete dynamical systems defined by such transformations are well studied. They include symmetries of the Yang-Baxter equations and their generalizations. In this paper we comment on two types of dynamical systems based on Cremona transformations. The first is the P1 case of Bellon et al. which pertains to the inversion relation for the matrix of Boltzmann weights of the 4-state chiral Potts model. The resulting dynamical system decouples completely to one in a single variable. The sub case z=x corresponds to the symmetric Ashkin-Teller model. We solve this case explicitly giving orbits as closed formulas in the number n of iterations. The second type of system treated is an extension from the famous example due to McMillan of invariant curves of area preserving maps in two dimensions to the case of invariant curves and surfaces of three dimensional Cremona maps that preserve volume. The trace map of the renormalization of transmission through a Fibonacci chain, first introduced by Kohmoto, Kadanoff and Tang, is considered as an example of such a system.

Tight-binding Hamiltonians with off-diagonal disorder are studied extensively in connection with localization phenomena. For instance, applying a random magnetic field to a square lattice amounts to assigning phases to the off-diagonal entries of H. A gauge transformation simplifies the resulting H to an equivalent Hamiltonian with apparently less disorder. This paper concerns the special case when non-zero entries of a Hamiltonian H are ± 1 between nearest-neighbor sites. Some thought shows that a one-dimensional chain Hamiltonian with this type of randomness can be transformed to that of an ordered chain by flipping the signs of selected basis functions, i.e. by a unitary transformation. Hence such a chain is not disordered at all. On the other hand, whether or not a similar H for a square lattice implies actual disorder is a topological question. The question can be put this way: Can one find a set of simple closed curves such that reversing the signs of basis functions inside the curves will change all non-zero H entries to +1? More generally one can ask about the extent of residual disorder and its effect on electronic properties of the model. We take up these questions on a periodic square lattice.

We estimate the number of metastable states of a self generated stripe glass, relevant for the formation of glassy doped Mott insulators. Using replica bound states, we demonstrate that the configurational entropy is the difference between the entropy of the stripe liquid and of an amorphous stripe solid with phonon-type excitations. Using simple scaling laws we then determine the relationship between the modulation length and the configurational entropy.

The oxidation of Mg(0001) was studied using LEED (Low Energy Electron Diffraction) and ESDIAD (Electron Stimulated Desorption Ion Angular Distributions) with supplemental information from STM (Scanning Tunneling Microscopy). In agreement with previous work, two kinds of oxide were observed: a sub-oxide consisting of subsurface oxygen and ionic MgO. From 0 to 8 L (Langmuir=1×10^-6 Torr Sec) oxygen exposure, the 1×1 LEED pattern obtained for clean Mg(0001) becomes diffuse and almost disappears, indicating that the initial stages of oxidation lead to a disordering of the surface. At higher oxygen exposure (~12 L) a diffuse 1×1 returns, showing that this new growth is in registry with the Mg substrate. The ESD O⁺ ion yield, which is initially near zero for substrate oxygen, increases dramatically at this point, indicating that the oxide is now on the surface. The desorbing O⁺ forms a normal emission pattern in the ESDIAD experiment at still higher oxygen exposures.

Mono- and heteronuclear clusters were synthesized by irradiating with gamma rays from a nuclear reactor aqueous solutions containing ionic precursors. Ag, Au/Ag and Ag/Pd clusters were produced and characterized by optical absorption and transmission electron microscopy measurements. Electron diffraction measurements show that the clusters have a crystalline fcc structure. Ag and Au/Ag clusters have a lattice parameter which coincides with the bulk. In the case of Ag/Pd clusters, a homogeneous alloy is formed whose lattice parameter closely follows Vegard's law.

The cluster size distribution is in the nanometer range, although coalescence processes lead often to large aggregates. Optical absorption spectra are in agreement with previously reported results, when the presence of large aggregates is taken into account.

We have prepared Nb/Dy/Nb trilayers to study size effects on the magnetic and superconductor properties and to look at the influence of the magnetic layer on a superconductor layer in a nanoscale system. We find size effects on the magnetic properties of the Dy layer for thickness less than 10 nm and size effects associated with the superconductor layer for Nb layers of thickness less than 50 nm. In addition increasing the thickness the Dy layer reduces the superconductor transition temperature of the Nb layer.

A unit cell of a spinel ferrite is just under 1 nm, actually about 0.8 nm. Particles approaching this size can be expected to possess properties that differ from the bulk material. Bulk polycrystalline Zn-ferrite (ZnFe₂O₄) has been ball milled; after 50 h the average particle size is about 13 nm. The ⁵⁷Fe Mössbauer spectra at T = 4.2 K with and without a large (52 kOe) magnetic field applied longitudinally establish that the antiferromagnetic structure of the bulk has become ferrimagnetic in the ten nanometer particles via the transfer of some Zn cations from the A (tetrahedral) sites to the B (octahedral) sites. In addition, there is a large non-collinearity in the magnetic structure.

We have fabricated PrBa₂[Cu_1-xGa_x]₃O₇ (PBGCO) ceramic samples for x = 0.05, 0.10, 0.15 and 0.20. The electrical resistivities of these samples are many orders of magnitude higher than that of PrBa₂Cu₃O₇ (Pr123) while the other physical properties remain much the same. PBGCO therefore may be a possible buffer layer for YBa₂Cu₃O_7-δ (Y123) superlattices/multilayers. For this reason YBa₂Cu₃O_7-δ/PrBa₂(Cu_0.8Ga_0.2)₃O₇ (Y123/PBCGO) and YBa₂Cu₃O_7-δ/Y_0.4Pr_0.6Ba₂Cu₃O₇ (Y123/YPBCO) superlattices were fabricated and their transport properties were studied. It is found that although PBGCO is better for insulating Y123 films in trilayers, inter-Y123 layer coupling exists between separated Y123 layer.

Low temperature surface adsorption at thermodynamic equilibrium is studied using a lattice model. So far, we have investigated fcc (100) and bcc (110) surfaces, with a finite number M of sites in one direction, and an infinite number of sites in the other, and no periodic boundaries. Here, we consider fcc (110) surfaces with first- (V), second- (W) and third- (U) neighbor adsorbate-adsorbate interaction energies, considered as arbitrary parameters. Adsorption is assumed to be on-site, or at locations commensurate with the substrate. Thus adsorbates are treated as monomers. A monomer in the gas phase has a chemical potential μ′ that depends on the gas pressure. With V₀ representing the lattice-adsorbate interaction energy, then μ′ and V₀ appear as the combination μ =μ′ + V₀. While keeping the temperature sufficiently low, the gas pressure is gradually increased, accompanied by an increase of μ. The surface begins to fill and phases with increasing coverage are observed until the surface is fully covered. The patterns depend both on the substrate and the adsorbates, and consequently on the interaction energies. A phase is characterized by the coverage θ₀, and the numbers per site, θ, β and γ of first- second- and third-neighbor adsorbate-adsorbate, respectively. The occupational characteristics of the phases and most of the transitions between phases are found to fit exact analytic expressions in M, allowing the exact extrapolation to the infinite two-dimensional surface. We have numerically investigated all possible relative values that V, W, and U may have whether attractive or repulsive. A set of phases occurs when V, W, and U are found to satisfy a certain set of inequalities defining an energy region. We have obtained all possible energy regions and the possible phases occurring in every region. Within a given energy region, let {θ₀₁, θ₁, β₁, γ₁} and {θ₀₂, θ₂, β₂, γ₂} be two consecutive phases. The value μ₁₂ of μ at the transition is the one for which the entropy is a local maximum. In all cases, (θ₀₂-θ₀₁) μ₁₂=-V(θ₂-θ₁)-W(β₂-β₁)-U(γ₂-γ₁). Consider an adsorption system with no surface reconstruction and at thermodynamic equilibrium. Providing first-, second- and third- neighbor interactions are predominant, it is sufficient to determine experimentally the patterns and the pressure at the transitions, to obtain a number of linear relations between the various interaction energies. Since there are four interaction energies, V₀, V, W and U, there may be cases where it is possible to determine all of these energies.

A three-dimensional sixteen-vertex model on the diamond lattice describing proton ordering in KDP-type crystals is shown to exhibit both 1^st- and 2^nd-order phase transitions. The model's critical behavior was found, using analyses of series expansions and Monte Carlo simulations. When the transition is 2^nd-order, critical exponents belong to the universality class of the three-dimensional Ising model.

A simple model for an electron in two dimensions interacting with an elastic distortion field shows interesting polaron-like behaviors. The model consists of a single tight-binding electron in a classical polarization field. The distortion potential ϕ and electron amplitude ψ evolve by coupled dynamical equations derived from a Lagrangian with cross term gϕ|ψ|². The equations of motion for ψ and ϕ are integrated together in time. For large enough g, the electron self-traps and a bound polaron-like object exists with time independent ϕ in the shape of a well and ψ a stationary state in the well. There is a critical value g_min below which the polaron well disappears. Spectral densities obtained by transforming double-time Green functions into show mass enhancement and lifetime broadening effects. The model, such as it is, can be extended easily to arbitrary, confined geometries in two or three dimensions.

We consider a model of cars driving along a compressible or erodible road. Each car is modeled as a mass supported above a wheel by a damped spring. We find conditions under which a flat road surface is unstable and gives way to a regular pattern of corrugations. The instability is mitigated by the damping of the cars' springs and also by the tendency of the road surface to harden as it is compressed. Similar phenomena occur in railroad rails and in calendaring of paper.

We propose a theoretical model to account for the increase in the effect of both magnetic induction and presence of a nonequilibrium state in layered superconductors which behave as intrinsic Josephson junctions. Using this model, we investigate the phase dynamics with a magnetic field applied parallel to the superconducting layers and show that the increase in the coupling between the superconducting layers due to the increase in the effect of magnetic induction and the departure from equilibrium significantly modifies the dispersion of the Josephson plasma mode in intrinsic Josephson junctions from that in conventional stacks of Josephson junctions.

Angle and temperature dependent torque magnetization measurements are reported for the organic superconductor κ-(ET)₂Cu(NCS)₂, at extremely low temperatures (~ T_c/10³). Magneto-thermal instabilities are observed in the form of abrupt magnetization (flux) jumps. We carry out an analysis of the temperature and field orientation dependence of these flux jumps based on accepted models for layered type-II superconductors. Using a simple Bean model, we also find a critical current density of 4 × 10⁸A/m² from the remnant magnetization, in agreement with previous measurements.

We have performed systematic quantitative measurements of the critical current density J_c(B, T) for YBCO thin films proton-irradiated to various dosages using the MOI technique. Results for external magnetic fields up to 400Oe and temperature ranging from 5 to 80K are presented for a 600nm thick YBCO film irradiated by proton up to a dosage of 10¹⁶cm^-2 at 500 keV. While no significant differences were observed for T>50K, the J_c values consistently increased with the increasing external field for T<50K.

We have fabricated nano-scaled planar superconductor-insulator-superconductor Josephson junctions using focused ion beam (FIB) with beam spot size ~5 nm. To study the effectiveness of this fabrication technique and for the purpose of comparisons, a variety of samples have been made based on high temperature superconducting (HTS) YBa₂Cu₃O_7-δ electrodes. The insulators are either vacuum or silicon dioxide. The samples showed current-voltage (IV) characteristics typical of a resistively shunted junction (RSJ). We will discuss various aspects of the processing methods and the physical significance of the junction characteristics.

Recently the remarkable discovery of field-induced phase transition from antiferroelectric (AFE) to Ferroelectric (FE) in ferroelectrics excited the scientific community in this field. This finding has both scientific and technological implications to us. In this paper, we studied the changes of the crystal structure when an electrical field is applied to it. The X-ray diffraction pattern reflecting these changes was discussed by theoretical calculation.

The phase transition from the paraelectric phase to ordered phases in Pb_1-x(Li_1/2La_1/2)_x(Zr_1-yTi_y)O₃ with compositions close to the ferroelectric - antiferroelectric - paraelectric triple point in the Ti-content - temperature phase diagram is studied. X-ray diffraction is used to identify two - phase (antiferroelectric and ferroelectric) nuclei embeded in a paraelectric matrix. The relation between these two-phase nuclei in paraelectric phase and the diffuseness of the phase transition is discussed.