Within a special kind of slave-boson mean-field approximation, which allows for the symmetry-broken states appropriate for a bipartite lattice, it is found that there is a phase transition to the antiferromagnetic insulator for the symmetric periodic Anderson model, and the critical value of the hybridization matrix element is consistent with the quantum Monte Carlo result of Jarrell, Akhlaghpour, and Pruschke [Phys. Rev. Lett. 70, 1670 (1993)]. Our results suggest that the magnetism is itinerant near the critical value of the hybridization matrix element, and it smoothly crosses over to the local-moment regime as the hybridization decreases to zero.

Low-frequency electronic Raman scattering in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ has been investigated over a wide compositional region, 0≤x≤0.34, covering the insulating, superconducting, and nonsuperconducting metallic compounds. We have found that the scattering intensity for the (xy) polarization predominates over that of the (x’y’) polarization in the low-doping region below x≤0.15, but vice versa at higher doping levels. The results indicate that anisotropy in the effective-mass tensor (or k-space dispersion) of the carriers changes systematically with hole doping, in contradiction with the simple band picture.

Raman measurements were performed on single-phase samples of ${\mathrm{Bi}}_{2.1}$${\mathrm{Sr}}_{1.9\mathrm{-}\mathit{x}}$${\mathrm{La}}_{\mathit{x}}$${\mathrm{CuO}}_{\mathit{y}}$ (0≤x≤1.0) and ${\mathrm{Bi}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{Sr}}_2$${\mathrm{CuO}}_{\mathit{y}}$ (0.05≤x≤0.6). Significant modifications of the spectra, becoming more pronounced with increasing x, are noted with respect to the undoped ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_{\mathit{y}}$ in the energy ranges of 450–465 ${\mathrm{cm}}^{\mathrm{-}1}$ and 610–650 ${\mathrm{cm}}^{\mathrm{-}1}$. The experimental results provide important data related to the origin of the incommensurate modulation structure and its relation to the superconductivity of the ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_{\mathit{y}}$ system.

In a recent paper [R. Coussement et al., Phys. Rev. B 45, 9755 (1992)] it was suggested that for a long-lived state that can undergo a Mössbauer transition, if the homogeneously broadened width, rather than the natural width, exceeds the solid-state-induced inhomogeneous width, it should be possible to observe resonant emission and absorption. On the other hand, it is expected that the probability of absorption from a spectral distribution of total width Γ should be proportional to ${\mathrm{\Gamma }}_{\gamma }$/Γ, where ${\mathrm{\Gamma }}_{\gamma }$ is the radiation width of the absorbing nucleus. Generally Γ is expected to be larger than ${\mathrm{\Gamma }}_{\gamma }$ because of the possibilities of internal conversion, transitions between magnetic sublevels, etc., all of which contribute to homogeneous broadening and should reduce the Mössbauer effect. The present Comment addresses questions of principle, mathematical assumptions, and other aspects of the work of Coussement et al. and demonstrates that a correct statistical treatment recovers the conventional more stringent requirement on the broadening.

The physical assumptions made to calculate resonant absorption for the nuclear transitions from long-lived isomeric (τ>1 ms) states by Coussement et al. [Phys. Rev. B 45, 9755 (1992)] differ qualitatively from those made to calculate nuclear resonant absorption for transitions from the conventional short-lived (τ<0.1 ms) states. It is for this reason that one cannot mathematically deduce the resonant fraction for the case of the long-lived states by mere extrapolation of the formulas from the case of short-lived ones.

Ab initio calculations of the structural properties of a recently discovered body-centered orthorhombic phase of Si are presented. This high-pressure phase (denoted Imma after its space group) was observed at pressures between the transition pressures for the β-Sn and simple hexagonal (sh) phases of Si. Total-energy calculations indicate that the optimal Imma structure varies with cell volume from the β-Sn structure at large volumes to the sh structure at small volumes when all three structures are described in terms of the Imma unit cell. The energy of the Imma phase is found to be lower than or equal to the energies of the β-Sn and sh phases for all cell volumes.

The results of many-body modeling are reported for polaronic defect excitations in the nickel-based halogen-bridged chain compounds, exemplified by NiBr. One-electron band structures and Raman scattering amplitudes, for electron and hole polarons and bipolarons, triplet excitons, negative, neutral, and positive solitons, are qualitatively described. A singlet exciton is found to be unstable with our model parameters, due to the large on-site Hubbard repulsion. Most significantly, the local defect excitations exhibit a substantial lattice distortion relative to the undistorted antiferromagnetic spin background. The stoichiometric lattice symmetry is therefore broken locally, resulting in Raman activity, which is unexpected on the basis of the stoichiometric ground-state symmetry.

The thermopower of concentrated Kondo systems based on Ce-Cu, Ce-B, and Yb-Cu intermetallic compounds doped with nonmagnetic impurities like Al, Ag, Au, and Ga, is discussed using the single-impurity Anderson model with more than one scattering channel. Interference between the resonant and nonresonant scattering is important for relating the theory to the experimental data. Correlation effects are present in the resonant channel and are accounted for by a numerical renormalization-group method. The large enhancement of the thermopower over the free-electron values reflects the renormalization of the characteristic energy scale of the elementary excitations from the Fermi energy, ${\mathrm{\epsilon }}_{\mathit{F}}$, to the Kondo energy, ${\mathit{T}}_{\mathit{K}}$. The characteristic shape of the experimental curves reflects the change in the structure of the low-energy excitations as the temperature passes through the Kondo temperature. The observed sensitivity of the sign of the thermopower to small changes of the local environment around the magnetic ions is attributed to the interference between resonant and nonresonant scattering.

We find that Co/Cu multilayers exhibiting giant magnetoresistance (GMR) also show giant magnetoresistive 1/f noise. The largest component of this noise (which peaks at magnetic fields maximizing the GMR derivative) is accurately predicted from a fluctuation-dissipation relation using the out-of-phase GMR in an ac field. A second noise mechanism, connected with defects in the antiferromagnetic order but not related to overall magnetization fluctuations, was found near zero field.

The stability of bunched fluxon states in inductively coupled Josephson junctions is analyzed for fluxons of both equal and opposite polarity. We demonstrate that unipolar fluxons may form stable bunched states if the velocity is above a given threshold. The stability of these bunched states is based on the existence of two characteristic velocities in the coupled system. The result of analytical stability analysis is in excellent agreement with the results of numerical simulations.

The optical reflectivity spectra for the bc surface of a large ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{z}}$ crystal were investigated over a wide energy range (30–30 000 ${\mathrm{cm}}^{\mathrm{-}1}$) at room temperature and 6 K. In the spectrum for E∥c, only the phonon peaks were observed without any electronic contribution down to 30 ${\mathrm{cm}}^{\mathrm{-}1}$, whereas for E∥b a Drude-like spectrum was observed. Even below ${\mathit{T}}_{\mathit{c}}$ the spectrum for E∥c does not show any remarkable change except for a small change in the phonon spectrum. This indicates an extremely small plasma frequency for E∥c, less than 30 ${\mathrm{cm}}^{\mathrm{-}1}$, and thus quite a large mass anisotropy ${\mathit{m}}_{\mathit{c}}^{\mathrm{*}}$/${\mathit{m}}_{\mathit{b}}^{\mathrm{*}}$>${10}^4$. Such a small plasma frequency supports the conduction model along the c axis, that the supercurrent flows by tunneling through insulating (semiconductor) layers such as the Bi-O layers.

Temperature-dependent ion-channeling experiments with He ions were performed on Y-Ba-Cu-O single crystals and c-axis or a-axis oriented Y(Eu)-Ba-Cu-O thin films. Angular yield curves (scans) were measured above and below ${\mathit{T}}_{\mathit{c}}$ in the different sublattices. For both, [001] and [100] crystal directions anomalous changes in the scans with respect to their halfwidths ${\mathrm{\Psi }}_{1/2}$ and minimum yield values ${\mathit{X}}_{\min }$ were observed in the oxygen sublattice by cooling below ${\mathit{T}}_{\mathit{c}}$. In [001] direction also ${\mathrm{\Psi }}_{1/2}$ in the scan from Cu revealed an anomalous increase at ${\mathit{T}}_{\mathit{c}}$. No special effect was observed in the Y(Eu)-Ba sublattice. The observations are discussed in terms of structural distortions above ${\mathit{T}}_{\mathit{c}}$ which are removed upon cooling into the superconducting state. The values of the displacement amplitudes were determined in Monte Carlo simulation calculation fitting the experimental data. The main effects are displacements of the apical oxygen atoms of 3.5 pm and of 1.5 pm in the remaining oxygen sublattice.

Thermal noise is known to cause a finite resistance ${\mathit{R}}_{\mathit{p}}$ of overdamped Josephson junctions even in the zero-current limit. By extending the treatment of Ambegaokar and Halperin the magnetic-field dependence of this resistance is analyzed for arbitrary current-phase relations. We show that the measurement of the magnetic-field dependence of ${\mathit{R}}_{\mathit{p}}$ allows the determination of the magnetic-field dependence of the critical current in the temperature regime where thermal-noise rounding effects prevent the application of the usual methods. Applying this technique the correlation function of the supercurrent distribution of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ grain-boundary junctions could be determined with a spatial resolution of 130 nm. The correlation function indicates that the supercurrent distribution along the grain boundary is spatially nonuniform on a sub-μm scale.

Based on the concept of self-organized criticality, we have developed an expression for the current decay of type-II superconductors: j=${\mathit{j}}_{\mathit{c}}$-${\mathit{j}}_{\mathit{c}}$[kT/${\mathit{U}}_{0\mathrm{}}$ln(1+βt/τ)${]}^{1/\mathrm{\beta }}$. This expression can be a general description of current decay in a wide range of j. Magnetic-relaxation experimental results in both high-${\mathit{T}}_{\mathit{c}}$ superconductors (single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$) and conventional type-II superconductors (multifilamentary wire of ${\mathrm{Nb}}_3$Sn) have shown excellent agreement with our theoretical predication. Several microscopic parameters such as the characteristic energy and the characteristic time for these type-II superconductors have been extracted from the experimental data.

We have measured the current-voltage characteristics of an ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ multilayer in perpendicular magnetic fields. A dislocation-mediated melting behavior of the two-dimensional vortex lattice (VL) is identified. Below the melting field, the vortex motion is found to undergo a crossover from elastic motion to plastic motion when the activation energy for the collective creep exceeds the formation energy of dislocation pairs in the VL at low currents. Above the melting field, the vortices are in a pinned liquid state with the I-V curves characterized by a linear behavior at small currents, changing to a nonlinear behavior at large currents.

It is shown that normal-state in-plane tunneling experiments in the high-${\mathit{T}}_{\mathit{c}}$ cuprates can provide evidence for the existence of a submerged band containing nondegenerate fermions. These carriers arise in a model described by two two-dimensional holelike subbands with the chemical potential lying close to the top of the lower band. The resulting jump in the density of states near the Fermi level manifests itself in the observed asymmetry in tunneling conductance with respect to the bias voltage in metal-insulator-cuprate junctions. In the off-stoichiometry systems the jump also results in peaks in ${\mathit{d}}^2$I/${\mathit{dV}}^2$. Implications of the model for the symmetric cuprate-insulator-cuprate junctions are discussed as well.

The reduction in transition temperature that occurs as oxygen is removed from ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ is accompanied by a rapid decrease in the size of the heat-capacity anomaly at ${\mathit{T}}_{\mathit{c}}$. We treat the fluctuation effects in the context of XY-like behavior and demonstrate that their dependence on the transition temperature agrees in detail with a relationship between ${\mathit{T}}_{\mathit{c}}$ and the hole concentration proposed by Schneider and Keller. Further, we show that this empirical relationship can be understood by combining scaling arguments with the nearly antiferromagnetic Fermi-liquid model of Monthoux and Pines.

Raman scattering from intermolecular A-${\mathrm{C}}_{60}$ optic modes in ${\mathit{A}}_3$${\mathrm{C}}_{60}$ thin films (A=Rb, K, and Na) yields a single broad band, whose peak frequency increases with decreasing alkali-metal mass. This very weak scattering is associated with disorder-induced contributions from a weighted phonon density of states. The broad band is assigned to vibrations of alkali-metal ions primarily in tetrahedral interstitial sites of the ${\mathrm{C}}_{60}$ fcc host lattice. Changes in Raman intensity with alkali-metal and thermally varied lattice constants are attributed to A-C distance dependent modulation of ${\mathrm{C}}_{60}$ electronic states.

A low-density polymorph of silica, α-cristobalite, has been observed to have a negative Poisson ratio, i.e., when subjected to a uniaxial compression, this crystal contracts in the transverse direction. This behavior is uncommon. Other low-density polymorphs of silica, which have a similar chemical environment, have been reported to have positive Poisson ratios. We have investigated the elastic behavior of several silica phases: α-quartz, β-quartz, and α-cristobalite, with first-principles quantum-mechanical calculations, and with interatomic potentials. Our calculations confirm a negative Poisson ratio in α-cristobalite. We predict that α-quartz will have a negative Poisson ratio at high tensile strains, and that β-quartz will never assume a negative ratio. We compute the second-order elastic constants of α-quartz and α-cristobalite using interatomic potentials and apply them in discussing the anisotropy of Poisson ratios in these materials. We demonstrate that the rigidity of the ${\mathrm{SiO}}_4$ tetrahedral units is intimately related to the occurrence of negative Poisson ratios in crystalline forms of silica.

Models for lattice dynamical systems possessing first-order structural phase transitions have recently been constructed, and extensive molecular-dynamics calculations have been carried out for them. The mechanism producing the transition is that there are lower vibrational frequencies in the high-temperature structure than in the low-temperature structure, thereby increasing the entropy of the high-temperature phase. These structure-dependent frequencies are produced by anharmonicity in the interparticle interaction. This paper describes a mean-field theory for such entropy-driven transitions. It is based on making a Gaussian ansatz for the single-particle probability density function. Qualitatively, the results agree well with the molecular-dynamics simulations. We use the theory to make a more extensive survey of the parameter space than has been done with the simulations. We find that there are three different intervals for the strength of the anharmonicity, in each of which the high-temperature behavior of the order parameter is different in an important way. This change is a possible explanation for the hysteresis found in the simulations.