Exact diagonalizations of two-dimensional $t-J$ clusters reveal dominant hole-hole correlations at distance √2 (i.e., between holes on next-nearest-neighbor sites). A singlet $d_{x^2-y^2}$ pair operator is proposed to account for the spatial extension of the two-hole-bound pair beyond nearest-neighbor sites.

We discuss the reproducible dependence of tunneling conductance on voltage in Nb/${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$ (single-crystal) point-contact junctions. The large and symmetric deviations from the background conductance observed at voltages greater than the gap are interpreted in the framework of inelastic electron tunneling at the surface of the high-temperature superconductor. The peaks of the odd second derivative of the tunneling current (${\mathit{d}}^2$I/${\mathit{dV}}^2$${)}_{\mathrm{odd}}$, shifted of the sum of Nb and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$ energy gaps, show a good correspondence with the peaks of the generalized phonon density of states G(ω) determined from neutron-scattering data. The amplitude of the relative deviations of the conductance from the background is explained, as a first approximation, by means of inelastic tunneling at the surface of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$ assuming a large electron-phonon coupling strength λ for selected phonon modes and, simultaneously, a small potential barrier U and a small thickness δ of the region where the inelastic process occurs.

Magnetic-hysteresis curves of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ single crystals are found to exhibit scaling features. The scaling parameters, the full penetration field ${\mathit{H}}_{\mathit{p}}$, and the associated magnetization ${\mathit{M}}_{\mathit{p}}$, are determined experimentally as the merging point between the virgin magnetic curve and the M-H loop envelope. Scaling is performed by dividing M and H by ${\mathit{M}}_{\mathit{p}}$ and ${\mathit{H}}_{\mathit{p}}$, respectively. For magnetic fields higher than ${\mathit{H}}_{\mathit{p}}$, the M-H loops, once scaled, lie on a unique envelope, independent of temperature up to the irreversibility line. This result suggests that there is no evidence of any phase transition for H>${\mathit{H}}_{\mathit{p}}$. This scaling has been performed on the data obtained for two ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ single crystals, one oxygenated under atmospheric pressure and the other one under high oxygen pressure. The results have shown that pinning in the latter crystal is more effective at intermediate magnetic fields, H∼${\mathit{H}}_{\mathit{p}}$, but less effective at fields H significantly higher than ${\mathit{H}}_{\mathit{p}}$.

The I-V characteristics of the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconductive films are measured over wide dynamic ranges. The analysis is based on the facts that (a) the critical temperature ${\mathit{T}}_{\mathit{c}}$ varies with the current, and (b) the resistivity near ${\mathit{T}}_{\mathit{c}}$ follows a power-law dependence on the temperature difference from ${\mathit{T}}_{\mathit{c}}$. The data are found to be scaled very well near ${\mathit{T}}_{\mathit{c}}$. The exact form of the scaling function, which can also be fitted to most of the previously published results, is proposed.

We have successfully synthesized the superconductor (${\mathrm{Sr}}_{1.5}$${\mathrm{Bi}}_{0.5}$)(${\mathrm{Y}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$)${\mathrm{Cu}}_2$${\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }}$, the crystal structure of which is similar to that of ${\mathit{T}}^{\mathrm{*}}$-phase (Nd,Sr,Ce${)}_2$${\mathrm{CuO}}_4$. This structure is also believed to be without the double Bi-O layer as in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$(L,Ce${)}_2$${\mathrm{Cu}}_2$${\mathrm{O}}_{\mathit{y}}$, and the coupling between layers in (Sr,Bi${)}_2$(Y,Ce${)}_2$${\mathrm{Cu}}_2$${\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }}$ is stronger than in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$(L,Ce${)}_2$${\mathrm{Cu}}_2$${\mathrm{O}}_{\mathit{y}}$. The transition temperature is actually increased by about 8 K. This result seems to be different from that in Tl-based superconductors. It is also found that the ionic radius of the A site in the Cu perovskite unit A${\mathrm{CuO}}_{3\mathrm{-}\mathrm{\delta }}$ is critical to the formation of the T, T’, ${\mathit{T}}^{\mathrm{*}}$ phases.

We have performed dc magnetization studies in the c-axis-oriented bulk ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$ compound. The penetration depth ${\lambda }_{\mathit{a}\mathit{b}}$(T) is extracted from reversible magnetization data M(H) taking into account fluctuations of the vortices. An ‘‘irreversibility line’’ is quantitatively compared with predictions of flux-line-lattice (FLL) melting models. The results strongly suggest that the ‘‘irreversibility line’’ near ${\mathit{T}}_{\mathit{c}}$ is associated with melting of FLL caused by thermal fluctuations.

Magnetic and calorimetric measurements reveal antiferromagnetic ordering of ${\mathrm{Gd}}^{3+}$ ions in the tetragonal compounds ${\mathrm{TlSr}}_2$${\mathrm{GdCu}}_2$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and (${\mathrm{Pb}}_{0.5}$${\mathrm{Cu}}_{0.5}$)${\mathrm{Sr}}_2$${\mathrm{GdCu}}_2$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, as well as in the vacuum-annealed orthorhombic compound ${\mathrm{Pb}}_2$${\mathrm{Sr}}_2$${\mathrm{GdCu}}_3$${\mathrm{O}}_{8+\mathit{y}}$ (y≃0). The Néel temperatures ${\mathit{T}}_{\mathit{N}}$(Gd) of 2.3–2.4 K of these semiconducting compounds are almost identical to those for several other high-${\mathit{T}}_{\mathit{c}}$ Gd cuprates, with similar double-${\mathrm{CuO}}_2$-layer structures. Correlation of ${\mathit{T}}_{\mathit{N}}$ with the Gd-Gd distance in various Gd cuprate systems indicates a highly anisotropic magnetic coupling. By taking into account another common feature in terms of a broad Schottky-like specific heat below ${\mathit{T}}_{\mathit{N}}$, the expected magnetic entropy of ln(2J+1)=R ln8 can be obtained.

For over ten years the existence of a ferroelectric phase transition in ${\mathrm{K}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$${\mathrm{TaO}}_3$ has been the object of a strong controversy despite early Raman results which revealed a splitting of the soft ${\mathrm{TO}}_1$ mode at low temperature. We report here the results of an inelastic-neutron-scattering study of a 3.5 at. % Li crystal which show, for q≠0, a corresponding splitting of the TA branch between a-polarized and c-polarized phonons and no condensation of the TA mode. The phonon assignment is based on measurements made in the presence of a bias electric field. These results confirm the existence of a cubic-to-tetragonal transition in ${\mathrm{K}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$${\mathrm{TaO}}_3$ even for relatively low concentrations. They reveal the order-disorder nature of this transition and the strong coupling of c-polarized TA phonons to the spontaneous polarization. A strong anisotropy of the dispersion surface is also shown to explain the asymmetric shape of the phonon peaks.

The Ca 2${\mathit{p}}^{\mathrm{-}1}$→3${\mathit{s}}^{\mathrm{-}1}$ fluorescence transition in ${\mathrm{CaF}}_2$ has been measured as a function of excitation energy in the threshold region of the 2${\mathit{p}}^{\mathrm{-}1}$3d absorption. The exchange splitting of the final Ca 3${\mathit{s}}^{\mathrm{-}1}$3d state, reached after populating intermediate atomiclike 2${\mathit{p}}^{\mathrm{-}1}$3d states, could be determined, together with the differential 3d electron screening, comparing a 2p and a 3s hole. It is demonstrated that the 2${\mathit{p}}_{1/2}^{\mathrm{-}1}$3d→2${\mathit{p}}_{3/2}^{\mathrm{-}1}$ Coster-Kronig transition is energetically allowed, when the excited state is of ${\mathit{t}}_{2\mathit{g}}$ symmetry. Additionally we obtain direct spectroscopic evidence that the 2${\mathit{p}}^{\mathrm{-}1}$3d states are populated in non-spin-conserving transitions (ΔS≠0).

The differential thermal analysis of a single crystal of pyridinium tetrafluoroborate was performed revealing two solid-solid phase transitions at 205 and 240 K. The real part of the dielectric permittivity was measured between 190 and 290 K. The large anomaly at the higher phase transition is interpreted as the Curie point of a new ferroelectric crystal.

We report the observation of the time evolution of incoherent resonant scattering from the 14.4-keV level of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ excited with synchrotron radiation. The measured time spectra are consistent with a model with two components. In this model, one component has a significantly enhanced decay rate and is the result of recoilfree excitation. The other component has the natural decay rate for an isolated nucleus and results from excitation with recoil.

The nonlinear dynamics of optical pulse propagation is considered in the spectral region above the resonance frequency of a longitudinal exciton (${\mathrm{\omega }}_{\mathit{L}}$) where two different polaritons, UP and LP, propagate through a crystal at the same frequency. The Kerr nonlinearity leads to interaction between UP and LP. It is shown that a UP wave packet can propagate through the crystal as a soliton and provide a ‘‘potential’’ within which the LP is trapped. Numerical calculations for CuCl crystal, in the spectral region of the lowest exciton resonance, have been performed to examine the conditions under which this trapping effect can take place.

We present evidence for a new type of giant magnetoresistance in (111) cobalt-copper superlattices with atomically smooth interfaces. We propose that the lowered dimensionality of the structure leads to an enhancement of the scattering of conduction electrons from paramagnetic interfaces obeying a Langevin-like saturation at very high fields, well beyond the switching field of the Co layers. The findings help to explain similarities in magnetotransport behavior with recently reported granular systems as well as differences with antiferromagnetically coupled multilayers.

High-resolution analytical electron microscopy (EM) on as-deposited dilute Fe(Co)-Ag films showed Fe(Co) clusters embedded in a Ag matrix suggesting that these materials are not uniform substitutional alloys as it was previously believed. The cluster sizes observed from the EM images are slightly larger than those calculated from magnetization curves, and this has been attributed to the interface roughness existing between the magnetic clusters and the nonmagnetic matrix. The magnetic properties and magnetoresistance values in these films are also discussed relative to the high-resolution electron-microscopy results.

We are developing a superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ analog of the rf SQUID. A thin partition with a micrometric orifice is placed inside a toroidal container filled with liquid helium; ac and dc superflows through the orifice are induced by rotating the whole torus, which is the inertial member of a torsional oscillator. The torus oscillation amplitude vs drive curve shows a small but reproducible staircase pattern, which is detectable between 1.4 and 2.05 K, but is not modulated by a superimposed steady rotation. We discuss this behavior in terms of the occurrence of multiple phase slips.

We propose a possible new superconducting state between the metallic and Wigner solid phases for the repulsive two-dimensional electron gas. This is demonstrated using a statistical transmutation followed by average flux and random phase approximations. The superconductivity is related to the spin-unpolarized half-filled quantum Hall state, and is a fermion analogy of anyon superconductivity.

We derive a wave function for a single hole in the t-J model that is an approximate solution to the Schrödinger equation to all orders in the number of excited spin waves in the quasiparticle, for arbitrary values of t/J. The self-consistent Green-function equation used by Schmitt-Rink, Varma, and Ruckenstein and Kane, Lee, and Read arises as a consistency condition for the existence of a solution to the Schrödinger equation. The approximation provides a wave function for the strong-polaron problem due to electron-phonon coupling as well. The wave function predicts the same dipolar variation of the average staggered spin deviation at large distances, at the band minimum, as does the semiclassical wave function of Shraiman and Siggia, but the quasiparticle state is not orthogonal to the bare-hole state (${\mathit{z}}_{\mathit{k}}$≠0).

We present specific-heat C measurements of high-purity polycrystals of ${\mathrm{U}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Th}}_{\mathit{x}}$${\mathrm{Be}}_{13}$ for 0≤x≤0.052 down to 90 mK. The low-temperature data reveal extremely large residual linear contributions to the specific heat of these superconductors. In addition, we observe a change in C(T) for T<${\mathit{T}}_{\mathit{c}}$ where two transitions emerge with thorium doping x.

We discuss the crossover from the quantum critical, z=1, to the quantum disordered regime in high-${\mathit{T}}_{\mathit{c}}$ materials in relation to the experimental data on the nuclear relaxation, bulk susceptibility, and inelastic neutron scattering. In our scenario, the spin excitations develop a gap Δ∼1/ξ well above ${\mathit{T}}_{\mathit{c}}$, which is supplemented by the quasiparticle gap below ${\mathit{T}}_{\mathit{c}}$. The above experiments yield consistent estimates for the value of the spin gap, which increases as the correlation length decreases.

We present magnetization curves for Bi-Sr-Ca-Cu-O crystals which exhibit an anomalous second peak. Between 20 and 40 K the peak disappears gradually with time. At lower temperatures (18 K) the peak is absent in the short-time limit and it is gradually built up with time. A smooth universal function relates the magnetization at the peak (400 Oe) with that at the minimum (200 Oe) for all isotherms and at any given time, thus demonstrating the absence of the anomalous peak in the short-time limit at all isotherms.