International Journal of Modern Physics B

The marginal Fermi liquid (MFL) which describes the anomalous normal state of the cuprates is extended to a d-wave superconductive state. A phenomenological collective excitation, P₀(ω,T), is taken such that it gives rise to, after renormalization, the MFL behavior in the normal state. With the P₀, the renormalized excitation spectrum and pairing function in superconductive state are calculated self-consistently within the Eliashberg formalism. In the superconductive state, the self-energy deviates from the normal state MFL ω/T-scaling and develops a new energy scale of ω₀≈2Δ, where Δ is the pairing amplitude.

The doping dependence of the bose condensation energy is investigated by using our SU(2) holon-pair boson theory. It is shown that the bose condensation energy U results from the interplay between the charge and spin degrees of freedom and that the doping dependence of U at T=0K is well fit by the relation, with Δ₀, the spin gap at 0K; x, the hole doping concentration and α, a constant.

In a solid, transport of electricity can occur via negative electrons or via positive holes. In the normal state of superconducting materials experiments show that transport is usually dominated by dressed positive hole carriers. Instead, in the superconducting state experiments show that the supercurrent is always carried by undressed negative electron carriers. These experimental facts indicate that electron-hole asymmetry plays a fundamental role in superconductivity, as proposed by the theory of hole superconductivity.

We have examined the effective interaction of two fermions in terms of the T-matrix and shown that there are two different series of ladder diagrams in describing the effective interaction: (1) the exchange ladder diagrams giving rise to the T-matrix for the triplet state with poles for attractive interaction and (2) the Hartree ladder diagrams describing the multiple scattering on the Hartree potential and producing the T-matrix for the singlet state with poles for repulsive interaction. It is verified that a bound state of two fermions in the singlet state exists only in the presence of repulsive interaction between them. We further argue that the potential in the Schrodinger equation should be spin-dependent to describe both the singlet and triplet states.

The binding of two fermions in a degenerate Fermi gas is examined in terms of the T-matrix. We deal with effective T-matrix of two fermions in the degenerate Fermi gas in terms of the scattering amplitude. This makes approach valid for the retarded as well as non-retarded interactions. We show that for the interaction of a pair of fermions in a quiescent Fermi sea there are two different series of the ladder diagrams-Exchange and Hartree diagrams, describing a bound state and producing two different T-scattering matrices. The exchange series of the ladder diagrams corresponds to the T-matrix in the triplet-state. This T-matrix contains the pole solution corresponding to the bound state for the attractive interaction only. In contrast, the Hartree diagrams lead to the T-matrix of a pair of fermions in the singlet-state, which have a pole solution corresponding to the bound state of a pair of fermions for the repulsive interaction. Thus, we are led to assert that the bound state of a pair of fermions in the singlet-state in the quiescent Fermi sea can form for the repulsive interaction only and there is no bound state in the singlet-state from the attractive interaction.

The necessary condition of superconducting pairing with large total pair momentum is mirror nesting that is a rise of pair Fermi contour due to matching of some pieces of the Fermi contour and an isoline of the pair-relative-motion kinetic energy. Such a condition may be satisfied, at definite total pair momenta, due to a special feature of electron dispersion. In the case of perfect mirror nesting a rise of the superconducting ordering turns out to be possible up to arbitrary small value of the pairing interaction strength.

When phonons strongly mix with electron charge fluctuations with low frequencies, the phonon mediated attractive interaction between electrons is strongly enhanced. The occurrence of the mixing has been indicated by the neutron scattering experimental results that the dispersion of the in-plane Cu–O bond-stretching mode in the high T_c cuprate superconductors is strongly softened near the zone boundary. We propose that the phonon mediated attractive interaction strongly enhanced by the vibronic effect can form a basis for the phonon mechanism of high temperature superconductivity. With the Eliashberg theory and with the electronic structure determined by ARPES and the electronic dielectric function obtained by the softened dispersion of the in-pane Cu–O stretching mode, we calculated the transition temperature and the order parameter at the transition temperature. The order parameter is of the d(x²-y²) symmetry and the transition temperature is well in excess of 100 K.

A recent unification of the BCS theory with that of the Bose–Einstein condensation (BEC) through a "complete" boson-fermion model is discussed as a generalization of the "BCS-Bose crossover" picture of superconductivity. Good first-principles T_c predictions in 2D are calculated with no adjustable parameters for the so-called "exotic" cuprate superconductors of the "Uemura plot", without abandoning the phonon interaction mechanism. The only condition is that one depart moderately from the perfect electron-/hole-Cooper-pair symmetry to which BCS (as well as the "crossover") theory are restricted by construction.

Slave-boson mean-field calculation is carried out analytically for doped CuO₂ conduction planes, described by the extended Emery model which is parameterized by Cu–O charge transfer energy Δ_pd, Cu–O hopping t₀, O–O hopping t′ and Coulomb repulsion U localized on Cu site, taken as infinite. At zero doping δ, finite small t′ expands the range of stability of the covalent, conducting state, characterized by renormalized band parameters Δ_pf and t, on the expense of insulating t=0 state, which, however, remains stable at larger Δ_pd. For sufficiently large Δ_pd, Δ_pf saturates at 4|t′|. Finite doping suppresses the insulating state nearly symmetrically with respect to its sign. The regime with Δ_pf close to 4|t′| fits remarkably well ARPES spectra of Y123, Bi2212 and LSCO, and, in the latter case, explains the observed strong doping-dependence of the effective Cu–O hopping.

A new class of doped layered superconductors has been discovered recently: the metallochloronitrides. Intercalating metallic atoms (Na, Li) into the layered parent compounds MNX (M=Zr, Hf and X=Cl, Br) superconductivity was found below a transition temperature that can be as high as T_c~26 K. We show that the dominant contribution to the pairing interaction is electronic and comes from the screened dynamical Coulomb interaction characteristic of layered systems. With use of parameters for Li_0.48(THF)_yHfNCl we show that the calculated T_c is in quantitative agreement with experiment.

Xu et al. observed enhanced Nernst effect and Iguchi et al. observed patched diamagnetism, both well above T_c in underdoped high-T_c superconductors (HTSCs). A new mechanism is proposed here, which seems to naturally explain, at least qualitatively, these observations, as well as the d-wave nature and continuity of pseudogap and pairing gap, the tunneling conductance above T_c, as well as T*(x), T_ν(x), T_c(x), etc. This mechanism combines features of dynamic charged stripes, preformed pairs, and spin-bags: At appropriete doping levels, the doped holes (and perhaps also electrons) will promote the formation of anti-phase islands in short-range anti-ferromagnetic order. On the boundary of each such island reside two doped carriers; the unscreened Coulomb repulsion between them stabilizes the island's size. Superconductivity results when such "pre-formed pairs" Bose-condense.

We studied the tunnel junctions of s-wave superconductors in the presence of nonmagnetic impurities and uncondensed quasiparticles. Contrary to general belief, we found that the tunneling conductance in an SIN junction made of strong-coupling superconductors is modified by uncondensed quasiparticles. Also, the Josephson current in an SIS junction can be drastically reduced by these particles from the standard result of Ambegaokar and Baratoff. In weak-coupling superconductors, the conventional junction behaviors are restored.

We discuss Scanning Tunneling Microscopy and Spectroscopy (STM/S) measurements at very low temperatures in single crystals of the non magnetic borocarbide superconductors RNi₂B₂C (R=Y, Lu, T_c=15.5 and 16.5 K) and in MgB₂. The tunneling spectra in some regions of the surface show a clear reduction of the anisotropy of the superconducting gap.

A Bethe–Salpeter treatment of Cooper pairs (CPs) based on an ideal Fermi gas (IFG) "sea" produces unstable CPs if holes are not ignored. Stable CPs with damping emerge when the BCS ground state replaces the IFG, and are positive-energy, finite-lifetime resonances for nonzero center-of-mass momentum with a linear dispersion leading term. Bose–Einstein condensation of such pairs may thus occur in exactly two dimensions as it cannot with quadratic dispersion.

Cuprate-planes are not necessary and essential for high-temperature superconductivity: doped Sr₂YRuO₆ superconducts fully at 23 K in its SrO layers once the Ru stops librating (the material begins superconducting at ≈ 49K). GdSr₂Cu₂RuO₈ superconducts near 45 K in its SrO layers, not in its cuprate-planes. By examining the charge transfer, we find that T_c increases with the number of layers n and with pressure p in the HgBa₂Ca_n-1Cu_nO_2n+2 compounds, as does the charge in the BaO layers (and not the charge in the cuprate-planes), indicating that the superconductivity is in the BaO layers. Four superconductors, including PrBa₂Cu₃O₇, were successfully predicted to superconduct not in their cuprate-planes, but in their BaO or SrO layers. The original claims that the superconductivity resides primarily in the cuprate-planes are invalid.

Most of the proposed models of high-temperature superconductivity (HTSC) are based on the short-range electron-electron correlations or/and on a short-range electron-phonon interaction. However, in the cuprates the screening is poor due to a low carrier density, layered crystal structure, and high ionicity of the lattice. We develop further the bipolaron model of HTSC, which explicitly takes into account the long-range origin of all interactions. The long-range electron-phonon (Fröhlich) interaction binds carriers into real space pairs (small bipolarons) with surprisingly low mass but sufficient binding energy, while the long-range Coulomb repulsion keeps them from forming larger clusters. The model has explained many key features of cuprates. It is shown here that real-space pairing takes place in many cuprates at variance with some (incorrect) criteria of the BCS-BEC crossover.

By using a low energy effective Lagrangian of the U(1) slave boson mean field theory we examine the vortex binding-unbinding transition (KT transition) from a doped Mott insulator at low temperatures in a d-wave superconductor. It is shown that the interaction potential between vortices is modified by a "Doppler energy shift" resulting in additional attractive interaction, as a consequence of supercurrent fluctuations coupled to the particle-hole excitations of the massless Dirac fermions.

Within a joint experimental and theoretical study the density-density response of the prototypical linear chain compound Sr₂CuO₃ has been investigated by means of polarization and momentum dependent electron energy loss spectroscopy (EELS) and exact diagonalizations including also the continued fraction technique for the determination of spectral densities such as the optical conductivity and the loss function, respectively. It has been shown that the lowest charge excitations of this 1D charge transfer insulator cannot be described properly within the usual pdσ-model for cuprates which is based on the Cu 3d_x²-y² and the corresponding O 2pσ states, only. The lowest energy charge excitations are dominated by O 2pπ final states within the CuO₄ plaquette plane. A detailed analysis of the EELS-data allows a classification into two types of charge transfer excitations with predominant π and σ character and various sizes of internal localization and dispersion.

The electronic structure of La₂CuO₄ has been studied by first-principles cluster calculations before and after doping. Clusters containing up to five planar copper atoms were investigated using the density functional method. At variance with band-structure calculations, we have been able to determine the charge and spin distributions from molecular orbitals expressed as linear combinations of atomic orbitals localised at the nuclear sites. Doping is achieved by subtracting an electron from the cluster which, although there is a change of spin state, produces a charge distribution which is remarkably similar to the charge distribution of the peripheral charge method which does not involve changes in spin state. The peripheral charge method is designed to simulate doping by placing a carefully chosen set of point charges beyond the periphery of the cluster and relies on the supposition that the hole distribution is closely related to the charge distribution. More importantly the peripheral charge method enables charge distribution changes to be obtained for fractional changes in doping. The results show that with fixed nuclear positions, doping not only depopulates the 2p_σ orbitals but also affects other orbitals. This redistribution of charges influences the electric field gradients and hyperfine coupling parameters. The theoretical values for the field gradients are compared to values derived from experiments.

We apply a perturbation approach to study the quarter-filled extended Hubbard model at strong coupling limit. An effective Hamiltonian up to sixth order in t/U and t/V (t defines electron hopping, U defines on-site Coulomb interaction, and V defines nearest-neighbor Coulomb interaction) for one-dimensional chains was obtained. The spin-spin correlation functions were involved, which can be obtained after comparing to the ground state energy numerically obtained from the phase diagram.

All the high T_c superconductors are structurally composed of two different blocks, the perovskite and the rock salt. Based on this fact, we developed a model to calculate the interaction between the two structural blocks. But the model could not deal with the superconductors with one Cu–O plane, such as Bi₂Sr₂CuO_y (2201) phase and HgBa₂CuO_4+δ (1201) phase. Here we modify our model, dealing with HgBa₂CuO_4+δ in a different way in which the unit cell is still divided into two blocks, and the combinative energy between the blocks in HgBa₂CuO_4+δ with different δ value is calculated. It is shown that the reaction of the different blocks also closely related with the superconductivity in this compound.

We propose a microscopic model Hamiltonian to account for impurity doping induced insulator-superconductor transition and the coexistence of antiferromagnetism and superconductivity in the high-T_c cuprates. The crossover from non Fermi liquid to Fermi liquid regime characterized by delocalization of d electrons on Cu sites is discussed.

The phase diagram and thermodynamic properties of the Hubbard model in one dimension are calculated numerically within the generalized self-consistent approach over a wide range of temperature T, magnetic field h, interaction strength U/t and electron concentration n. The temperature variation of the transverse magnetization, double occupied sites and chemical potential at various n, U/t and h is also analyzed. The wave vector q for the magnetic order parameter in the vicinity of the half-filling in the metallic phase shows a crossover from incommensurate magnetic phase (0<q<π) into the state with strong antiferromagnetic correlations (q=π) as temperature is increased. Overall our numerical results in the approximate theory at finite temperatures are in good quantitative and qualitative agreement with the Bethe-ansatz results.

We propose an explanation of several experimental features related to the "pseudogap" in high T_c cuprates in terms of a spin-charge gauge theory. In this approach, based on a formal spin-charge separation applied to the t-J model, the low energy effective action describes gapful spinons (with a theoretically derived doping dependence of the gap m_s~J(δ|ln δ|)^1/2, where δ is the doping concentration) and holons with "small" Fermi surface (∊_F~tδ) interacting via a gauge field. The main effect of gauge fluctuations is to introduce a dissipation ~T/χ, where χ is the diamagnetic susceptibility. The competition between the two energy scales, m_s and T/χ, is the root in our approach of many phenomena peculiar to in-plane transport properties of the "pseudogap phase". Furthermore the gauge interaction induces binding of spinon and holon into an "electron" resonance. This binding introduces another energy scale, the recombination rate, which dominates the out-of-plane resistivity, yielding an insulating behavior. A good agreement is found between the experimental data and theoretically derived doping and temperature dependence of resistivity, both in-plane and out-of-plane, and spin relaxation.

The accurate analytical and numerical calculations of the electronic band structure and ground state properties of strongly correlated electrons within the Hubbard model are performed by constructing convergent perturbation theory for general interaction strength and electron concentration. We test the developed perturbation approach about mean field solution in the extreme conditions of one dimensionality for entire parameter space of electron interaction U/t and electron concentration n. The many-body perturbation formalism up two second order about the generalized self-consistent field (GSCF) Hamiltonian goes beyond the range of applicability of standard perturbation theory by incorporating systematically the effect of the random-phase-type perturbation techniques and controlled expansion of the energy functional for general U/t and n. The second order perturbation correction vanishes at small and large U/t limit and performed calculations of the ground state energy show a next to the perfect numerical agreement with the Bethe-ansatz results.

We model electron-doped high T_c materials using the t-t′-t′′-J model. This model is solved on a cluster with 32 sites using the method of exact diagonalization. Our purpose is to study the symmetry and asymmetry between hole- and electron-doped high T_c materials by comparing the results with those of the t-J model. In the electron-doped model with one charge carrier, we find a strong quasiparticle peak at (π,0). Compared to the t-J model, the bandwidth is larger, reflecting the fact that the charge carrier moves more freely in the electron-doped model. In the two-carrier model the ground state has robust d_x²-y² symmetry. This is in contrary to the two-hole t-J model whose ground state is a competition between low-energy states with d_x²-y² and p symmetries. The spatial distribution function of the carriers shows that they move almost freely on different sublattices.

We study the properties of photogenerated states of strongly correlated electron systems (SCES) with using the 'excited-state' t-J Hamiltonian. The lowest energy state in the two-photon excited manifold is calculated for the 20 site two-dimensional square lattice at half filling. The analysis of the off-diagonal order suggests that the d-wave superconducting state is photogenerated. We propose an optical method using the virtual transition to avoid the temperature growth arising from the optical excitation. We also investigate the transient behavior of the photoexcited state in SCES, which distinctively reflects the spin-charge separation in SCES.

We investigate the superconductivity in a two-band model by using the renormalization group approach. We discuss the superconducting gap function in the two-band model.

The thermodynamic properties: specific heat and entropy are studied as a function of temperature and doping within the two-dimensional Hubbard model with various U/t=4-12. Quantities are calculated using the finite-temperature Lanczos method with additional phase-averaging for a system of 4×4 sites. Results show that the entropy at low T reaches a maximum near half-filling at the electron density n~1±0.15 in the whole regime of studied U/t.

We study the symmetric Anderson–Holstein (AH) model at zero temperature with Wilson's numerical renormalization group (NRG) technique to study the interplay between the electron-electron and electron-phonon interactions. An improved method for calculating the phonon propagator using the NRG technique is presented and applied to the AH model as the onsite Coulomb repulsion U and electron-phonon coupling constant g are varied. As g is increased, the phonon propagator is successively renormalized, and for g≳g_co crosses over to the regime where the mode splits into two components, one of which approaches back to the bare frequency and the other develops into a soft mode. The initial renormalization of the phonon mode, as g is increased from 0, can be either positive or negative depending on U and the hybridization Δ.

¹³C pulsed nuclear magnetic resonance (NMR) measurements have been performed on LuNi₂B₂C single crystals to investigate the vortex structure and dynamical behavior. The spectrum in the superconducting state significantly broadens by local field inhomogeneity due to a vortex lattice and then evolves to double peaks at low temperature. Both peaks shift toward low magnetic field due to imperfect field penetration whereas linewidth increases because of larger field inhomogeneity. Both the linewidth, reflecting vortex fluctuation, and the transverse relaxation rate 1/T₂, probing slow motion of vortices, indicates significant thermal fluctuation of vortices in this low T_c and isotropic superconductor.

Enhancing the pinning force in high-T_c superconductors can be achieved by externally introduced periodic magnetic dots. We have numerically calculated the interaction between the magnetic dot and the vortex in high-T_c superconductors. The London equation is used to generate two-dimensional vortex lattice. In the matching condition, we calculate the attraction force between the magnetic dots and vortices at various conditions. It is found that the magnitude of the pinning from the dots is compatible with the intrinsic pinning force of YBCO thin film materials.

We have studied effects of microscopic disorder (normal-state resistivity ρ_n) and dimensionality (film thickness t) on the vortex phase diagram at low temperature T on the basis of measurements of DC and AC complex resistivities for amorphous (a)-Mo_xSi_1-x films with different ρ_n and t. For thick films (t=100 nm) we have commonly observed the vortex-glass transition (VGT) down to low T~0.05T_c0 and high fields up to B~0.9B_c2(0), where T_c0 and B_c2(0) are the mean-field transition temperature and upper critical field at T=0, respectively. In the limit T=0, the VGT line B_g(T) is independent of T and extrapolates to a field below B_c2(0), indicative of the presence of a quantum-vortex-liquid (QVL) phase at T=0 in the regime B_g(0)<B<B_c2(0). We find that the width of the T=0 QVL phase, [B_c2(0)-B_g(0)]/B_c2(0), grows as the film becomes more resistive. This result is consistent with the view that the QVL phase is driven by strong quantum fluctuations, which are enhanced with increasing disorder. In the mixed state of the 6-nm-thick film, both the DC resistivity and vortex relaxation time follow the activated T dependence, suggestive of two-dimensional VGT.

We visualize the vortex chains in Bi₂Sr₂CaCu₂O_8+y by Bitter decorations and differential magneto-optical observations. Bitter decorations on a sample with intentionally introduced field gradient enable us to determine the transition field from the vortex chain state to chain + lattice state. We also demonstrate using magneto-optical technique that we can control the density and the direction of vortex chains by the strength and the direction of the in-plane field. Precise determination of the spacing between vortex chains offers a unique way to quantitatively estimate the anisotropy parameter in highly anisotropic superconductors.

For type II superconductors, Josephson has shown that vortices moving with velocity v_L create an effective electric field E′= -v_L×B_V. By definition the effective electric field is gradient of the electrochemical potential, what is the quantity corresponding to voltage observed with the use of Ohmic contacts. It relates to the true electric field E via the local chemical potential μ as E′=E-∇μ/e. We argue that at low temperatures the true electric field in the bulk can be approximated by a modified Josephson relation E=(v_S-v_L)×B_V, where v_S is the condensate velocity.

The magnetic phase diagram of slightly overdoped La_2-xSr_xCuO₄ (x=0.17) is characterised by a field-induced hexagonal to square transition of the vortex lattice at low fields (~ 0.4 Tesla) [R. Gilardi et al., Phys. Rev. Lett.88, 217003 (2002)]. Here we report on a small angle neutron scattering study of the vortex lattice at higher fields, that reveals no further change of the coordination of the square vortex lattice up to 10.5 Tesla applied perpendicular to the CuO₂ planes. Moreover, it is found that the diffraction signal disappears at temperatures well below T_c, due to the melting of the vortex lattice.

We study vortex lattice structure of high temperature superconductors by using the Ginzburg–Landau model. The structure of the vortex lattice is oblique at the temperatures well below the transition temperature T_c, where the mixed s–d state is expected to have the lowest energy. Whereas, very close to T_c, the d_x²-y² wave is slightly lower in energy, and a triangular vortex lattice recovers. The coexistence and the coupling between the s- and d-waves account for the upward curvature of the upper critical field curve H_C2(T).

Experimental data on the field and time dependencies of the isothermal magnetization hysteresis for Tl₂Ba₂CaCu₂O₈ single crystals with fields applied parallel to the c-axis are reported. From the M–H measurements, we determined the first peak field (H_p), the onset field of the second peak (H_on), the second peak field (H_sp), and the irreversible field (H_irr). Using these physical parameters, we have drawn a vortex phase diagram for this system. As observed in BSCCO(2212) system, below the irreversibility line, the H–T plane is divided into three distinct phases: the vortex lattice, the entangle phase, and the quasi 2D pancake vortex states in the temperature interval 30 K≤T≤60 K.¹ From the relaxation of remanent magnetization measurements, we have obtained the flux creep activation barrier U(j) as a function of j using the method developed by Malay et al. Within the collective pinning theory, it is argued that T≤30 K, the pancake vortices are pinned individually, while for T≥42 K, the vortex state is characterized by individually pinned 3D vortex strings. At T~30 K, a crossover from 2D pancake to 3D collectively pinned state was observed and shows a smooth crossover to 3D single vortex state at higher temperatures.

Phase diagram of vortex states of high-T_c superconductors with sparse columnar defects is obtained by Monte Carlo simulations. The Bragg–Bose glass phase characterized by hexagonal Bragg peaks and the diverging tilt modulus is observed at low doses for the first time. As the density of defects increases, the melting temperature increases. When the density of defects further increases, the transition to the Bose glass phase occurs. The interstitial liquid is observed between these glass phases and the vortex liquid phase.

A novel Kosterlitz–Thouless phase and continuous melting transitions are proposed for Josephson vortex systems induced in layered superconductors by strong parallel magnetic fields. The mechanism of the phenomenon is revealed and the phase diagram with a multicritical point is presented.

We performed large-scale simulations in the current-driven three-dimensional frustrated anisotropic XY model in the presense of point-like defects with resistively-shunted junction dynamics. A moving Bragg (BrG) glass is observed applying the current in the equilibrium BrG state. The first-order phase transition from the moving BrG to the moving smectic is clarified. The effect of disorder become weak once the vortex matter is set in motion.

Maximum-Entropy (ME) analysis is applied to muon-spin-rotation (μSR) data of RBa₂Cu₃O_y (RBCO; R = Ho, Eu) vortex states. Our focus is on the prospect of antiferromagnetism (AF) associated with the vortex core. The ME transforms are fitted by two μSR signals: the vortex signal and the grain-boundary (GB) signal. Below T_c, the GB signals are well fitted by Gaussians for both EuBCO (T_c = 94 K) and underdoped HoBCO (T_c = 66 K). Below 0.5T_c, EuBCO and HoBCO vortex signals are best fitted using Lorentzians, instead of expected Gaussians. An estimate for an effective Neél temperature of an AF vortex core is about 30 K. Our AF-core search results for RBCO vortex states are discussed considering a magnetic origin of cuprate superconductivity.

In Magnet/Superconductor Nanostructures it is possible to control magnetism and conductivity at the nanoscale by using localized magnetic fields from structural (dots, stripes, wires) and topological (vortices's, skyrmions and domain walls) textures in magnet and/or superconductor subsystems of the hybrid material. We review experimental and theoretical results in the field of nanostructured magnet/superconductor systems.

We investigate the roles of flux pinning on vortex motion at low temperatures (T) in two-dimensional (2D) superconductors using two thin (4 nm) amorphous Mo_xSi_1-x films with different pinning strength. We measure the T dependence of resistance R at fields B below the critical field B_c of the 2D superconductor-insulator (SI) transition. For both films we observe the very small T-independent R at T→0 in certain field regions below B_c, suggesting quantum motion of a small number of vortices (dislocations) in the 2D vortex-glass phase in the presence of current. For the film with weaker pinning, the activation energy decreases at T<0.1 K.

We review some of the problems still affecting photoemission as a probe of high-temperature superconductivity, as well as important recent results concerning their solution. We show, in particular, some of the first important results on thin epitaxial films grown by laser ablation, which break the monopoly of cleaved BCSCO in this type of experiments. Such results, obtained on thin LSCO, may have general implications on the theory of high-temperature superconductivity.

The coexistence of magnetic and superconducting order in rare-earth-nickel-borocarbides (RNi₂B₂C with R = Tm, Er, Ho, Dy) and in Ru-2122 rutheno-cuprates (R = Eu and Gd) has been recently reported. Thermal conductivity measurements for magnetic superconductors from these families are discussed.

We have examined the isotope effect in superconductivity due to the repulsive interaction. The equation for the temperature of transition to the superconducting state is derived in terms of the temperature Bethe–Salpeter equation for a bound state of two fermions in the spin-singlet state. The effective interaction in the equation for T_c is written in terms of the effective scattering amplitude, thus providing the validity of the equation for the non-retarded interaction. The superconducting phase transition occurs due to non-retarded repulsive inter-particle interaction. In contrast, additional retarded electron-phonon interaction contributes to the pair-breaking effect. We have shown that the isotope effect coefficient α is negative in this case.

We investigated the origin of a non-linear temperature dependence of resistivity ρ_ab(T) in the a-b planes of YBCO, frequently observed in optimally doped twinned YBCO single crystals and thin films and in untwinned YBCO crystals along the chain direction. We performed the measurements of ρ_ab(T) in c-axis oriented YBCO thin films that were subjected to low temperature annealing at 120–140°C (400–420 K) in argon. Our results have shown that redistribution of oxygen at a fixed oxygen content in the chain layers of YBCO with a non-linear ρ_ab(T), leads to an increase of T_c, an increase of resistivity and a reduction of the non-linear component in ρ_ab(T).

We summarize here some of our recent results on the superconducting fluctuation effects above T_C in different low- and high-T_C superconductors, at high reduced-temperatures and magnetic fields. These results confirm our proposal that the collective behaviour of the fluctuating Cooper pairs in the short-wavelength fluctuation regime is dominated by the uncertainty principle, which imposes a limit to the shrinkage of the superconducting wave function when T increases well above T_C or H becomes of the order of H_c2(0), the upper critical magnetic field amplitude extrapolated to T = 0 K.

High-energy spin collective modes are often observed in inelastic neutron scattering experiments for high-temperature superconductors. Motivated by the recent scanning tunneling microscopy (STM) observation of the "checkerboard" pattern in the tunneling conductance in superconducting Bi₂Sr₂CaCu₂O_8+δ, we study the effects of these high-energy dynamic spin collective modes on the local density of states (LDOS). Unlike other scenarios recently proposed, we do not assume the pinning of the collective mode. The exchange coupling between the superconducting electrons and the collective modes produces high energy peaks in the LDOS. The spatial dependence of the LDOS at high resonant energy suggests a possible detection of resonant magnetic mode with the STM.

We report charge dynamics of a quantum spin-1/2 chain (SrCuO₂) over the complete Brillouin zone using the momentum tunability of inelastic X-ray scattering. Excitations at the insulating gap edge are found to be highly dispersive. Observed dispersions are consistent with charge fluctuations involving holons unique to 1-D spin-1/2 quantum systems.

The tunneling spectrum of an inhomogeneous d-wave superconductor is discussed in the framework of self-consistent slave-boson mean-field theory. Distinct from the usual BCS-type mean-field theory, an electron is now described using both fermionic and bosonic degree of freedom. We show that one can define two types of tunneling spectra in this theory, which also corresponds to two ways of calibrating the STM spectra. In good agreement with the experimental observation, we show that one type of tunneling spectrum remains inhomogeneous while the other type shows a remarkable degree of homogeneity at low energy, despite the underlying disorder. Physical implications of this result, in particular in relation to thermodynamic measurement such as heat capacity, are discussed.

The in-plane resistance of single crystal non-magnetic Lu_xY_1-xNi₂B₂C and magnetic Lu_xGd_1-xNi₂B₂C samples as a function of the magnitude and direction of the magnetic field and temperature have been measured. The superconducting upper critical field H_c2(T) is anisotropic for both. A positive curvature in H_c2(T) near T_c(x) is clearly observed only for the non-magnetic impurity. At low temperature H_c2(T) for the sample with magnetic impurities is strongly reduced. Thermopower, thermal conductivity and resistivity as a function of temperature are also reported for Lu_xGd_1-xNi₂B₂C.

The Hall effect in epitaxial YBa₂Cu₃O_7-x films in perpendicularly applied magnetic fields B from 1 to 6 T reveals a sign reversal of the Hall coefficient for B ≤ 3 T. We analyze our experimental data by using the full quantitative expressions based on the renormalized superconducting fluctuation model. We propose an approach how the vortex pinning can be considered in the Hall coefficient, based on the observation that flux pinning mainly affects the longitudinal conductivity in the low-temperature part of the superconducting transition.

We have investigated the magnetic and magnetotransport properties of monophasic double perovskites Sr₂FeMO₆(M = Mo, W). Magnetic measurements indicate that SFMO is a ferromagnet and SFWO is an antiferromagnet with T_N = 35 K at H = 5 T. Large magnetoresistance ratio (MR) of ~ 22% (H = 3 T) at room temperature (RT) was observed in the SFWO compound. However, the SFMO compound did not show any significant MR even at high fields and RT (MR~1%; H = 3 T and 300 K). The changes observed by physical measurements are supported by band structure calculations to explain the interaction between the 3d(Fe), 4d(Mo) and 5d(W) orbitals of transition metal ions and oxygen ions.

Measurements of magnetization and of resistivity under pressure on Ba_1-xSr_xVS₃ single crystals show that Sr-induced chemical pressure progressively weakens the antiferromagnetic correlations responsible for the metal-insulator transition. The antiferromagnetic ordering temperature, T_X = 30 K at x = 0, vanishes at x = 0.08. At larger x, a ferromagnetic order appears with T_C = 21.5 K at x = 0.18. The key-parameter favoring the metallic state is found to be the compression of the VS₆ octahedra and we predict a metallic state at 0 K for a V–S distance of 2.33 Å. The stabilization effect of the metallic state provided by the chemical pressure is supported by first principles band structure calculations performed by using the Vienna ab initio simulation package (VASP) within the projector augmented–wave (PAW) method in the local density approximation (LDA) + U scheme.

We a describe a measurement of the spin Knight shift (K_s) around the middle point between two vortices as a function of magnetic field (H) up to 28 T in a high-T_c superconductor TlSr₂CaCu₂O_6.8. At low temperatures, K_s increases substantially with increasing field, which indicates that the quasiparticle states with an ungapped spectrum are extended outside the vortex cores in a d-wave superconductor. The density of such states is found to be , with N₀ being the normal-state density of states and H_c2 being the upper critical field.

We investigate momentum dependence of charge excitations across the effective Mott gap in a quasi-zero dimensional (q-ZD) model cuprate Li₂CuO₂, using high resolution inelastic X-ray scattering by working near a resonance. In comparison to quasi-one dimensional (q-1D) systems such as Sr₂CuO₃ or SrCuO₂, momentum dependence is much weaker in Li₂CuO₂. Particle-hole pair excitations at the gap edge in these low dimensional Mott systems are found to be strongly dependent on the effective dimensionality (or topology) of the lattice and the strength of electron–electron interaction.

We report momentum dependence of charge excitations across the effective Mott gap in quasi-low dimensional model cuprates with different effective dimensionalities (CuGeO₃ and SrCuO₂), using high resolution inelastic X-ray scattering by working near a resonance. Particle-hole pair excitations at the gap edge in these compounds are found to be strongly dependent on the effective dimensionality of the lattice and the strength of electron–electron Coulomb interaction.

We report the first tunneling experiments performed by means of point-contact technique on the ruthenate-cuprate RuSr₂GdCu₂O₈ (Ru1212) compound. Spectroscopy has been carried out on polycrystals characterized by superconducting transition at T_C(ρ = 0) ≃ 26 K and magnetic transition at T_M ≃ 133 K. A chemically etched copper tip has been used like normal counterelectrode. Very stable N-S contacts (N = Cu, S = Ru1212) have been obtained, clearly showing dI/dV spectra in the Andreev reflection regime. Temperature behaviour of the spectra has been measured in the range 4.2 K ≤ T ≤ 30 K. The dI/dV spectra seem to indicate important depairing effects in the Ru1212 and we discuss possible explanation.

A series thin film sample of YBCO with different value of T_c was studied by high-resolution X-ray diffraction. Two different threading dislocations, the in-plane twist and the out-of-plane tilt, were studied carefully. It is found that the value of T_c is much more sensitive to the substrate normal tilt than to the in-plane twist. Dislocations with different Burgers vectors are suggested to exert different influence on the value of T_c of the YBCO thin film. The screw dislocation in the film is strongly influences the properties of the YBCO.

The self-energy analysis of the frequency-dependent infrared conductivity σ(ω) of MgB₂ in normal state is presented. Experimentally obtained σ(ω) is inverted to yield the self-energy of electrons. From the extracted self-energy, other physical properties such as the effective interaction between electrons can also be computed. We suggest from the self-energy analysis that the small electron-phonon coupling constant of MgB₂ obtained previously can be attributed to an underestimate of the plasma frequency.

μSR measurements were performed on HgBa₂Ca₄Cu₅O_y (Hg-1245) samples with T_c of 108 K. Rapid enhancement of muon-spin depolarization rate and losses of initial asymmetry were found in ZF-μSR time spectra below 60 K. These indicate enhancement of magnetic correlation between Cu spins. Moreover, muons precession signals were observed below 45 K, indicating that the Hg-1245 samples were in long range ordered state. These results reveal that antiferromagnetic order is developing in the Hg-1245 superconductor below T_c.

We discuss how superconductivity can emerge from (1)quasi–1D spin-Peierls system and (2) stripe-ordered state, with an emphasis on the role played by global antiferromagnetic fluctuations. A parallelism between the two cases based on physical and symmetry considerations is noted.

Using a high-sensitivity cavity perturbation technique (40 to 180 GHz), we have probed the angle dependent interlayer magneto-electrodynamic response within the vortex state of the extreme two-dimensional organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂. A previously reported Josephson plasma resonance [M. Mola et al., Phys. Rev.B62, 5965 (2000)] exhibits a dramatic re-entrant behavior for fields very close (< 1°) to alignment with the layers. In this same narrow angle range, a new resonant mode develops which appears to be associated with the non-equilibrium critical state. Fits to the angle dependence of the Josephson plasma resonance provide microscopic information concerning the superconducting phase correlation function within the vortex state. We also show that the effect of an in-plane magnetic field on the temperature dependence of the interlayer phase coherence is quite different from what we have previously observed for perpendicular magnetic fields.

We report systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) on Bi₂Sr₂Ca_n-1Cu_nO_2n+4 (n = 1-3) to study the electronic structure near the Fermi level (E_F) and the low energy excitation. Comprehensive ARPES data on the sudden break in the energy dispersion near E_F (kink) as a function of momentum, temperature, and number of CuO₂ layers indicate that the coupling of electrons with Q = (π,π) magnetic mode is dominant in the superconducting state for multi-layered cuprates.

In this contribution, the peak position of the spectral function in the ARPES spectrum has been calculated for several gap symmetries and electronic band structures. As a result, the d–wave, s + id and s* + id symmetries appear to be consistent with ARPES results. An additional comparison with previous calculations of the specific heat in normal and superconducting states as well as the single particle tunnelling current, reveals the existence of two characteristic temperatures, intrinsically associated with the Van Hove Singularity.

The explosive compaction technique has been used for manufacturing bulk superconducting components, e.g. three dimensional plates and axisymmetric billets, rods, discs and tubes, with high density and good electrical and magnetic characteristics. The high pressures and temperatures developed in a very short time result in sintering and in fracturing of the original grains, inducing primarily line defects that would provide flux pinning centers in Type II superconductors. Densified superconducting ceramic YBCO bulk components of various geometries were produced by this dynamic technique and the whole process was simulated by the explicit finite element code LS-DYNA3D. In this paper, experimental and numerical results of the fabricated superconducting bulks are reported and discussed. Applications are also briefly outlined.

We have developed a method for the reproducible fabrication of superconducting scanning tunneling microscope (STM) tips. We use these tips to form superconductor/insulator/superconductor tunnel junctions with the STM tip as one of the electrodes. We show that such junctions exhibit fluctuation dominated Josephson effects, and describe how the Josephson product I_cR_N can be inferred from the junctions' tunneling characteristics in this regime. This is first demonstrated for tunneling into Pb films, and then applied in studies of single crystals of NbSe₂. We find that in NbSe₂, I_cR_N is lower than expected, which could be attributed to the interplay between superconductivity and the coexisting charge density wave in this material.

Experimental studies of the pairing state of cuprate superconductors reveal asymmetric behaviors of the hole-doped (p-type) and electron-doped (n-type) cuprates. The pairing symmetry, pseudogap phenomenon, low-energy spin excitations and the spatial homogeneity of the superconducting order parameter appear to be non-universal among the cuprates, which may be attributed to competing orders. We propose that the non-universal pseudogap and nano-scale variations in the quasiparticle spectra may be the result of a charge nematic (CN) phase stabilized by disorder in highly two-dimensional (2D) p-type cuprates. The CN phase is accompanied by gapped spin excitations and competes with superconductivity (SC). In contrast, gapless spin excitations may be responsible for the absence of pseudogap and the presence of excess sub-gap spectral weight in the momentum-independent quasiparticle spectra of n-type cuprates. The physical implications and further verifications for these conjectures are discussed.

The temperature and field dependence of the penetration depth was determined from muon spin rotation (μ⁺SR) measurements on a single crystal of YBa₂Cu₃O₇ having a superconducting transition at T_c ≈ 91.3 K. Data were acquired at applied magnetic fields of 0.05, 1.0, 3.0, and 6.0 Tesla, yielding results inconsistent with any pairing state requiring nodes, including d-wave pairing. These data are, however, completely consistent with s-wave (or extended s-wave) superconductivity, with clear evidence of field-dependent, temperature-activated vortex pinning. Our results confirm the s-wave character originally observed in 1989, and show that the features of μ⁺SR (and microwave) data used by other authors as evidence for d-wave superconductivity are instead due to temperature- and field-dependent vortex pinning/reordering, resulting in significant distortion of the flux lattice.

We discuss the pseudogap phenomena in High-T_c superconductors. The superconducting fluctuation is taken into account as an origin of the pseudogap. Then, the T-matrix approach has been widely used. We report a development along this line. The higher order correction, which is important to capture the effects of the SC fluctuation more precisely, is investigated. It is shown that the vertex correction, which is not included in the self-consistent T-matrix approximation, enhances the pseudogap.

The orientational anisotropy of the pseudogap closing field H_pg is investigated in magnetic fields up to 60 T applied parallel (||ab) and normal (||c) to CuO₂ planes in Bi₂Sr₂CaCu₂O_8+y. We find that, in contrast to large field anisotropy related to the orbital motion of Cooper pairs below the upper critical field H_c2, the temperature-independent and small anisotropy of H_pg is due to anisotropy of the g-factor: , indicating correlations only in the spin-channel.

Using Anderson impurity model to describe the magnetic moments of Ni doped in high-T_cd-wave cuprates, the impurity state of Ni at the vortex core is studied. We find that the existence of the impurity resonance is insensitive to the vortex core although the resonant energy is sightly changed, which might help to examine the dominant mechanism of experimentally observed impurity state.

We discuss the static fluctuations of the d-wave superconducting order parameter Δ in CuO₂ planes, due to quasiparticle scattering by charged dopants. The analysis of two-particle anomalous Green functions at T = 0 permits to estimate the mean-square fluctuation δ² = <Δ²> - <Δ>², averaged in random dopant configurations, to the lowest order in doping level c. Since Δ is found to saturate with growing doping level while δ remains to grow, this can explain the collapse of T_c at overdoping. Also we consider the spatial correlations <Δ(0)Δ(R)> for order parameter in different points of the plane.

Point contact tunneling data are reported in a multilayered high-T_c cuprate (Cu,C)Ba₂Ca₃Cu₄O_12+δ with T_c = 117 K. The tunneling spectra in the superconducting state (T ≪ T_c) display spectral features such as well-defined superconducting gap peak at ±Δ as well as dip-hump structures beyond the peaks. In some cases, the spectra with two-gaps have been observed, indicating the coexistence of two inequivalent superconducting layers. The statistical distribution of superconducting gap magnitude suggests two distinct kinds of superconducting gaps that may originate from two inequivalent CuO₂ planes, a characteristics of multilayered cuprates with n ≥ 3.

We have studied the effect of Zn substitution for Cu, on the superconducting (SC) and magnetic properties of RuSr₂Eu_2-xCe_x(Cu_1-xZn_x)₂O₁₀ (Ru-1222) for x = 0, 0.01 and 0.025. Similar to all other Zn doped HTSC materials, Zn reduces T_C, from T_C = 38 to 26 K for x = 0 and 0.01 respectively. The system with x = 0.025 is not SC down to 4.2 K. On the other hand, the magnetic state of the Ru sublattice is not significantly affected by the presence or absence the SC State. Thus, the two states are practically decoupled. The wide ferromagnetic hysteresis loops which open at 5 K close themselves at elevated temperatures and the remanent magnetizations (M_rem) and the coercive fields (H_C) become zero around T_irr = 80 K. Surprisingly, at T_irr < T < T_M a reappearance of the M_rem and H_C (with a peak at 120 K) is observed for all three samples studied. We argue that the Ru-1222 system becomes (i) Anti-ferromagnetically (AFM) ordered at T_M. (ii) At T_irr < T_M, weak-ferromagnetism (W–FM) is induced by the canting of the Ru moments. (iii) At T_C (T_M/T_C ~ 4), the system becomes SC and both SC and W–FM states coexist.

Polycrystalline samples of Nd(Sr_2-yBa_y)(Cu_3-xMo_x)O_z (0 ≤ y ≤ 0.5, 0.1 ≤ x ≤ 0.3) with tetragonal structure were synthesized and investigated by powder X-ray diffraction, electrical resistivity, magnetic susceptibility and thermoelectric power measurements. Contrary to the absence of superconductivity of the Ba-free samples, superconductivity can be achieved by partially substituting Ba for Sr in semiconducting NdSr₂(Cu_3-xMo_x)O_z (0.15 ≤ x ≤ 0.3). Thermoelectric power measurements reveal that all the samples are in the underdoped state and the partial substitution of Ba for Sr results in an increase of the carrier density on the superconducting layers.

The reaction of superconducting YBa₂Cu₃O_y powder with water is studied kinetically. The initial rate law is found as where [S]_o is the initial surface area of the solid, a and b is found to be 0.94 ± 0.02 and 0.318 ± 0.002, respectively. For 100.7 mg powder with a mean radius of 0.0265 mm at pH 7.00 and 298 K, R_o is 2.21 × 10^-4mol · min^-1. Using a = 1 and b = 0.3, the rate constant, k, obtained at 283, 298 and 313 K is 0.64, 1.84 and 4.91 × 10^-3mol · min^-1 · cm^-2 · M^-0.3, respectively. The activation energy is 50.0 kJ · mol^-1 and pre-exponential factor is 1.09 × 10⁶mol · min^-1 · cm^-2· M^-0.3, as calculated from the Arrhenius equation.

Single electron transistors (SETs) has been proposed as a future basis for nanoelectric components. SETs, which utilize Coulomb blockade effect arising from the electrostatic charging energy of a single electron, are one of the promising candidates for future integrated circuits due to their extremely low power consumption and high-density integration. Here, we report on the study of the formation mechanism of a nanometer-scale Au chain on a patterned SiO₂ based wafer by controlling the reaction of raw materials to form citrate Au sol and aminosilane/dithiol treated SiO₂/Si wafer. Three gold colloidal particles (~15 nm) were aligned in a chain to form a one-dimensional current path which was bridged on a 80 nm gap between source and drain metal electrode. The SET device exhibited a Coulomb blockade effect at 40 K.

Results are presented of a local structural study of the nearest neighbor Cu–O distance distribution in underdoped but superconducting La_1.875Sr_0.125CuO₄, La_1.85Ba_0.15CuO₄ and La_1.85Sr_0.15CuO₄. Atomic pair distribution function analysis of neutron powder diffraction data has been carried out to obtain local structural information over a wide temperature range. We observe an anomalous increase in the Cu–O bond length distribution at low temperature that may be associated with the presence of an inhomogeneous charge distribution. The increase at low temperature has an onset temperature which correlates with observations of charge and spin freezing seen by other probes.

We study the effect of electron correlation in the armchair carbon nanotubes (CNT). We model the carbon nanotube as a tubule with electrons confined to the surface of the tubule by an attractive δ-function potential. We derived the pair interaction potential between two electrons in the tubule incorporating short-range and exchange correlation. Dispersions of plasmon modes at different values of orbital momentum (ℓ) and single-particle excitations are derived as well. We find that the plasma modes are not Landau damped and the lowest mode has acoustic behavior. We also find that the multiple scattering of an electron on the plasma acoustic mode leads to the so-called plasmaron quasiparticle and self-localization of an electron in the polarization well. We also show that there is a competition between superconducting correlation and self-localization due to the electron plasma interaction.

We have fabricated metal doped PrBa₂Cu₃O₇ (PBCO), i.e. PrBa₂(Cu_1-xM_x)₃O₇ (PBCMO) with M = Al, Fe, and Ni, and x = 0.05, 0.10, 0.15, and 0.20. X-ray data indicate no significant second phase for Ni-doped samples until 15% doping level is reached. No second phase was found in Al- and Fe-doped samples up to the 20% doping level. All the doped samples are in orthorhombic structure and their lattice parameters are very close to those of YBa₂Cu₃O_7-δ (YBCO). However, for Al or Fe doped samples the difference in lattice parameters a-b has decreased from that of PBCO. At 77 K the electrical resistivity for Al- or Fe-doped samples is several orders in magnitude higher than that of PBCO. Therefore these compounds may be better buffer-layer materials for YBCO superconducting electronic circuits and devices.

We have investigated physical properties of the valence fluctuating compound YbInCu₄ at high pressures and low temperatures. The first-order valence transition temperature T_V ~ 42 K at ambient pressure is completely suppressed for pressures above 2.49 GPa. Present electrical resistivity data in the pressure-stabilized high-temperature (HT) phase exhibit successive shoulders at around 20 and 2.4 K. The former can reasonably be compared with the Kondo temperature T_K ~ 25 K estimated from the static-susceptibility data at ambient pressure. For pressures above 2.27 GPa, the ac-susceptibility showed a clear peak at 2.4 K, which can be explained as the onset of a long-range ferromagnetic ordering. ⁶³Cu pure-quadrupole-resonance in the HT phase showed that the value of spin-lattice relaxation time T₁ and the almost T-independent behavior are hardly affected with pressure up to 1.3 GPa, in contrast to the large increase in quadrupole frequency.

The electrical resistivity of single crystalline HoNi₂B₂C has been measured at high pressure and magnetic fields. The three anomalies in the magnetoresistance due to metamagnetic transitions are observed both at ambient and high pressures. It is found that the metamagnetic transition fields increase with increasing pressure. The temperature dependence of electrical resistivity is strongly dependent on magnetic field. Non Fermi liquid behavior is observed near the metamagnetic transition fields. But the normal Fermi liquid behavior recovers after completing the phase transition. The Grüneisen parameters are also calculated to examine the stability of electronic state.

We investigated the transport properties of MgB₂ while changing the contents of excess Mg. The samples containing almost no excess Mg showed the highest T_c and the sharpest transition width (ΔT_c). A residual resistivity ratio (RRR) of ~ 5.8, and a magnetoresistance (MR) of 12%, at 40 K, were obtained for this stoichiometric sample. Moreover, no upturn of resistivity in a low temperature region at 10 Tesla was observed. However, the samples containing appreciable amounts of excess Mg showed quite different behaviors; the values of ΔT_c, RRR and MR were much larger. Surprisingly, big upturn appeared in this Mg-excess MgB₂

We present the first direct studies of photoinduced Cooper-pair breaking and subsequent superconducting state recovery dynamics by means of femtosecond time-resolved optical-pump terahertz probe spectroscopy focusing on the superconducting state recovery dynamics in MgB₂. The superconducting state recovery proceeds on the timescale of several hundred picoseconds and shows a strong temperature dependence. In particular, upon increasing the temperature the recovery time first decreases, reaches a minimum, followed by a quasi-divergence upon further increasing the temperature towards T_c. Moreover, the recovery time shows virtually no intensity dependence, even though the photoexcitation fluence is changed by over an order of magnitude. This suggests that pair recovery is goverend by a phonon-bottleneck mechanism, where the lifetime of high energy phonons (ℏω > 2Δ) is governed by their anharmonicity.

The (Ru_1-xNb_x)Sr₂(Eu_1.5Ce_0.5)Cu₂O_z(0 ≤ x ≤ 0.5) compounds have been synthesized and characterized by X-ray diffraction, electric transport, and magnetic susceptibility measurements. We find that there is no significant change in the superconducting transition temperature when Nb is substituted for Ru up to x = 0.5. Unlike RuSr₂(Eu_1.5Ce_0.5)Cu₂O_z, bulk Meissner effect is observed in the field-cooled magnetization measurements of the Nb doped samples. The experimental results are discussed in conjunction with a reduction of the internal field caused by the Nb substitution for Ru, based on the spontaneous vortex phase interpretation.

We report on the superconducting properties in a thick amorphous Mg_xB_1-x film, which was prepared by coevaporation of pure Mg and B. Measurements of the current-voltage characteristics and the AC complex resistivity in magnetic fields indicate the presence of the vortex-glass transition. Based on the data, we construct the possible phase diagram in the field-temperature plane. We find that there is the large vortex-liquid phase, which persists down to low temperatures.

Highly epitaxial thin films of YBCO have been obtained on silicon wafers using a Eu₂CuO₄/YSZ (yttrium-stabilized ZrO₂) double buffer. Our results showed that application of such a double buffer can significantly enhance the epitaxy of grown YBCO. It also leads to an excellent surface morphology. The average surface roughness was found less than 5 nm in a large range. The results of X-ray small angle reflection and positron spectroscpy demonstrate a very clear and flat interface between YBCO and buffer layers. The Eu₂CuO₄/YSZ double buffer could be promising for coating high-T_C superconducting films on various reactive substrates.

The discovery of superconductivity brought with it the promise of a miracle solution to many technological problems encountered by the electrical power industry. That discovery was at Leiden in 1911. Since then, engineering designs and prototypes have been developed for the use of superconductive materials in electric power transmission, transformers, and machinery. The development of superconducting magnetic energy storage systems also held great promise. Superconductivity was even used to build marine propulsion systems and levitated track vehicles. Despite that, and despite the financial support of governments for prototype developments, the only commercial application of the technology, outside of laboratories, is for MRI magnets. Similar experience is encountered in superconducting applications to electronics, although some success has been achieved in the communication industry. The discovery of high temperature superconductivity, despite its promise, did not significantly change the situation. The developments will be reviewed, and some of the reasons why superconductivity is still mainly confined to the laboratory will be given with the view of what we, as scientists, can do in order to enhance and hasten the commercial adoption of superconducting technology.

Studies on time-resolved dynamics of optically-thick YBa₂Cu₃O_7-δ microbridges biased with supercritical current pulses I of nanosecond duration and synchronously excited by femtosecond optical pulses have been carried out. The switching process is explained by the transient vortex dynamics in the superconducting stripe. It was found that the amplitude of the photoresponse increased linearly with the increase of laser fluence up to < 30 μJ/cm² and with the increase of the bias current up to ~ 1.9I_c. Calculations of quantum responsivity showed that our YBa₂Cu₃O_7-δ bridges biased with I > I_c could driven by optical pulses of comparatively weak fluence in the range of few tens of mW.

We report the effects of quantum and thermal fluctuations on the stability of phase-locking in one-dimensional long Josephson junction (LJJ) devices, involving high temperature superconductors. Accounting for both the magnetic induction effect and the charging effect, we determine the zero temperature (T = 0) stability phase diagram using renormalization group (RG) analysis. This phase diagram shows that the in-phase mode is stable but some out-of-phase modes are unstable against quantum fluctuations. At finite T, all stable phase-locked modes (at T = 0) are unstable, but its stability is maintained within the decoherence length which decreases inversely with T.

We discuss properties of hybrid systems which consist of an array of magnetic nanorods (nanowires) embedded in/or covered by a superconductor film. Such hybrid nanostructures are expected to exhibit fascinating characteristics, which are important from both fundamental and applications point of view.

The widespread use of energy-efficient, environmental-friendly and secure local power stations, or mini power plants (MPPs) has become a world-wide trend. Disperse power generation has been developed against centralized generation in central power stations/plants with a particular view on the utilization of renewable energy sources. Exceeding the stage of the development of individual superconducting power devices (SPDs), more and more concern is being focused around all superconducting (AS) integrated systems (ASIS) consisting of a set of individual SPDs being integrated into one system with a common cryogenic environment. Such a system possesses with substantial additional benefits compared to and exceeding those of the individual SPDs. The concept of AS MPP model of the order of 10 kW developed by the author's team is presented in this paper.

We have determined the spin polarized density of states (DOS) of 3-d amorphous Gd_xSi_1-x in the quantum critical regime of a tunable Metal-Insulator Transition (MIT). Using a spin polarized BCS DOS we fit the data and extract the spin polarization P.

X-ray Absorption Fine Structure (XAFS) measurements of the colossal magnetoresistance (CMR) sample La_0.79Ca_0.21MnO₃ at high fields indicate a decrease in the width parameter of the pair distribution function, σ, as the applied magnetic field is increased for T near T_c. The change in σ² from the disordered polaron state varies approximately exponentially with magnetization irrespective of whether the sample magnetization was achieved through a change in temperature or the application of an external magnetic field. This suggests a more universal relationship between local structure and the sample magnetization than was previously indicated.

Superlattices consisting of ferromagnetic SrRuO₃ and superconducting YBa₂Cu₃O₇ [YBCO] layers of a thickness between 5 nm and 60 nm have been analyzed with respect to their magnetic [T_mag] as well superconducting [T_C] ordering temperature. The reduction of both is compared with previous results obtained in YBCO/La_2/3Ca_1/3MnO₃ superlattices. Whereas the reduction of T_C is comparable for both ferromagnetic materials the suppression of ferromagnetism is more pronounced in the case of the SrRuO₃-based superlattices. This result is discussed in terms of a competition of the order parameters giving rise to ferromagnetism and superconductivity and the role of interface roughness.

Ferroelecromagnet Pb(Fe_1/3Nb_2/3)O₃ has been synthesized and its crystal structure and dielectric, electrostrictive and magnetic properties have been investigated. The structure of PFN ceramics examined by X-ray diffraction was found perovskite-like. The dielectric constant and dielectric loss as a function of temperature at different frequencies was measured. A para-ferroelectric transition at a temperature of 403 K was found and the maximum dielectric constant at a frequency of 500 Hz at the transition temperature is as high as 9050. The piezoelectric strain at room temperature was examined. The magnetization measurements as a function of temperature in the temperature region from room temperature to 648 K with different applied magnetic fields revealed that the compound is antiferromagnetic.

Lead magnesium niobate-lead titanate (PMN-PT) is an intriguing candidate for applications in many electronic devices such as multi-layer capacitors, electro-mechanical transducers etc. because of its high dielectric constant, low dielectric loss and high strain near the Curie temperature. As an extension of our previous work on Ta-doped PMNT-PT aimed at optimizing the performance and reducing the cost, this paper focuses on the effect of Pb volatilization on the dielectric properties of 0.77Pb(Mg_1/3(Nb_0.9Ta_0.1)_2/3)O₃-0.23PbTiO₃. The dielectric constant and loss of the samples are measured at different frequencies and different temperatures. The phase purity of this compound is determined by X-ray diffraction pattern. It is found that the volatilization during sintering does influence the phase formation and dielectric properties. The best condition is sintering with 0.5 g extra PbO around a 4 g PMNT-PT sample.

Valence state of the doped Ru at the Mn sites in Pr_0.5Sr_0.5MnO₃ has been studied using Ru K-edge X-ray absorption near-edge structure (XANES) spectroscopy. In comparison with XANES of reference ruthenates, it is found that the Mn-site doped Ru is dominantly in tetravalent state. This result suggests that the change of charge carrier density by Ru doping is not sufficient to understand drastic enhancement of ferromagnetism observed in the transport properties of Ru-doped Pr_0.5Sr_0.5MnO₃. The prime role of Ru in Mn³⁺–O–Mn⁴⁺ networks is discussed in terms of the valency effect and the magnetic interactions between doped Ru and Mn ions.