We argue that the natural variable to establish the crossover from Cooper-pair-based superconductivity to Bose-Einstein condensation of bound-electron pairs is the product ${\mathit{k}}_{\mathit{F}}$ξ of Fermi electron wave vector times the coherence length for two-electron correlation, and that Cooper-pair-based superconductivity is stable against bosonization down to ${\mathit{k}}_{\mathit{F}}$ξ≃2π. We also propose that the experimental plot by Uemura et al., relating the superconducting temperature ${\mathit{T}}_{\mathit{c}}$ to the Fermi temperature ${\mathit{T}}_{\mathit{F}}$ for a variety of ‘‘exotic’’ as well as conventional superconductors, can be rationalized by correlating the ratio ${\mathit{T}}_{\mathit{c}}$/${\mathit{T}}_{\mathit{F}}$ to ${\mathit{k}}_{\mathit{F}}$ξ. It appears in this way that the high-${\mathit{T}}_{\mathit{c}}$ superconductors lie in the plot by Uemura et al. near the ‘‘instability’’ line ${\mathit{k}}_{\mathit{F}}$ξ=2π.

We employ the Bardeen-Cooper-Schrieffer theory to calculate the frequency-dependent interlayer conductivity of a superconducting bilayer, the two layers of which are coupled by weak single-particle tunneling. The effect of the superconducting transition on the normal-state absorption band is to blue-shift and broaden it, while causing the peak absorption to be reduced and the line shape to become asymmetric.

A nonzero superfluid density is equivalent to the occurrence of a Meissner effect and therefore signals superconductivity. A recent theorem shows that in the case of a spectrum with a gap the superfluid density is equivalent to the Drude weight. This theorem is employed to compare approximate calculations of the superfluid density in the two-dimensional attractive Hubbard model using the Hartree-Fock approximation with exact diagonalization calculations of the Drude weight. Direct comparison of the approximate results with recent finite-temperature quantum Monte Carlo calculations is also made. The approximate results are found to be quantitatively accurate for all fillings, except close to half-filling.

Weak-coupling expansions (conserving approximations) are carried out for the attractive Holstein and Hubbard models (on an infinite-dimensional hypercubic lattice) that include all band-structure and vertex correction effects. Quantum fluctuations are found to renormalize transition temperatures by factors of order unity, but may be incorporated into the superconducting channel of Migdal-Eliashberg theory by renormalizing the phonon frequency and the interaction strength.

The upper critical field for coupled layered superconductors with local pairs is analyzed within the mean-field approach for the anisotropic negative-U Hubbard model. Effects of reduced dimensionality are investigated by solving the Ginzburg-Landau equations to obtain critical fields for different anisotropy ratios. The Harper’s-type system of equations is solved analytically for some special field values. Limitations of Peierls substitution are also discussed.

The study of fluctuation effects on the properties of a conventional superconductor with a strong electron-phonon interaction is continued. The correction to the specific heat of the metal above the critical point is calculated. It is proved that the phenomenological Ginzburg-Landau theory can be applied perfectly well to a system with a retarded interaction.

Aspects of magnetic-flux dynamics in different types of samples of the high-temperature superconductor ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ have been investigated in magnetic fields below 1 Oe and at 77 K. The experiments were carried out in an arrangement including a field coil, a flat sample perpendicular to the field, and a radio frequency-superconducting quantum interference device (RF-SQUID) along a common axis. For epitaxial thin films the Meissner effect was established. For a ceramic sample it was found that field changes of order ${10}^{\mathrm{-}2}$ Oe affect the flux distribution inside the sample. The observations can explain why RF-SQUID’s made from this ceramic operate in the nonhysteretic mode. The central object was an epitaxial film with a large density of defects. In this film the dynamics of the mixed state show features expected in ‘‘spin-glass’’ models.

Observations show that ${\mathit{T}}_{\mathit{c}}$ depression of the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ system resulting from disorder caused by partial replacement of Cu by metallic dopants Fe, Ni, Zn, and Ga, in small concentrations, is significantly faster than in the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ system. Another feature of ${\mathit{T}}_{\mathit{c}}$ suppression in the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ system is that the effect of Fe and Ni is much stronger in this system than that of Zn. These features are analyzed in terms of the hole occupancy of the doubly charged impurity atoms in two circumstances—first when the impurity is isolated, and the other when it is in the crystal environment. On this basis the observations support a mechanism of ${\mathit{T}}_{\mathit{c}}$ suppression wherein the moments of the impurity acquire effectively more isolated-atom-like character in the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ system and wherein the pairing interaction is also suppressed directly.

A two-band model has been applied to explain the observed behavior of the thermoelectric power of Bi-2:2:2:3. The model invokes the Cu-O planes, which contribute to the metallic conduction of holes, while Bi-O planes contribute to the semiconductorlike behavior of electrons. The results indicate a good fit to the observed data and are consistent with the recently reported scanning-tunneling-spectroscopy studies of Bi-O layers in Bi cuprates.

Magnetic and superconducting properties of single-Tl-O-layered ${\mathrm{TlSr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$${\mathit{R}}_{\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_7$ [R=Gd (x=0.6,1.0);Ho,Yb (x=1.0)] have been studied. The antiferromagnetic ordering temperature of Gd moments in nonsuperconducting ${\mathrm{TlSr}}_2$${\mathrm{GdCu}}_2$${\mathrm{O}}_7$ is 3.1 K, which is the highest among the corresponding nonsuperconducting parent cuprates. The materials based on Ho and Yb do not order magnetically down to 1.5 K. Superconductivity (${\mathit{T}}_{\mathit{c}}$=60 K) and antiferromagnetic ordering (${\mathit{T}}_{\mathit{N}}$=1.8 K) coexist in ${\mathrm{TlSr}}_2$${\mathrm{Ca}}_{0.4}$${\mathrm{Gd}}_{0.6}$${\mathrm{Cu}}_2$${\mathrm{O}}_7$. The low value of the effective magnetic moment (2.9${\mathrm{\mu }}_{\mathit{B}}$) of Yb along with the lattice constant anomaly suggest that Yb ions are in a mixed valence state in ${\mathrm{TlSr}}_2$${\mathrm{YbCu}}_2$${\mathrm{O}}_7$.

Measurements of the in-plane magnetic-field penetration depth for the n-doped high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ are reported. The measurements were performed at 4 GHz by a microstrip resonator technique on thin films obtained by laser ablation. The temperature dependence of the penetration depth is carefully analyzed and the energy gap-ratio is estimated. The results confirm that ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ behaves as a BCS-like superconductor.

The generation of coherent phonons upon photon-induced pair breaking observed by Kutt, Albrecht, and Kurz in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ may be taken as evidence of a role of static deformations in the binding of Cooper pairs, as recently proposed by the present authors. Quantitatively, the size of the lattice deformation brought on by superconductivity as observed by neutron scattering is consistent with the authors’ hypothesis.

The experimental evidence for twelvefold quasicrystals is reviewed. It is demonstrated that these structures can be explained reasonably well by a square-triangle random tiling model, contrary to the suggestion by Min and Wu in two recent publications [Phys. Rev. B 45, 10 306 (1992) and Phys. Rev. Lett. 65, 3409 (1990)].

We present measurements of the thermoelectric power S(T) from 2–300 K in the material ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{U}}_{\mathit{x}}$${\mathrm{Pd}}_3$, with x=0, 0.05, 0.1, 0.2, 0.3, and 0.4. The general behavior for x=0.05,0.1 is reminiscent of Kondo materials, characterized by a ‘‘giant,’’ positive contribution that is linear in T at low temperatures, and is independent of x. This suggests that the Kondo scale is also independent of x, in contrast to the results of other measurements. The high-temperature thermopower changes from positive to negative as x is varied from 0.1 to 0.3, suggestive of a crossover or transition in electronic structure near x=0.2. The spin-glass transition of the x=0.3,0.4 alloys is signaled by a zero crossing of S, similar to that observed for conventional spin glasses.

We study the three-dimensional quantum Ising spin glass in a transverse magnetic field following the evolution of the bond probability distribution under renormalization-group transformations. The phase diagram (critical temperature ${\mathit{T}}_{\mathit{c}}$ vs transverse field Γ) we obtain shows a finite slope near T=0, in contrast with the infinite slope for the pure case. Our results compare very well with the experimental data recently obtained for the dipolar Ising spin glass ${\mathrm{LiHo}}_{0.167}$${\mathrm{Y}}_{0.833}$${\mathrm{F}}_4$, in a transverse field. This indicates that this system is more appropriately described by a model with short-range interactions than by an equivalent Sherrington-Kirkpatrick model in a transverse field.

The one-hole spectral weight for two chains and two-dimensional lattices is studied numerically using a method of analysis of the spectral function within the Lanczos iteration scheme: the Lanczos spectral decoding method. This technique is applied to the t-${\mathit{J}}_{\mathit{z}}$ model for ${\mathit{J}}_{\mathit{z}}$→0, directly on an infinite-size lattice. By a careful investigation of the first 13 Lanczos steps and the first 26 ones for the two-dimensional and the two-chain cases, respectively, we find several interesting features of the one-hole spectral weight. A sharp incoherent peak with a clear momentum dispersion is identified, together with a second broad peak at higher energy. The spectral weight is finite up to the Nagaoka energy where it vanishes in a nonanalytic way. Thus the lowest energy of one hole in a quantum antiferromagnet is degenerate with the Nagaoka energy in the thermodynamic limit.

Recent experiments on rare-earth multilayers have led to its being important to understand in detail the phase diagrams of models with competing interactions and spin anisotropy. Therefore we study the ground state of the XY model with competing first- and second-neighbor interactions and sixfold anisotropy, D. Infinite series of long-period phases are found to be stable. Particular attention is given to the evolution between the continuous spin and the discrete spin limits as D is increased from zero to infinity.

We have measured the tunneling density of states in ultrathin high-sheet-resistance, 16 Ω<${\mathit{R}}_{\mathit{N}}$<17 kΩ, granular Pb films in perpendicular magnetic fields, H. At the lowest ${\mathit{R}}_{\mathit{N}}$, states appear within the superconducting energy gap at low fields (H<0.1 T) indicating the presence of vortex cores. At the highest ${\mathit{R}}_{\mathit{N}}$, no states appear in the gap in fields as high as 2 T indicating that the field-induced vortices do not have cores. A simple model implies that the average Josephson coupling energy between the grains of the film is comparable to the condensation energy of a single grain at the ${\mathit{R}}_{\mathit{N}}$ separating these two regimes.

We have investigated the room-temperature oxygen depletion in the near-surface region of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ samples kept under high vacuum. Results obtained from secondary-ion-mass-spectroscopy and four-probe-conductivity measurements are presented. The depletion process is limited by oxygen out diffusion in the samples with diffusion coefficients of ${10}^{\mathrm{-}18}$–${10}^{\mathrm{-}19}$ ${\mathrm{cm}}^2$/s. The reverse oxygen in-diffusion process occurs in oxygen atmosphere. The oxygen out-diffusion process can be accelerated by direct ultraviolet light (UV) irradiation of the samples in vacuum. The in-diffusion process is accelerated if UV light is used in oxygen atmosphere to produce ozone and atomic oxygen.

The electron-phonon interaction in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ is investigated, beginning with a self-consistent linear-combination-of-atomic-orbitals band calculation, and using a tight-binding parametrization to obtain derivatives of the Hamiltonian matrix with respect to atomic displacements. The phonon spectrum is calculated on the basis of the shell model. We solve the anisotropic Eliashberg equations and obtain a high ${\mathit{T}}_{\mathit{c}}$ in accord with experiment, but also a large oxygen-isotope effect. The gap function is calculated on the different sheets of the Fermi surface. It is strongly anisotropic but nodeless.