International Journal of Modern Physics B

A superconducting superlattice of quantum stripes with a particular shape that gives the amplification of the critical temperature is called "superstripes". This superlattice is characterized by: a) the metallic stripe width L of the order of the de Broglie wavelength of electrons at the Fermi level, L~λ_F, and b) the hopping of pairs between the stripes larger than the hopping of single electrons. "Superstripes" can be realized artificially to get new room temperature superconductors (RTS). This particular mesoscopic heterostructure at the atomic limit has been found as a self organized network of charges, in a short time and space scale, in doped cuprate perovskites near the micro-strain quantum critical point for "superstripes" formation.

Two distinct energy scales, the d-wave superconducting order parameter, and the charge segregation in the form of stripes in cuprates are unified in the framework of the bipolaron theory as a result of the formation of mobile bipolarons in the normal state and their Bose-Einstein condensation. Within the theory both the d-wave superconducting order parameter and striped charge distribution result from the bipolaron (center-of-mass) energy band dispersion rather than from any particular interaction.

Recent experimental results have evidenced a remarkable structure in the k-dependence of the d-wave superconducting order parameter of underdoped high-T_c cuprates near a node. Here, it is pointed out that deviations from a simple d-wave superconducting spectrum induce corrections to the superconducting DOS and the low-T asymptotic behavior of various electronic properties in the superconducting state at intermediate energies. Such corrections depend on the theoretical scheme employed to describe the deviations.

According to the recent neutron scattering data on YBCO, it has been concluded that the incommensurability of the spin dynamics is a common feature among oxide high-T_c superconductors, which cannot be explainable by Fermi surface anomaly. In this paper, we speculate a possible scenario of the mechanism of the superconductivity based upon an assumption of the stripe domain structures; RVB-like spin singlet formation plays a primary role for the attractive force for the Cooper pair formation of dopant holes existing in the adjacent charge chain site, which is separated by the spin stripes.

Crystallographic features of the transitional state in La_1-xSr_xTiO₃ around x=0.05 have been examined by transmission electron microscopy. In the transitional state, the insulating O′ regions with the size of about 3nm are distributed in the metallic O* matrix. Note that the O′ and O* phases have the Pbnm structures with the Jahn-Teller distortion and without it, respectively. Because the hole concentration of the O* phase is higher than that of the O′ phase, the transitional state is therefore characterized by the static electronic phase separation. In addition, it was also found that the crystal structure of the O′ phase should involve not only the (3z²-r²)-type Jahn-Teller displacement with q=0 but also the (x²-y²)-type one with q=<1/2 1/2 0 >_c in the cubic Brillouin zone.

We report hydrostatic and uniaxial pressure dependence of T_c and HTT to LTO transition temperature (T_d) for La_1.85Sr_0.15CuO₄ superconducting crystals. It was found that the T_c increases and T_d decreases with pressure along a-axis and vice versa along c-axis. The T_c does not show any change for pressure along (110) axis. The T_c increases (dT_c/dP=+1.54 K/GPa) and T_d decreases (dT_d/dP=-44 K/GPa) under hydrostatic pressure. These anisotropic effects are discussed based on the variations in the amplitude of the tilt angle of oxygen octahedra, which would affect the pinning of fluctuating stripe order.

We present a unified theory for the resonance peak and low-energy incommensurate response observed in high-T_c cuprate superconductors. The origin of both features is purely magnetic and they represent universal features signaling the existence of an incommensurate spin state both below and above the superconducting transition temperature. We argue that the resonance peak is the reflection of commensurate antiferromagnetism. Our theoretical scenario gives an account of the main universal features observed in various families of superconductors and predicts those that have not been observed yet experimentally.

The metallic phase in doped cuprate perovskites is determined by both the hope doping δ and the micro-strain ε of the planar Cu-O bond length. The micro-strain ε in the CuO₂ plane has been measured by Cu K-edge EXAFS and x-ray diffraction using synchrotron radiation. The critical micro-strain ε_c for the onset of local lattice distortions (LLD) and stripe formation has been determined. The strain quantum critical point (QCP) is found at (ε_c,δ_c). The superconducting critical temperature is measured as a function of two variables T_c(ε,δ) and it reaches its maximum at the strain QCP. The superconducting phase occurs in the region of critical fluctuations around this QCP. The critical fluctuations near the strain QCP drives the self-organization of the metallic plane forming a particular superlattice of quantum stripes called "superstripes" that favors the amplification of the superconducting critical temperature.

A new class of self organized networks is described that is relevant to understand the emerging order in a large number of complex systems such as growing biological systems, the web, and heterogeneous phases of correlated condensed matter systems formed by doping. The Bose and Fermi quantum distributions are shown to be the right tool to describe the two extreme limit distributions, the fermionic Cayley-tree network and the bosonic scale-free network. In this new large class of self-organized networks the two different types of self organization coexists, maintaining the same 'ergodic' nature and are described by a 'mixed' quantum distribution.

An extremely low noise RF preamplifier cooled with liquid helium has been developed and is used to perform Cu-NQR measurements for Hg-1223 (T_c=134 K) at temperatures from 4.2 K to 145 K. At temperatures from 4.2 K to 20 K the spectra are centered at 15 MHz with a spectral width of approximately 2 MHz, similar to the Cu-NQR spectra of the other members of the HgBa₂Ca_n-1Cu_nO_2n+2+δ series. The Cu-NQR spectra in this temperature range exhibit some features indicating an unresolved splitting but they can still be fitted quite well by a set of two pairs of ^63/65Cu lines, as expected for the 1223 structure. The spectra at 40 K and above clearly show a structure that can not be fitted suitable by a set of two pairs of ^63/65Cu lines. However, a fit with a set of four pairs of ^63/65Cu lines is in good agreement with the experimental results. Furthermore, the spectra at 80 K, 85 K and 90 K show a strong variation with respect to each other which might indicate a structural change like the formation of a superstructure, as discussed for other high-T_c compounds, instead of a simple disorder in the crystal lattice.

Using new resonance results, we investigate the relation between the charge and spin ordering features seen by a.o. neutron scattering and ARPES, and the NMR/NQR signal intensities (wipe-out effects), relaxation rates and line shapes in hole doped La₂NiO₄, La₂CuO₄, Bi₂Sr₂CaCu₂O₈ and electron doped NdCeCuO₄. We indicate what is understood and which questions are still open, with especially a focus on the observation that the wipe out effects happen in both hole and electron doped systems and continue down to the lowest measured temperatures. The latter feature might be the signature of a glassy dynamics of an entirely new kind, associated with the special nature of stripe order.

The NMR and neutron scattering experiments on YBa₂Cu₃O₇ with Li and Zn impurities are compared using a simple framework exhibiting short-range antiferromagnetic correlations. The model parameters are determined by fitting the NMR rate of the pure system, and the effective impurity potential is approximated by a static extended form. The results of the calculations are compared with the experimental data, and their implications are discussed.

We study the unconventional insulating state in A₄C₆₀ by means of the dynamical mean-field theory. The interplay between the electron-electron correlation and the Jahn-Teller interaction determines the properties of these compounds. The system is a Mott-Jahn-Teller insulator. In that state, conduction between molecules is blocked by on-site Coulomb repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and important excitations (such as optical and spin gap) are found to be essentially intra-molecular. Experimental values of the optical and spin gaps are recovered by our calculations.

The presence of incommensurate spin structures in the half-filled triangular Hubbard model, where frustration leads to a competition among different magnetic phases, is investigated using both the slave-boson technique, and exact diagonalization of finite clusters. We also investigate the metal-insulator transition which, due to the lack of perfect nesting, takes place at a finite value of U. Within the slave-boson approach, as the interaction grows the paramagnetic metal turns into a metallic phase with incommensurate spiral ordering. Increasing further the interaction, a linear spin-density-wave is stabilized, and finally for strong coupling the latter phase undergoes a first-order transition towards an antiferromagnetic insulator. No trace of the intermediate phases is instead found in the exact diagonalization results.

I study the evolution of the antiferromagnetic ground state of the half-filled Hubbard model in the presence of a charge modulation induced by the dimerization of the crystal field, making use of the slave-boson approach. I also discuss how the model can be extended to describe the neutral-to-ionic transition which occurs in some organic mixed-stack donor-acceptor crystals.

The interplay of strong local electron-lattice and electron-electron interaction is analyzed in the two-dimensional Hubbard-Holstein model using an effective single band Hamiltonian. The effect of the strain field acting on the CuO₂ plane and the local cooperative pseudo Jahn Teller vibronic electron-lattice coupling appropriate to cuprate superconductors at optimum doping (i.e.; at doping larger than 1/8) are included via the effective next near neighbor electronic hopping integral t′ (determined by the strain field) close to the local vibronic frequency ω₀ (involved in the pseudo Jahn Teller electron lattice coupling). The system shows a characteristic feature of the physics of complex systems, the competition and fluctuation between two states: 1) a first phase of charges self trapped in domains with local lattice distortions (a generalization to a many polaron system of the idea of single intermediate polaron) and 2) a second phase of free carriers.

Lanthanum cuprate (La₂CuO₄) is an antiferromagnetic insulator, which becomes superconducting after introducing interstitial oxygen by high-pressure, chemical or electrochemical oxidation. We report here the flux growth of superconducting single crystal in an ambient atmosphere, which the morphology is remarkably different from that of insulating crystal. The behavior of superconductivity is studied by AC-susceptibility measurement. A phase separation makes a strong effect on the superconductivity. The phase separation mainly occurs at the temperature range of 270 to 190 K and the separation is reversible in the solid state. Superconducting transition temperature (Tc) depends on the temperature, the time and the cooling process. Tc reaches an equilibrium value after several hour treatment at constant temperature, which indicates the completion of the phase separation. It increases to a higher value level only by 30 minute cooling from 260 to 230 K, which suggests the stripe nucleation and growth. A mechanism of the stripe formation is discussed based on the staging phase diagram of La₂CuO_4+δ. Parallel intergrowth of the oxygen-rich phase and the oxygen-poor phase takes place alternatively in the miscibility gap, in result of self-assembling a composite structure in the duplex single crystal.

We have carried out the measurements of the magnetoresistance (MR) of YNi₂B₂C single crystals (T_c=15.3 K and H_c2(5.5 K)~4.7 T) in a wide range of temperature and magnetic field, T_c+1<T<T_c+145 K and 0<H<8 T. In the normal state, the in-plane MR, Δρ(H, T)=ρ(H, T)-ρ(0, T), changes from negative to positive as the temperature decreases. A pronounced sign-reversal is observed at T=80 K with H=4 T. A negative MR supports the presence of magnetically-related scattering and could signify considerable suppression of the spin fluctuations most likely associated with nearly magnetic metals in the presence of an external field. Such novel behavior reveals the possibility of pseudo-gap formation, which plays the role of unconventional superconducting mechanism, as indicated by the recent ¹¹B and ¹³C NMR studies [1]. Our results are also in quantitative agreement with the short-range magnetic moment dependence upon temperature and applied field.

The effect of hydrostatic pressure on the properties of the La_0.75Ca_0.25MnO₃ manganite have been studied by means of spectroscopic techniques. Raman and infrared (IR) spectra of this pseudocubic manganese perovskite have been collected for the first time over a wide pressure range (0–14 GPa) using a diamond anvil cell. Preliminary results have been obtained also in the case of the Sr_1.5La_0.5MnO₄ layered perovskite. The Raman measurements allow us to follow the pressure evolution of the phonon spectrum of the MnO₆ octahedron vibrational modes. Through the IR measurements we study the pressure dependence of the high-frequency octahedron stretching mode and of the broad background associated to polaronic absorption. Raman results show an abrupt discontinuity at P*~7.5 GPa in both frequency and width of some of the observed phonons. The infrared absorption measurements shows a remarkable increase of the polaronic absorption which do not completely screen the phonon peak even at the maximum pressure. Our results, which are in agreement with recent high pressure diffraction data, do not support the possibility of obtaining a metallization transition in the La_0.75Ca_0.25MnO₃ manganite at high pressure.

Recent investigations in the context of the standard and extended t-J models using computational techniques are here described. In regimes of couplings producing a one-hole dispersion compatible with photoemission data for the parent insulator, the one-hole states have strong antiferromagnetic correlations across-the-hole, particularly at momenta (0,π)-(π,0). This effect is rationalized as a tendency toward spin-charge separation in two-dimensions at short distances. At finite hole density, doped stripes are observed. The elementary building block of the stripes resembles the spin arrangement around individual holes. This suggests that the seed of doped stripes already exist in the unusual properties of the insulating parent compound. Stripes are conjectured to originate in a "frustrated spin-charge separation" effect, different from other theories valid at large J/t discussed in previous literature.

Polarised EXAFS measurements on the Cu K edge in a Pr-doped BSCCO (2212) insulating single crystal have been made at room temperature at the SRRC, Taiwan. Measurements have been confined to only the E//a, E//a^∧45 and E//b directions within the Cu-O₂ plane as most of the action is here. While the spectra in the E//a and E//b look very similar, the E//a^∧45 spectrum does look a little different. EXAFS analysis shows that Cu-O (planar) distances as well as the Debye Waller factor values in different directions turn out to be slightly different indicating a possibility of presence of some in-plane distortion. This appears to be very much in agreement with our XANES results on this system and our EXAFS results on Pr-doped YBCO (123) system reported earlier, which also indicate the presence of some distortion. Presence of Pr is speculated to be the cause for producing this distortion. This would be discussed in some detail taking into account the possibility that Pr, besides quenching holes, may also be displacing the in-plane oxygens.

We have investigated the charge ordering in an oxygen doped La₂CuO₄ crystal by diffuse high-resolution x-ray diffraction using synchrotron radiation. The micro-strain ε in the CuO₂ plane is close to the micro-strain ε₀ for the formation of static charge ordered phases. The crystal shows the co-existence of a first phase with an insulating crystal of charge strings and a second superconducting phase of "supestripes". The first insulating phase is due to charges at the critical doping 1/8 self trapped into a crystal of ordered strings of finite length (~60 Å) and a commensurate period of 4 lattice units. The superconducting phase shows diffuse diffraction peaks with short coherence length due to fluctuations of stripes with an incommensurate period of about 4.8 unit cells and length longer than 500 Å.

Significant elements in the case for high-temperature superconductivity originating in the charge-reservoir layers are presented, including: (i) the successful predictions that PrBa₂Cu₃O₇, Gd_1.6Ce_0.4Sr₂Cu₂TiO₁₀, Pr_1.5Ce_0.5Sr₂Cu₂NbO₁₀, and Eu_1.5Ce_0.5Sr₂Cu₂TiO₁₀ all superconduct; (ii) the fact that muons stopped in the SrO layers of Sr₂YRu_1-uCu_uO₆ detected flux expulsion, (iii) the observed superconductivity of PrBa₂Cu₃O₇ when its c-axis lattice constant is longer than 11.77 Å, and (iv) the failure of Gd_2-zCe_zCuO₄, Cm_2-zTh_zCuO₄, and Ba₂GdRu_1-uCu_uO₆ to superconduct.

Assuming the exchange of antiferromagnetic spin fluctuations as the Cooper pairing mechanism we calculate the doping dependence of the resonance peak seen in inelastic neutron scattering and the magnetic coherence effect. We find that the resonance peak in the magnetic susceptibility, Im χ(q,ω), appears only in the superconducting state at ω_res and that it scales with T_c. Magnetic coherence is a result of an interplay between a d-wave order parameter and the kinematic gap ω₀. We analyze the structure of Im χ below T_c, the doping dependence of ω₀ and ω_res, and the consequences for the optical conductivity.

Single-phase samples of HgBa₂CuO_4+δ(Hg1201) are studied by thermoelectric power (TEP) measurements. The results show a dip in T_c(p) at a hole density p~1/8, similar to the one seen for the La_1.875Ba_0.125CuO₄ and related systems. The observation of this dip could be a signature of stripe phase in Hg1201 superconductor. The data are discussed in terms of 1D versus 2D stripe phases.

In the paper the topological model of the charge ordering and magnetic ordering in La_2-xMe_xCuO₄ is suggested. The model assumes that the spin polarons appear in the compound La₂CuO₄ at the doping. The La substitution leads to the hole appearance in the p-band of the oxygen in CuO₄ unit. This carrier is in the medium of the magnetic moments of Cu ions and it interacts with this moments to form the spin polaron. At x>0.05 the polarons are mobile and they are arranged in the stripes at the temperature drop.

We performed systematic measurements of magnetic susceptibility on single crystals of La_1.875Ba_0.125-xSr_xCuO₄. The dependence of the superconducting transition temperature on Sr-concentration demonstrates a step-like pattern upon doping at x~0.08 as the crystal structure changes from low-temperature tetragonal (LTT) to low-temperature orthorhombic (LTO) phase at low temperature. Upon cooling, an anomalous upturn in the susceptibility was observed at the structural phase transition between the LTT-LTO phases under the magnetic field parallel to c-axis.

We performed point-contact experiments on high-quality La_2-xSr_xCuO₄ polycrystalline samples with 0.116 ≤ x ≤ 0.186 by using very sharp Au tips (~ 2μm of diameter). In most cases we observed conductance curves indicating the occurrence of Andreev reflection. The stability of the contact allowed us to study the temperature dependence of these curves up to T_c. The well reproducible results give no evidence of a pseudogap even in the underdoped samples. We fitted the curves at various temperatures by using the generalized BTK model by Tanaka and Kashiwaya allowing for different symmetries of the order parameter, and we obtained the best fit in the (s+d)-wave case. All the symmetries give similar values of the isotropic component of the gap Δ_is, which, at low temperature, is always greater than the anisotropic one Δ_an and has a different behaviour in temperature. In the overdoped region, Δ_is obtained from Andreev reflection almost coincides with the gap determined by quasiparticle tunneling or ARPES. In the under-doped regime the Andreev gap decreases with x, while, following recent measurements, the gap measured by ARPES rapidly increases at the lowering of x. These results seem to confirm in LSCO the hypothesis, recently appeared in literature, of the existence of two distinct energy scales, one associated with the pair formation and the other related to the achievement of the phase coherence in the condensate.

Two equilibrium Cu-O bond lengths coexist in the superconductive CuO₂ sheets of the high T_c copper oxides because of a discontinuous change in the volume of localized and delocalized electrons at a cross-over from localized to itinerant electronic behavior. Cooperative bond-length segregation and a progressive ordering of the two bond lengths with decreasing temperature result. Transformation from local fluctuations to long- range order in a traveling charge- density wave occurs at a critical point T_crit≈T_c for a hole concentration x≈ 1/6 per Cu atom in the superconductive CuO₂ sheets. Extensive experimental evidence for the model is cited.

Using high-resolution x-ray scattering, we have demonstrated the existance of quenched disordered charge stripes in a single crystal of La_5/3Sr_1/3NiO₄ at low temperatures. Above the second-order transition critical scattering was observed due to fluctuations into the charge stripe phase. The charge stripes are shown to be two dimensional in nature both by measurements of their correlation lengths (ξ_a≈185 Å, ξ_b≈400 Å, ξ_c≈25 Å.) and by the critical exponents of the charge stripe transition. The charge stripes are disordered and that the length scale of the disorder is quenched. Detailed measurements of the correlation lengths at higher temperatures, just below the charge stripe melting temperature, display an anisotropic broadening indicative of an order-disorder transition. We ascribe these results to a smectic-nematic phase transition at approximately 225 K. Such electronic quantum liquid-crystal phase transitions have been theoretically predicted before, but these results provide the first experimental evidence for them in charge stripe nickelates.

Topological aspects of the stripe structure in manganese oxides are discussed in terms of the "winding number" w associated with the Berry-phase conncection for e_g orbitals acquired by the parallel transport through the periodic array of Jahn-Teller centers. For La_1-xCa_xMnO₃, w is shown to characterize both the three-dimensional spin-charge-orbital structures in the antiferromagnetic phase for x≥1/2 and the charge-orbital stripes in the two-dimensional ferromagnetic phase for <1/2.

In this paper we will show that the microscopic origins of the "high-energy" pseudogap and of the peculiar hole-concentration dependence of the electronic entropy correspond to a phase change between a phase consisting of the small Fermi surfaces and a phase with a large Fermi surface. In real materials, however, the phase change is not sharp due to the finite spin-correlation length for local antiferromagnetic ordering. Further we will show that the dopant hole carriers in cuprates behave like heavy fermions.

The extended Hubbard model with the nearest-neighbor Coulomb interaction on the square lattice is studied within the mean-field approximation. The stable states for 8×8 sites lattice with the periodic boundary condition and the electron filling n=0.875, as well as for 10×10 lattice and n=0.80, are obtained. For 8×8 lattice where quarter-filled straight stripe is expected because of the long-range Coulomb interaction, the cross stripe phases become stable.

One- and two-dimensional attractive Hubbard models in the presence of magnetic field h are studied in a wide range of on-site interaction strength U, temperature T and electron concentration n by means of the generalized self-consistent field (GSCF) approach with renormalized chemical potential. Pairing, the smooth crossover from the BCS regime into the Bose condensation state and the inhomogeneous superconductivity with the non-uniform order parameter are investigated. The excitation spectrum, the momentum distribution function and thermodynamic properties, including the concentration of double occupied sites, the kinetic energy, and the chemical potential, are calculated. The numerical calculations are found in a good agreement with the Bethe-ansatz results and quantum Monte-Carlo calculations in a wide range of system parameters U, n and h. The GSCF theory correctly displays the separation of the energy gap from the order parameter (crossover) as n or |U| decrease. The concentration of double occupied sites and the bound electron pairs n_pair are calculated and we found a simple relationship between the order parameter and n_pair, valid for the arbitrary U and all n in any dimension. The developed GSCF approach in magnetic field suggests a smooth transition into the inhomogeneous superconducting state.

Our recent experimental studies on the so-called 1/8 anomaly in the Bi-2212 and Y-123 phases and also on the new anomaly in the overdoped region of the La-214 phase are reviewed. In the partially Zn-substituted Bi-2212 and Y-123 phases, we have found anomalous suppression of superconductivity (SC) at p (the hole concentration per Cu in the CuO₂ plane) ~1/8, and it has been found from the muon-spin-relaxation (μSR) measurements that the magnetic correlation between Cu spins is enhanced singularly at p~1/8. Furthermore, we have found giant suppression of SC singularly at x=0.21 in the 1% Zn-substituted single-crystals of La_2-xSr_xCu_1-yZn_yO₄ (LSCZO), and it has been found from the μSR measurements that the magnetic correlation is also enhanced at x=0.21. These results suggest that the dynamical stripe correlations of holes and spins exist in the Bi-2212 and Y-123 phases as well as in the La-214 phase and that they tend to be pinned by a small amount of Zn, leading to quasi-static stripe order and the suppression of SC at p~1/8 in the Bi-2212, Y-123, La-214 phases and also at x=0.21 in LSCZO. If this tendency is the case in a wide range of p, it is possible to understand the universal experimental results in the high-T_c cuprates that Zn is a strong scatterer in the unitarity limit and that the Zn substitution markedly suppresses SC.

We found that insulating and conducting electron strings may arise in oxide materials with not very wide bands, like HTSC, due to electron-phonon and electron-electron interactions including antiferromagnetic correlations. We estimate the string length and the number of particles self-trapped into a single string taking into account the typical parameters of cuprates, like La₂CuO₄. The conducting strings typically may have a small doping (n<0.3) and are created mostly due to the long-range electron-phonon interaction associated with ionic bonding of Oxides.

We examine the possibility of high-T_c superconductivity mediated by the elementary excitation of an electron stripe which is realized by neutralizing local polarization mainly caused by the c-displacement of ions on the CuO₂ plane. Under the translation invariance of the electronic state of the neutralization charges, it is found that both itinerant and localized electrons and/or holes can coexist in the stripe phase. If the frequency of the localized charge oscillation tends to soften along the stripe direction, superconductivity with T_c around 100 K can be realized, due to the enhancement of the coupling constant.

We present the first infrared conductivity spectra on single crystals of oxygen deficient cuprate La_8-xSr_xCu₈O₂₀. The infrared conductivity for a sample with x= 2.24 behaves as a function of temperature according to a conventional Drude model. On the other hand, the low energy excitations for a sample with x=1.56 are itinerant polarons. Below T=100 K, polarons localize and this effect can be related to the anomalies present in the transport and magnetic properties around this temperature.

Electron-doped Nd_2-xCe_xCuO₄ and hole-doped Bi₂Sr₂CuO₆ have been studied in the infrared to discriminate between the existing models of the optical conductivity σ(ω) in metallic cuprates. In Nd_2-xCe_xCuO₄ an absorption peak clearly visible in the insulating phase softens for increasing x and for decreasing temperature, until it reaches the far infrared (FIR peak) at the insulator-to-metal transition. In the superconducting phase it persists beside the free-carrier Drude absorption, as shown by fitting procedures, and disappears only in the overdoped normal metal. Direct evidence for the presence of the FIR peak in a metallic cuprate comes from the reflectivity of a Bi₂Sr₂CuO₆ film, measured down to 10 cm^-1, where we could resolve the FIR peak from a normal Drude term as narrow as 35 cm^-1 at low T. This observation provides direct evidence for the coexistence of free and "bound" charges in a superconducting cuprate.

Using the one-band Hubbard Hamiltonian we determine various basic properties of the electron-doped cuprate superconductor Nd_2-xCe_xCuO₄ for a spin-fluctuation-induced pairing mechanism. We find a narrow range of superconductivity and, most importantly, like for hole-doped cuprates d_x²-y² -symmetry for the superconducting order parameter. The superconducting transition temperatures T_c(x) for various electron doping concentrations x are calculated to be much smaller than for hole-doped cuprates due to the different energy dispersion and a flat band well below the Fermi level. We find that lattice disorder may sensitively distort the d_x²-y² -symmetry via electron-phonon interaction yielding a finite isotope exponent α₀.

A phase separation model for stripes has found good agreement with photoemission experiments and with other studies which suggest a termination of the striped phase in the slightly overdoped regime. Here the model is extended in a number of respects. In particular, a discussion of the nature of the charged stripes is presented, suggesting how density waves, superconductivity, and strong correlations can compete with the quantum size effects inherent in narrow stripes. The anomalous doping dependence of the chemical potential is explained.

We present a minimal model for cuprate superconductors. At the unrestricted mean-field level, the model produces homogeneous superconductivity at large doping, striped superconductivity in the underdoped regime and various antiferromagnetic phases at low doping and for high temperatures. On the underdoped side, the superconductor is intrinsically inhomogeneous and global phase coherence is achieved through Josephson-like coupling of the superconducting stripes. The model is applied to calculate experimentally measurable ARPES spectra.

The significant role of spin stripes in HTSC system is emphasized. The evidence for spin density wave (SDW) nature of pseudogap is presented. The dependence of CuO₂ lattice modulation amplitude on the temperature and applied magnetic field is studied. The indication to possible SDW-SC resonance is obtained. The possible amplification of T_c with using of artificial magnetic stripes with taking into account their dynamic nature is proposed. An example of twofold amplification of T_c in thin single-crystalline film of La-based cuprate is presented.

We measured optical time-resolved photoinduced reflectivity in La_0.82Pb_0.18MnO_3+δ thin film as a function of temperature. The photoinduced signal consists of initial fast relaxation on 200-300 fs scale followed by a slower relaxation on 10-100 ps timescale and extremely slow response on the ns-μs timescale. All components show distinctive changes below metal-insulator transition. Amplitudes of the faster components increase with decreasing temperature below metal-insulator transition indicating a bottleneck in the relaxation. The slow components amplitudes show distinctive peak at ~200K indicating presence of localized states in the vicinity of the Fermi energy significantly below the metal-insulator transition.

A comparison of photoinduced absorption spectra in AMnO₃ with A=La³⁺, Nd³⁺, Ca²⁺ is presented. The spectra are analyzed in the framework of photon assisted polaron hopping. In contrast to LaMnO₃ and NdMnO₃ the photoinduced absorption spectrum of CaMnO₃ shows distinct contributions from photoexcited holes and electrons. Both contributions are identified, and our results imply that magnitude of electron-phonon coupling constant for electrons in Mn e_g orbitals in CaMnO₃ is significantly smaller than in LaMnO₃.

The doping dependence of the low-temperature spectral function is precisely determined from angle resolved photoemision (ARPES) measurements. It is found that, as the doping decreases, the maximum of the superconducting gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos(k_x)-cos(k_y) form. We suggest that this arises due to the increasing importance of long range interactions as one approaches the insulator. It is also shown that the shape of the spectral function at the (π, 0) point below T_c can be explained by the interaction of the electrons with a collective mode whose energy matches that of the magnetic resonance as obtained by inelastic neutron scattering data, and points to the intimate relation of magnetic correlatins to high T_c superconductivity.

We have studied the electronic structure of the stripe phase in La_2-xSr_xCuO₄ (LSCO) and the Cu-O chains in PrBa₂Cu₃O₇ (Pr123) and PrBa₂Cu₄O₈ (Pr124) using angle-resolved photoemission spectroscopy (ARPES). In LSCO with x=0.12, the spectral feature near the Fermi level (E_F) is almost flat from (π, 0) to (π, π/4), namely, along the stripe direction in LSCO. While the 1/4-filled chain in Pr123 has a band gap because of charge ordering, the metallic chain in Pr124 shows a dispersive feature which reaches E_F at ~(π, π/4) and a flat feature near E_F which is similar to that observed in the stripe phase of LSCO. Although Pr124 shows spectral-weight suppression near E_F due to the instability toward charge ordering, the suppression is imperfect and Pr124 has a substantial spectral weight at E_F probably due to the chain-chain coupling. This is in sharp contrast to LSCO with the perfect gap opening near (π, π/4). The difference between the stripe phase in LSCO and the coupled chains of Pr124 would be derived from the interaction between the stripe and the neighboring Cu spins which suppresses the charge ordering along the stripe.

The presence of stripes and the effect of adding non-magnetic impurities (NMI), such as Zn or Li, to high-Tc cuprates is studied applying Monte Carlo techniques to a spin-fermion model. It is observed that adding Li is qualitatively similar to diping with equa percentages of Sr and Zn. The mobile holes (MH) are trapped by the NMI and the system remains insulating and commensurate with antiferromagnetic (AF) correlations. This behavior persists in the region %NMI>%MH. On the other hand, when %NMI<%MH magnetic and charge incommensurabilities are observed. The vertical or horizontal holerich stripes, present when %NMI=0 upon hole doping, are pinned by the impurities and tend to become diagonal, surrounding finite AF domains. The %MH-%NMI plane is investigated. Good agreement with experimental results is found in the small portion of this diagram where experimental data are available. Predictions about the expected behavior in the remaining regions are made.

We report the resistive upper critical field H_c2(T) of the hole-doped two-leg ladder Sr_14-xCa_xCu₂₄O₄₁(x=12) single crystal, which becomes superconducting with T_c~5 K in a pressure range above ~3.0 GPa . Using a newly developed compact high-pressure cell, the magneto-transport measurements have been performed at temperatures down to ~140 mK under high pressure above 3.5 GPa. It is found that H_c2 (perpendicular to the ladder Cu₂O₃ plane) increases anomalously as the temperature is decreased having an upward curvature and shows no saturation down to T/T_c~ 0.03. Also, its temperature dependence is found to resemble that of the high-T_c superconducting oxide Tl₂Ba₂CuO₆. This fact suggests that the anomalous behavior of H_c2 has a common origin between the ladders and the two-dimensional high-T_c superconducting oxides.

Local lattice distortions in YBa₂Cu₃O_y single crystals are studied by polarized x-ray absorption spectroscopy (XAS) for E//ab and E//c orientations between 300 K and 20 K. From the Fourier transform analysis of the Cu K-EXAFS oscillations, we find that the out-of-plane Cu-O correlation shows an anomalous temperature dependence of radial distribution below T_l=240-250 K, indicating an increase of lattice fluctuations, while no anomalies are found for the Cu-Ba correlation. On further lowering temperature, the Fourier transform shows that the lattice fluctuation are reduced below T₂~ T*=140-150 K associated with pseudogap opening. Below T*, the radial distribution of oxygen atoms became asymmetric, indicating that the distorted domain is formed. A strong electron-lattice coupling is indicated by the anomalous local lattice distortions at the cross-over temperatures characterizing various electronic transitions in the title system.

A cluster spin-glass transition has been identified in La_1.97Ba_0.03CuO₄, thanks to the analogies of the acoustic spectrum of this sample with that of La_2-xSr_xCuO₄ (x=0.03 and x=0.06), which has a well established cluster spin-glass phase. Our measurements are in good agreement with the presence of antiferromagnetically correlated clusters which are separated by hole rich boundaries. These "charge rivers" are thought to be the precursors of stripes.

High quality single crystals of Ba_1-xK_xBiO₃ (x=0; 0.15, 0.2, 0.33, 0.42) have been investigated by Raman light scattering in the 5-300 K temperature range. All spectra are interpreted as phonon states that are introduced in Raman scattering by folding back of modes from different points of the zone boundary due to structural distortions. Direct comparison of our spectra with inelastic neutron scattering data provides clear evidence for the existence of local R-type tilt and breathing distortions together with some additional distortion possibly related to the M-point.

A single crystal of La₂NiO_4+δ with δ=0.11 giving a hole concentration favouring phase separation, has been studied by infrared reflectivity between 4 K and 300 K, from 20 to 7,000 cm^-1. The spectra cannot be fitted properly by usual models and both plasma frequency and integrated conductivity shows two anomalies with temperature interpretable in terms of charge and spin ordering and their arrangements into stripes.

We show an evidence for the superconducting stripes (superstripes) in the Bi2212 system by joint x-ray diffraction and angle resolved photoemission. The kink observed at k_y=0.4π in the energy distribution curves is shown to be related to a modulation of the Cu displacement out of the oxygen plane with a wavevector Q~(0.4π, 0.4π) that modulates the next-nearest neighbor hopping integral t′. The resulting Fermi surface reveals broken segments around the M points due to the modulation of the t′, associated with modulation of the electron-lattice coupling λ(ε) that depends on the micro strain ε of the CuO₂ plane. The present findings further enlightens the fact that the micro-strain, controlling the electron-lattice coupling λ(ε) is a critical parameter for the superstripes.

Cu K-edge XANES spectroscopy has been used to study local structure around the Cu-site in three-dimensional oxygen deficient perovkite La_8-xSr_xCu₈O₂₀ system. The combination of E//ab and E//c polarization and high resolution XANES spectra on high quality single crystals has allowed us to distinguish the various features associated with the complex Cu-O networks in the system which have then been compared with those of the La_2-xSr_xCuO₄ system.

We consider a possible mechanism of the charge stripe order due to the next-nearest neighbor exchange interaction J′ in the two-dimensional t–J model, based on the concept of the phase separation. We also calculate some hole correlation functions of the finite cluster of the model using the numerical diagonalization, to examine the realization of the mechanism.

Anisotropic Cu-O strain has been measured and a relation between the superconducting critical temperature and the strain is studied in the YBa₂Cu₃O_6+k (Y123). oxygen ordered YBa₂Cu₃O_6+k phases. We show that the critical temperature scales both with the strain along the a axis (ε_a,) and along the b axis (ε_b). While undoped, the system is close to the critical strain, ε_c~ 0.043. The formation of chains with doping gives rise to a decrease of the ε_b to take the system away from the critical point approaching the dynamical quantum stripe fluctuations (ε_b<ε_c) and the ε_a increases to approach the phase of superconducting stripes (ε_a>ε_c).

In YBa₂Cu₃O_x, distinct features are found in the pressure dependence of the transition temperature, dT_c/dp, and in ΔC_p· T_c, where ΔC_p is the jump in the specific heat at T_c:dT_c/dp becomes zero when ΔC_p· T_c is maximal, whereas dT_c/dp has a peak at lower oxygen contents where ΔC_p· T_c vanishes. Substituting Nd for Y and doping with Ca leads to a shift of these specific oxygen contents, since oxygen order and hole doping by Ca influences the hole content n_h in the CuO₂ planes. Calculating n_h from the parabolic T_c(n_h) behavior, the features coalesce for all samples at n_h≈0.11 and n_h≈0.175, irrespective of substitution and doping. Hence, this behavior seems to reflect an intrinsic property of the CuO₂ planes. Analyzing our results we obtain different mechanisms in three doping regions: T_c changes in the optimally doped and overdoped region are mainly caused by charge transfer. In the slightly underdoped region an increasing contribution to dT_c/dp is obtained when well ordered CuO chain fragments serve as pinning centers for stripes. This behavior is supported by our results on Zn doped NdBa₂Cu₃O_x and is responsible for the well known dT_c/dp peak observed in YBa₂Cu₃O_x at x≈6.7. Going to a hole content below n_h≈0.11 our results point to a crossover from an underdoped superconductor to a doped antiferromagnet, changing completely the physics of these materials.

We study the single-particle spectra of a model which shows an incommensurate charge-density wave (CDW) instability arising from the competition between phase separation and long-range Coulomb interactions. Starting from an open Fermi surface we find that the resulting CDW is oriented along the (1,0)- and (or) (0,1)-direction which (besides a purely one-dimensional structure) allows for a two-dimensional "eggbox type" charge modulation. This leads to a substantial enhancement of the van Hove singularities and the spectral weight of Fermi surface states near the M-points tends to be suppressed. Remarkably, a leading edge gap arises near these points, which, in the eggbox case, leaves finite arcs of the Fermi surface gapless. Extending this approach to the inclusion of dynamic incommensurate CDW fluctuations in the superconducting state we calculate the photoemission lineshape which agrees well with experimental data of underdoped Bi2212 compounds.

Deoxygenated Y_1-xCa_xBa₂[Cu_0.992⁵⁷Fe_0.008]₃O_6.1 compounds with calcium content of x=0.05, 0.08, 0.12, 0.16, 0.2, 0.26 were prepared by the standard ceramic method. The system exhibit insulator to metal transition. The onset of superconductivity is detected in the sample with x=0.26. The influence of doped holes on low temperature antiferromagnetic order has been investigated. Low temperature Mössbauer spectra consist of two sextets with relative ratio changing with hole concentration. The sextets are broadened, their shape and temperature dependence indicate the existence of magnetic order fluctuations with the relaxation time slightly higher than the observation time of the method ~10^-7s.

Molecular orbital calculations on the model clusters of cuprate superconductor are presented to show the mechanism of insulator to metal transition. Highest occupied molecular orbitals (HOMO) of the model clusters of one-dimensional chain showed that the metallic carrier will appear above 6% doping level, which is consistent with experimental results.

We have investigated the magnetic measurements on high quality single crystals of YBa₂Cu₃O_7-δ. Several magnetization hysteresis loops have been obtained for different temperature values, and as a function of the angle θ between the direction of the applied magnetic field and the c-axis of the sample. Measurements were performed at magnetic field up to 6 Tesla and various temperatures between 10 and 90 K. Magnetization hysteresis loops scale and shape are strongly dependent on the temperature values. Our results show that the magnetic properties are profoundly affected by flux lines pinning. The irreversible magnetization decreases as the applied magnetic field deviates from the c-axis (θ increases).

In the underdoped, overdoped, Zn-doped or stripe-forming regions of high-T_c cuprate superconductors (HTSC), the superfluid density n_s/m* at T→ 0 shows universal correlations with T_c. Similar strong correlations exist between 2-dimensional superfluid density and superfluid transition temperature in thin films of ⁴He in non-porous or porous media, and ⁴He/³He film adsorbed on porous media. Based on analogy between HTSC and He film systems, we propose a model for cuprates where: (1) the overdoped region is characterized by a phase separation similar to ⁴He/³He; and (2) pair (boson) formation and fluctuating superconductivity occur at separate temperatures above T_c in the underdoped region.

We review our recent x-ray and neutron scattering studies that reveal static diffuse scattering due to polarons in the paramagnetic phase of the colossal magnetoresistive manganites La_2-2xSr_1+2xMn₂O₇, with x=0.40 and 0.44. We show that the polarons exhibit short-range incommensurate correlations that grow with decreasing temperature, but disappear abruptly at the combined ferromagnetic and metal-insulator transition in the x=0.40 system because of the sudden charge delocalization, while persisting at low temperature in the antiferromagnetic x=0.44 system. The "melting" of the polaron ordering as we cool through T_C occurs with the collapse of the polaron scattering itself in the x=0.40 system. The polaron order is characterized by an ordering wave vector q=(0.3,0,1) that is almost independent of x for x≥0.40, and is consistent with a model of disordered stripes.

We describe a search for renormalization group fixed points which control a second-order quantum phase transition between a d_x²-y²-superconductor and some other superconducting ground state. Only a few candidate fixed points are found. In the finite temperature (T) quantum-critical region of some of these fixed points, the fermion quasiparticle lifetime is very short and the spectral function has an energy width of order k_BT near the Fermi points. Under the same conditions, the thermal conductivity is infinite in the scaling limit. We thus provide simple, explicit, examples of quantum theories in two dimensions for which a purely fermionic quasiparticle description of transport is badly violated.

We performed a comparative study on the colossal negative magnetoresistivity and the Hall effect in thin films of the manganese perovskite La_1-xCa_xMnO₃ with x=0.3 and x=0.67. The underdoped sample (x=0.3) undergoes a phase transition from a paramagnetic semiconductor to a ferromagnetic quasimetal at the Curie temperature T_C=280 K, while the overdoped compound (x =0.67) stays a paramagnetic semiconductor at all temperatures. Both materials show colossal negative magneto-resistivity, albeit on considerably different temperature- and field scales, depending on the magnetic interactions between neighbouring Mn ions. According to the Hall data, the charge carriers in the underdoped material are hole-type, partially compensated by an electron-type contribution. The overdoped system shows electronic carriers with a thermally activated concentration. The ordinary Hall effect is in both compounds superimposed by an anomalous Hall contribution with a sign opposite to the intrinsic charge-carrier type.

Static and dynamical properties of Cu spins in the overdoped single-crystals of La_2-xSr_xCu_1-yZn_yO₄ have been investigated by the muon-spin-relaxation (μSR) method. In the Zn-substituted single-crystal with x=0.21 and y=0.01, where the high-T_c superconductivity is anomalously suppressed, a coexisting state of statically ordered Cu-spins and localized holes has been found at low temperatures below about 2 K. The dynamical depolarization rate of the muon spin at 3.0 K has been found to be nearly proportional to minus-root-square of the external magnetic-field applied along the initial muon-spin polarization. This field-dependence can be explained by assuming the existence of an excited state of the Cu spins diffusing in a one-dimensional direction. The present study suggests a high possibility of the existence of a new one-dimensional stripe ordered state of the Cu spins and doped holes at x=0.21 in the overdoped region of La_2-xSr_xCu_1-yZn_yO₄.

High temperature superconductors have an unusual temperature (T) — doping (x) phase diagram, in which both antiferromagnetism and superconductivity occur. The spin gap and the superconducting gap are the two essential features of the T(x) plane which evolve with doping. A material allowing investigating the T-x plane in detail, from the underdoped (x<0.15) to the overdoped (x>0.15) regime, is the La_2-xSr_xCuO₄ superconductor. The temperature dependence of the resistivity strongly depends on the substitution ratio x. Overdoped films show metallic behavior, meaning that ρ(T) is linear above 100 K, while it saturates for lower temperatures. This saturation persists below T_c, when superconductivity is suppressed by sufficiently high pulsed magnetic fields. From the slightly overdoped (0.2<x) down to the strongly underdoped regime (x~0.03) we observe, instead of saturation, a ρ(T) dependence with logarithmic divergence in the field-induced normal state at T→0. For underdoped films (x<0.1) a superlinear ρ(T) behavior develops at intermediate temperatures (50 K<T<200 K), indicating the opening of the spin gap.

High-resolution, high-energy, X-ray diffraction results are presented for the study of weak charge ordering phenomenon. By utilizing X-rays in the 100 keV region the dramatic increase in the penetration depth allows for both bulk-sensitive and high-resolution measurements to be made. The strontium doped La₂NiO₄ system is a prototypical system in the understanding of strong electron-phonon coupling, and the resultant effects on material properties. At doping levels of 1/3 and 1/2 commensurate charge modulations are observed indicating real-space charge stripes. We have measured the correlation lengths of these charge stripes using both 100 keV X-rays and 8.3 keV X-rays. In comparing our results we have observed that the charge stripes appear to be well correlated in the near-surface region with correlation lengths ξ≈2400Å. However, our bulk sensitive measurements show that the charge stripes appear in a possible stripe glass phase with a correlation length of only ξ≈300Å. Our measurements on the 3D charge order manganite system Nd_0.5Sr_0.5MnO₃ show that the charge ordering appears to be well correlated in the bulk of the sample in contrast to our nickelate results.

Recent angle-resolved photoemission (ARPES) studies of the La_2-xSr_xCuO₄ (LSCO) compound provide a new piece of information on the evolution of the electronic structure in cuprate systems with doping. Our aim is to analyze that evolution by means of the spin polaron approach based on the string picture. We study the motion of a single mobile hole in the stripe phase of a doped antiferromagnet. Holes within the stripes are taken to be static, undoped antiferromagnetic domains between hole stripes are assumed to have an alternating staggered magnetization, as it has been suggested by neutron scattering experiments. The system is described by the t-t′-t′′-J model with realistic parameters and we compute the single particle spectral density. In this paper we concentrate on the filling level 1/12. Theoretical spectra and results of measurements performed at that hole concentration show reasonable agreement. In both cases we observe, along the direction (0,0)→(π,0), "splitting" of the spectrum and formation of new low energy states around (π,0). This splitting is also observed in the direction (0,π)→ (π,π). The spectra along the direction (π,0)→ (π,π) are much broader. That hinders us in assignment of any band for some range of k points in that direction. Finally, the spectra along the direction (0,0)→ (π,π) are very broad and it is also practically impossible to distinguish any sharp band.

In the framework of the t-J_z we discuss localized spin polarons for various original hole arrangements in different spin backgrounds. According to the so-called string picture we assume that local destruction in the magnetic structure caused by hole motion tends to localize holes and acts on them as a kind of well potential. We concentrate on the case of the hole concentration 1/8 for which stripes are experimentally observed. We explicitly take into account interaction between holes and consider several possible configurations, that represent: holes homogeneously distributed in the antiferromagnetic background, hole pairs (which correspond to spin bipolarons) homogeneously distributed in the antiferromagnetic background, hole pairs which form the stripe structure in the perfect antiferromagnetic background, and eventually holes in stripes which separate antiferromagnetic domains with alternating direction of the staggered magnetization. Spin polarons are constructed by means of some computer algebra. It turns out that lowest eigenenergy has a state that represents a single hole polaron at the anti-phase domain wall (ADW). On the other hand, the stripe phase is exposed to keen competition from holes binding in the antiferromagnetic background and the energy margin by which stripes win is small.

The phase diagram, the Bethe-ansatz ground-state properties, including the chemical potential μ, the spin (magnetic) and charge susceptibilities, are calculated within the one-dimensional Hubbard model in entire range of interaction strength (-∞< U/t<+∞), magnetic field (h≥ 0) and all electron concentrations (0≤n≤1). The continuous and smooth variation of μ with n and h in the vicinity of n=1 points on the gapless character of charge excitations at U<0 and provides rigorous upper and lower bounds for μ. The spin (magnetic) susceptability χ at half-filling changes discontinuously as U→0 and is strongly enhanced by electron repulsion, comparing with that of the non-interactig case. The compressibility κ_ch increases with n at U<0 and shows non-monotonous behavior with a dramatic increase at U>0. Variations of κ_ch^-1 in both repulsive and attractive cases qualitatively well reproduces corresponding behavior of charge stiffness.

We present spectral weight mapping in the momentum space for La_2-xSr_xCuO₄ using angle-resolved photemission spectroscopy. The spectral weight of the nodal states, which shows suppression in previous results, is enhanced and shows sharp peaks in the second Brillouin zone for the overdoped sample due to matrix element effects, although they remain much weaker than the other cuprates.

The single-particle Green's function for a striped t–J model is calculated using cluster a perturbation theory, where short-distance effects are accounted for by exact cluster diagonalization and long-distance effects by perturbation (in the hopping). Both site-centered and bond-centered stripe scenarios have been studied. The data for site-centered stripes quantitatively reproduce salient (ARPES-) features.