We have measured in-plane resistivity and magnetoresistance of insulating ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ (x∼0.63) crystals in magnetic fields up to 8 T. Mott variable-range-hopping transport ρ=${\mathrm{\rho }}_0$ exp{(${\mathit{T}}_0$/T${)}^{\mathrm{\alpha }}$} has been observed with α=1/4 at low temperatures suggesting the metal-insulator transition caused by Pr doping is due to carrier localization. The magnetoresistance in the Mott variable-range-hopping regime is found to be positive and anisotropic, which may be caused by a decrease of the localiztion length induced by the magnetic field.

We report on the spectral investigation of the time-resolved photoluminescence in insulating ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.4}$ (YBCO) thin films and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{z}}$${\mathrm{Lu}}_{\mathit{z}}$${\mathrm{Cu}}_2$${\mathrm{O}}_8$ (BSCLCO) single crystals. The characteristic decay times of the photoexcited states in YBCO are two orders of magnitude longer than in BSCLCO, and have a different dependence upon the emission wavelength. These observations provide insight into the static photoinduced phenomena recently found in YBCO.

In this paper we studied the flux motion in a high-${\mathit{T}}_{\mathit{c}}$ superconductor. For both field-cooled and zero-field-cooled cases, we numerically solved the flux motion equations and calculated various magnetizations M(t) changing with time t. The scaling behavior of the magnetization is obtained. There is a kink in the magnetization curve that corresponds to the flux front reaching the middle of the slab sample. The self-organized critical state (SOC) is related to the early time of the flux penetration and the effect of temperature on the SOC is discussed.

In a recent paper [Phys. Rev. B 47, 3005 (1993)], Carini et al. have reported on low-frequency Raman-scattering measurements made on samarium phosphate glasses. They analyze their data with the model of Martin and Brenig, which has been (and continues to be) traditionally used to account for the universal so-called ‘‘boson peak’’ dominating low-frequency Raman spectra of glasses. The aim of this Comment is to recall important work published in the past years, which has shown that the model of Martin and Brenig is not generally applicable to explain low-frequency Raman scattering in glasses.

Li and Torr [Phys. Rev. B 46, 5489 (1992)] have developed theoretical models to estimate the magnitude of gravitomagnetic effects in superconductors. The magnitude deduced, however, is grossly overestimated, because the authors have misinterpreted the meaning of the magnetic permeability, μ, as it is commonly used by experimentalists describing the Meissner effect in bulk superconducting samples. The authors have also proposed flawed models of the microscopic dynamics of superconductors to support their error. Ambiguity in the common usage of the magnetization field (M), magnetic susceptibility (χ), and permeability (μ), to describe the macroscopic and microscopic behavior of superconductors is partly to blame; a simple heuristic model is presented here to clarify the meaning of these terms.

The Auger-decay processes in solid ${\mathrm{SiO}}_2$ after resonant and off-resonant excitations around the Si 1s edge have been investigated using synchrotron radiation. It was found that the Si ${\mathit{KL}}_{2,3}$${\mathit{L}}_{2,3}$ Auger line splits into two peaks when the photon energy is close to that of the Si 1s→3p resonance. These two peaks are ascribed to the normal and resonant Auger-decay processes. It was also observed that the resonant Auger peak is shifted to higher energy in proportion to the photon energy. The resonant Auger peak is attributed to the spectator Auger decay following the Si 1s→3p excitation. The energy shift of the resonant Auger electron is interpreted in terms of the energy transfer from the excited electron in the conduction band to the Auger electron in the course of the Auger-decay process.

We report results of time-resolved photoinduced absorption experiments on oriented poly(2-methoxy-1,4-phenylene vinylene) films using probe energies from 0.2 to 2.3 eV. We observe two photoinduced absorption bands centered near 0.5 and 1.5 eV which form within our time resolution (<1 ps) and decay with a fast (ps) and a slow (ns) component. The relaxation dynamics for the two bands are identical, indicating they are from the same excited species. On the basis of the spectra, the dynamics, and the pump intensity and polarization dependences, we conclude that these absorption features are due to bound polaron pairs and not to singlet or triplet excitons as previously suggested.

We show that frequency-down conversion and elastic scattering treated as quasidiffusion can explain quantitatively the recent experimental results by Shields et al. and Monte Carlo simulations by Tamura. The ballistic component is demonstrated to be negligible in forming the exponential decay of the phonon signal observed experimentally. The decay is governed by the special bottleneck at some frequency ω¯ which depends upon the boundary conditions and belongs to the quasidiffusive region of the phonon spectrum. The bottleneck frequency separates the phonons which decay (ω>ω¯) and those which diffuse to the detector (ω<ω¯).

A mechanism resulting in biquadratic interlayer coupling is proposed and analyzed theoretically. This mechanism is connected with the magnetic-dipole field, created by magnetic layers with roughness. This field decays exponentially with the distance from the layer, but it shows oscillating behavior in the lateral direction. The scale of both exponential and oscillating dependencies corresponds to the scale of the interface roughness. The oscillating variation of the field makes 90° alignment of the magnetization energetically favorable in analogy to the Slonczewski mechanism. Computer simulations and estimates show that this mechanism can provide a coupling strength of the order of 0.01 erg/${\mathrm{cm}}^2$ for Fe films with 1 nm interlayer thickness.

${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer measurements show that the ${\mathit{T}}_{\mathit{c}}$ of 〈12〉-nm particles of ${\mathrm{MnFe}}_2$${\mathrm{O}}_4$ is higher than that of the 〈33〉-nm particles, which exhibit the bulk ${\mathit{T}}_{\mathit{c}}$ value (573 K). The ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ isomer shift as well as the internal field are, however, independent of the particle size, as is the cation distribution determined by Mn and Fe extended x-ray absorption fine structure and Mn x-ray absorption near-edge structure measurements at different temperatures. The results confirm that the higher ${\mathit{T}}_{\mathit{c}}$ of the 〈12〉-nm particles arises from finite-size scaling.

We have carried out dynamic simulations for Ising spin glasses on simple cubic lattices in dimension 3 with five different sets of near-neighbor interaction distributions. We find that the dynamic exponent in the power-law relaxation of the autocorrelation function at the ordering temperature varies considerably with the distribution chosen. This indicates that the standard universality rules do not appear to hold for these systems.

The Shubnikov–de Haas (SdH) measurements in the quasi-two-dimensional (2D) conductor (BEDT-TTF${)}_2$TlHg(SCN${)}_4$ have been performed to clarify the ground state. The SdH oscillations with frequencies of 663 and 4270 T are observed for H?${\mathit{b}}^{\mathrm{*}}$ above 10 T, which come from the closed orbit of the 2D Fermi surface and the magnetic-breakdown orbit, respectively. Below 3 T, three SdH oscillations with frequencies lower than 13 T are found. These low-frequency oscillations are interpreted to correspond to the small closed orbits caused by the nearly perfect nesting of the 1D Fermi surface. The results indicate that the ground state is a spin-density-wave state at low magnetic fields, but the nesting is removed by high magnetic fields. Two of the three low frequencies depend on temperature below 0.6 K. The observation shows that the Fermi surfaces change as temperature varies.

The spin-fluctuation spectrum of an extended t-J model appropriate to bilayer Y-Bu-Cu-O is investigated within a renormalized-mean-field approach. Strong antiferromagentic correlation between bilayers is found for realistic values of the interplane coupling in a regime where the in-plane susceptibility is strongly exchange enhanced even though the antiferromagentic correlation length is short. The influence of spin pairing on the spin-fluctuation spectrum, in this case via a d-wave order parameter, is included. The model predicts a strong energy dependence of the modulation in the vicinity of the characteristic energy for spin fluctuations.

We calculate corrections to the BCS gap equation caused by the interaction of electrons with the collective phase and amplitude modes in the superconducting state. This feedback reduces the BCS gap parameter Δ and leaves the critical temperature ${\mathit{T}}_{\mathit{c}}$ unchanged. The feedback effect is proportional to (Δ/${\mathrm{\varepsilon }}_{\mathit{F}}$${)}^2$, where ${\mathrm{\varepsilon }}_{\mathit{F}}$ is the Fermi energy. This is a negligible correction for type-I superconductors. However, in type-II superconductors the feedback effect is greatly enhanced due to smaller Fermi velocities ${\mathit{v}}_{\mathit{F}}$, and may be responsible for effects seen in recent experimental data on organic superconductors.

Sound velocities along high- and low-symmetry directions are measured with the goal of determining the full set of elastic constants in ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$. The velocity of quasitransverse mode (${\mathit{v}}_{\mathit{t}}$) with wave vectors q lying in the {010} plane shows anomalous behavior. Rotating q from the [100] to the [001] direction we registered, first, a strong lowering, and, then, a steplike increase (up to two times) of ${\mathit{v}}_{\mathit{t}}$. There is no reasonable explanation of the observed peculiarities in the framework of conventional elasticity theory and the effect can be accounted for by the existence of a two-dimensional soundlike collective mode of electronic origin that strongly interacts with lattice vibrations. A proposed phenomenological model of the effect is in qualitative agreement with the experimental data.

The spin and electronic spectra of the two-dimensional Hubbard model doped away from half-filling (n≃0.9) are calculated in a conserving fluctuation-exchange approximation for coupling U/t up to 7 and temperature down to 30 K. We find neither a spin-density wave nor a superconducting instability. The energy of the overdamped collective spin mode decreases as T decreases and U/t increases. The Wiedemann-Franz ratio of the thermal and electrical conductivities becomes smaller than the Sommerfeld value. We make comparison with experimental data on cuprates.

We derive rapidly convergent expressions for the Coulomb component of the pressure tensor of single-charge, partial-charge molecular, and point-dipole lattices in the Ewald formulation for both bulk and surface geometries. In the case of the pressure tensor, a general procedure for generating the series expansions is described. Some of these expressions are simple enough to be suitable for incorporation in molecular-dynamics and Monte Carlo molecular simulation computer programs covering a range of specific polar condensed phases. The surface-geometry formulas are more complicated than the corresponding bulk expressions because of the reduced symmetry.