By analyzing the Korringa contribution to the EPR linewidth of ${\mathrm{Gd}}^{3+}$ ions doped to high-quality ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ samples with various Sr content (x=0.08–0.35) a dependence of the density of states N(${\mathit{E}}_{\mathit{F}}$) on x has been extracted. The analysis of the observed dependence together with that for the critical temperature ${\mathit{T}}_{\mathit{c}}$(x) enabled us to study the relation between the coupling constant λ and N(${\mathit{E}}_{\mathit{F}}$) in course of Sr doping. We found that up to the optimum Sr concentration of x=0.15 the coupling constant λ increases correspondingly to increasing of the density of states. At the same time λ rapidly decreases in the overdoped region (x>0.15) while N(${\mathit{E}}_{\mathit{F}}$) continuously grows. The possible reasons for such a behavior are discussed.

We report measurements of the ab-plane resistivity (ρ) and Hall coefficient (${\mathit{R}}_{\mathit{H}}$) of crystalline thin films of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (0.05≤δ≤0.53) from below ${\mathit{T}}_{\mathit{c}}$ up to 400 K. In contrast to measurements on sintered samples, and some other recent work on thin films, the residual resistivity of these films (as extrapolated from high temperature) remains low for all δ. As oxygen is removed from the films, ρ increases and ρ(T) develops downward curvature below 300 K. The Hall angle (${\mathit{FTHETA}}_{\mathit{H}}$) however, continues to obey the relation, cot${\mathit{FTHETA}}_{\mathit{H}}$=${\mathit{AT}}^2$+B. As δ increases, the coefficient A decreases while B remains constant. Data for an oxygen-deficient single crystal of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ agree well with our film data. Measurements of ρ in the region of the superconducting transition in fields of up to 7 T, applied parallel to the crystallographic a, b, or c axes, are also reported. The well-known ‘‘field-induced broadening’’ of the ρ(T) curves becomes larger as δ is increased. There is evidence for a transition line in the field-temperature plane that has the 4/3-power-law behavior associated with critical fluctuations in the three-dimensional XY model.

A theory of vortex pinning in high-temperature superconductors by correlated disorder in the form of twin boundaries, grain boundaries, and columnar defects is described. Mapping vortex trajectories onto boson world lines leads to a ‘‘superfluid’’ flux liquid at high temperatures, as well as low-temperature ‘‘Bose-glass’’ and ‘‘Mott-insulator’’ phases, in which the flux lines are localized. Currents perpendicular to the average vortex direction act like an electric field applied to charged bosons, while currents parallel to the field act like an imaginary magnetic field in this approach. We discuss the equilibrium and dynamic properties of these phases, and propose a scaling theory for the flux-liquid to Bose-glass transition, at which the linear resistivity vanishes. Although the Bose-glass predictions share some features with vortex-glass behavior predicted for point disorder, the response to tilting the magnetic field in the two cases differs dramatically, thus allowing the two theories to be distinguished experimentally.

A model is developed for superconducting and charge-density-wave (CDW) correlations in the bismuth oxide superconductors ${\mathrm{BaPb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Bi}}_{\mathit{x}}$${\mathrm{O}}_3$ and ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$, starting from the Hubbard model and assuming a three-dimensional tight-binding band structure. Self-consistent equations for the superconducting gap (Δ), and the CDW gap (g), and the band filling (${\mathit{N}}_{\mathit{e}}$) are derived and solved numerically. Solutions for both Δ and g are obtained for 0.29≤${\mathit{N}}_{\mathit{e}}$≤0.38 and 0.80≤${\mathit{N}}_{\mathit{e}}$≤1.00 (where ${\mathit{N}}_{\mathit{e}}$=1.00 represents half-filling). These solutions are compared against the experimental data for ${\mathrm{BaPb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Bi}}_{\mathit{x}}$${\mathrm{O}}_3$ and ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$ and qualitative agreement is obtained.

We present a theoretical study of librational modes in solid ${\mathrm{C}}_{60}$ and doped solid ${\mathrm{C}}_{60}$. General results are derived by utilizing the space-group symmetries of the solids. At the Brillouin-zone center Γ, in solid ${\mathrm{C}}_{60}$ there should be in general five distinct libron frequencies in the simple-cubic phase, and ten in the so-called 2${\mathit{a}}_0$-fcc phase; in ${\mathit{M}}_3$${\mathrm{C}}_{60}$, where M=K,Rb, a single libron frequency is expected; in ${\mathit{M}}_6$${\mathrm{C}}_{60}$, one also expects a single libron frequency. The case of ${\mathit{M}}_3$${\mathrm{C}}_{60}$ is of particular interest since in the first approximation, the potential governing the libron motion contains a single undetermined parameter, and is thus completely determined by a measurement of the libron frequency.

Angle-dispersive powder-diffraction techniques with an image-plate area detector and synchrotron radiation have been used to perform a structural refinement of mercury telluride in its cinnabar phase. We find that the structure is site-ordered with fractional coordinates of (u=0.641(1),0,1/3) and (v=0.562(1),0,5/6) for Hg and Te, respectively, at 3.6 GPa. The structure is substantially different from that of HgS cinnabar, with coordination much closer to fourfold.

We show that quantum-mechanical molecular-dynamics simulations in a finite-temperature local-density approximation based on the calculation of the electronic ground state and of the Hellmann-Feynman forces after each time step are feasible for liquid noble and transition metals. This is possible with the use of Vanderbilt-type ‘‘ultrasoft’’ pseudopotentials and efficient conjugate-gradient techniques for the determination of the electronic ground state. Results for liquid copper and vanadium are presented.

We present molecular-dynamics investigations of medium-range ordering in amorphous transition-metal–metalloid alloys based on interatomic forces derived from tight-binding-bond theory. We demonstrate that the small-angle scattering observed in Fe-rich ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{B}}_{100\mathrm{-}\mathit{x}}$ alloys (x>75) arises from B-enriched regions of approximate composition ${\mathrm{Fe}}_{75}$${\mathrm{B}}_{25}$ embedded in an Fe matrix. The formation of B-enriched regions is due to the strong covalent Fe-d–B-p bond.

Circular dichroic x-ray photoemission spectroscopy (CDXPS) experiments have been performed on the 2p core-level spectra of polycrystalline Fe films deposited directly on a ${\mathrm{Ni}}_3$Fe (μ-metal) substrate that was magnetized by a low excitation field. Because of the polycrystalline state of the iron films, CDXPS studies in the remanent mode were not possible, thus the experiments were performed with the magnetic field switched on during the experiment. Our work on this polycrystalline iron gives some insight into the understanding of the magnetic circular dichroism mechanism. The results show a similar angular variation for the measured asymmetry, as observed on an Fe bcc (100) single crystal remanently magnetized in the (100) direction.

It is shown how the critical temperature and zero-temperature energy gap decrease by shifting the position of the singularity in the density of states (DOS) with respect to the Fermi energy. The enhancement of the critical temperature due to the singularity (compared with the constant DOS near the Fermi surface) is not as large for strong-coupling superconductors as in the weak-coupling limit. Hence, the singularity in the DOS cannot be the exclusive reason for large values of critical temperatures in high-${\mathit{T}}_{\mathit{c}}$ superconductors.

We study the manifestations of superconducting current-induced bound states in the ac Josephson effect in tunnel contacts. Due to the normal elelctron backscattering by the contact, the bound states have a two-level structure, which allows direct interlevel transitions driven by an electromagnetic field introduced in the dielectric layer. We find substantial nonequilibrium suppression of the Josephson current, and nonlinear current oscillations induced by a resonant applied electromagnetic field.

Numerical simulations of the multiple fluxon dynamics stimulated by an external oscillating force applied at a boundary of a long Josephson junction are presented. The calculated IV characteristics agree well with a recent experimental observation of rf-induced satellite flux-flow steps. The voltage spacing between the satellites and the main flux-flow step corresponds to the second harmonic of the external frequency. We find that the satellite flux-flow modes are characterized by collective excitations propagating through the fluxon chain. These dynamical states can be interpreted as fluxon density waves.

We show that the thermopower of alloys with nonisovalent substitution is described by renormalization due to electron-phonon-impurity interference. The thermopower of high-${\mathit{T}}_{\mathit{c}}$ oxides as well as ordinary metallic alloys is analyzed. For Bi- and Tl-based oxides the observed temperature dependence with a maximum at 100–150 K and subsequent decrease with a sign change results from hole scattering on excess ${\mathrm{O}}^{2\mathrm{-}}$ ions. The more complicated situation in Y-Ba-Cu-O is also discussed to explain the strong concentration dependence and different signs of the thermopower along the a and b axes. The electron-phonon coupling constant evaluated from experimental data has a reasonable value: 1–2.5.

We have studied the effect of uniaxial stress ${\mathrm{\sigma }}_{11}$ along the a direction, up to 150 MPa, on the thermoelectric power S(${\mathrm{\sigma }}_{11}$,T) and electrical resistance R(${\mathrm{\sigma }}_{11}$,T) of (Bi,Pb${)}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ single crystals from the transition temperature to room temperature. Both S and R increase monotonically with increasing ${\mathrm{\sigma }}_{11}$. Above ${\mathit{T}}_{\mathit{c}}$, S(${\mathrm{\sigma }}_{11}$,T) is positive and linear with T, and ΔS(${\mathrm{\sigma }}_{11}$,T)/ΔT=-41 nV/${\mathrm{K}}^2$ for all ${\mathrm{\sigma }}_{11}$. The extrapolated S(${\mathrm{\sigma }}_{11}$,T=0) increases with ${\mathrm{\sigma }}_{11}$, with ΔS(${\mathrm{\sigma }}_{11}$,0)/Δ${\mathrm{\sigma }}_{11}$=(1.5±0.4)μV ${\mathrm{K}}^{\mathrm{-}1}$ ${\mathrm{GPa}}^{\mathrm{-}1}$. The effect of ${\mathrm{\sigma }}_{11}$ on S(${\mathrm{\sigma }}_{11}$,0) is similar to the effect of a reduction in the hole density due to annealing under different oxygen atmospheres. S(${\mathrm{\sigma }}_{11}$,T) drops sharply at the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$(${\mathrm{\sigma }}_{11}$). We discuss our results in terms of several recently proposed models.

The magnetic properties of ${\mathrm{H}}_{\mathit{y}}$${\mathrm{Sm}}_{1.93}$${\mathrm{Si}}_{0.07}$${\mathrm{CuO}}_4$ (0≤y≤0.2) have been examined using ac-susceptbility measurements. It is shown that H doping lowers the temperature of the antiferromagnetic transition of the Sm ions. This fact together with the observed hydrogen-induced changes of the temperature dependence of the resistivity suggests that the doped hydrogen plays the role of electron donor in the compound. Simultaneously, a substantial decrease of the strength of the magnetic anomaly at 12 K in χ’(T), found recently for ${\mathrm{Sm}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$, was observed by hydrogenation.

The spin-1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain has been studied by Hida [Phys. Rev. B 46, 8268 (1992)]. We point out an error in his bosonized Hamiltonian representation of this model and also show that his numerical results can be well explained by use of the corrected form of the bosonized Hamiltonian.

The appearance of new states in the inverse-photoemission spectrum of a 4×4 Hubbard cluster upon hole doping is explained by symmetry-related selection rules.

We reexamine whether or not including lattice effects in a simple model of a Kondo impurity is sufficient to explain observed field effects in metamagnetic ${\mathrm{CeRu}}_2$${\mathrm{Si}}_2$.

The matter under discussion is the effect of the softening of the elastic constants on the metamagnetic transition in ${\mathrm{CeRu}}_2$${\mathrm{Si}}_2$. In the preceding Comment it is argued that some assumptions used in our original paper are theoretically unjustified. However, keeping in mind the experimental evidence for our assumptions, our feeling is that what is needed is a more sophisticated theory.

Frequency and time-domain experiments are conducted to present additional confirmation of the slow speed of electromagnetic waves in a dense strongly scattering medium. A wide-band microwave signal is propagated through randomly distributed glass spheres, and the speed is defined as the time at which 50% of the pulse energy arrives. Thus, the speed was measured directly without the use of the transport mean free path and the diffusion coefficient. The speed of an incoherent pulse in the Mie resonance scattering region is found to be less than 40% of the vacuum speed for a fractional volume of 11% consistent with the energy velocity. Since the incoherent wave in a dense medium is the diffuse wave, the velocity presented in this paper corresponds to the velocity of the diffuse wave and it is different from the ballistic velocity. The speed of coherent waves is found to be almost constant and is quite different from the energy velocity.