A longitudinal dipolon dispersion relation has been calculated including screening. It is found that the dipolon field is strongly screened by the free electrons thus removing the wave-vector dependence.

We consider the important points raised by Pimpale and Paranjape (PP) in their comments on our paper [Phys. Rev. B 43, 7756 (1991)]. It is emphasized that we disagree with their conclusion. At the outset, we indicate that the results obtained by PP show an interesting contradiction because of two reasons. First, the changes obtained by PP in the first-order approximation on the dipolon frequencies by interaction with plasmons do not depend, by any means, on the plasmons or on any parameter characterizing the free electrons which are present. In order words, PP obtain the screening on the dipolons due to plasmons but the screening term which they have deduced does not depend on the source, namely, the plasmons. Second, the results derived by PP do not retrieve the proper limiting results corresponding to the dipolons when the plasmons are ‘‘switched off’’ although they retrieve the proper limiting results when the dipolons are ‘‘switched off.’’ The PP’s present interesting contradiction can be resolved if one compares their results with those derived by one of us (R.R.S.) for the plasmons interacting with dipolons. We discover that an important term is missing in PP’s formalism. The present expressions are found to retrieve the expected results in the limiting situations when the dipolons or the plasmons are ‘‘switched off.’’ Furthermore, in contrast with PP’s results, we show that the corrected results depend, as expected, on the free electrons incorporated in the formalism. More importantly, the results, in contrast with PP’s results, do not completely remove, but modify the wave-vector dependence of the dipolon dispersion relations due to the presence of plasmons. We also discuss other inconsistencies in the calculations given by PP.

We have performed first-principles local-density-approximation calculations on aluminum-magnesium intermetallic compounds. Experimental studies of this system have shown that complex topologically close-packed structures with large unit cells are favored over simpler planar structures. We show that this is due to the opening of a quasigap in the density of states at the Fermi level; our results for total energies confirm that this lowers the energy. These electronic effects resemble those measured and calculated for quasicrystals which have similar chemical compositions and structures.

We study the Anderson model of localization in three dimensions with different probability distributions for the site energies. Using the Lanczos algorithm we calculate eigenvectors for different model parameters like disorder and energy. From these we derive the singularity spectrum typically used for the characterization of multifractal objects. We demonstrate that the singularity spectrum at the critical disorder, which determines the mobility edge at the band center, is independent of the employed probability distribution. Assuming that this singularity spectrum is universal for the metal-insulator transition regardless of specific parameters of the model we establish a straightforward method to distinguish localized and extended states. In this way we obtain the entire mobility-edge trajectory separating regions of extended states from regions of localized states in the energy-disorder phase diagram. The good agreement with results from transfer-matrix calculations for all probability distributions used corroborates the applicability of our criterion.

We have measured the giant magnetoresistance (GMR) and the magnetization of a melt-spun granular sample of ${\mathrm{Cu}}_{87}$${\mathrm{Co}}_{13}$. The Cu matrix contains very small particles of Co that exhibit superparamagnetism. Although the GMR is due to these small superparamagnetic particles, we find that the GMR does not vary quadratically with the magnetization. This unexpected result is attributed to the presence of a range of sizes for the superparamagnetic particles. Assuming a simple distribution of particle sizes, we calculated the magnetic-field dependence of the GMR and find excellent agreement with experiment.

We developed a method for obtaining effective cluster interactions in substitutional binary alloys from first-principles total-energy calculations. The method reproduces not only the formation energies, but also the ground-state line and relative energies of different structures. These quantities are crucial if the energy parametrization is to be used to compute the temperature-composition phase diagram of the alloy system. We applied the method to the Pd-V alloys, and obtained insight into the energetics of this complex alloy system. We showed that pair cluster interactions of long range (seventh or eighth nearest neighbor) are necessary to reproduce the ground-state line of the fcc Pd-rich superstructures.

We develop a method that provides an analytical expression of the free-electron spin susceptibility for arbitrary temperature. The result can be further reduced by using the low-temperature approximation, which shows that the oscillation decays exponentially as exp(-π${\mathit{T}}^{\mathcal{’}}$${\mathit{k}}_{\mathit{Fr}}$) in the long-range limit, and the wave number of the oscillation varies as ${\mathit{k}}_0$∼${\mathit{k}}_{\mathit{F}}$[1-(${\mathrm{\pi }}^2$/12)${\mathit{T}}^{{\mathcal{’}}^{}2}$], where ${\mathit{T}}^{\mathcal{’}}$ is the normalized temperature with respect to the Fermi energy. The result contradicts what Darby has proposed, especially in the long-range region. On the other hand, in the short-range region, where the Sommerfeld expansion method is valid, numerical study shows that Darby’s result agrees with ours fairly well.

Measurements of the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ NMR spectrum, isotropic Knight shift (${\mathit{K}}_{\mathit{i}}$), and spin-lattice relaxation rate (1/${\mathit{T}}_1$) in an unoriented powder sample of ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C are reported in the normal and the superconducting states. In the normal state, ${\mathit{K}}_{\mathit{i}}$ varies linearly from (+5.7±0.7) × ${10}^{\mathrm{-}4}$ at 300 K to (+4.9±0.7)×${10}^{\mathrm{-}4}$ just above the superconducting transition temperature. The anisotropic Knight shift, if present, is substantially smaller than ${\mathit{K}}_{\mathit{i}}$. An anomalous increase in (${\mathit{T}}_1$T${)}^{\mathrm{-}1}$ with decreasing temperature (T) is found in the normal state. From the behavior of the NMR linewidth in the superconducting state, the low-temperature limit λ(0)=(108±5) nm is found for the penetration depth. At the superconducting transition, 1/${\mathit{T}}_1$ drops rapidly and no Hebel-Slichter peak is seen.

Neutron-diffraction techniques have been used to study the magnetic structure of the superconductor ${\mathrm{ErNi}}_2$${\mathrm{B}}_2$C (${\mathit{T}}_{\mathit{c}}$=11 K). The experimental results, obtained on single crystals, show that below approximately 7 K this compound is in an incommensurate modulated antiferromagnetic state, with wave vector 0.553a*, that coexists with superconductivity. The field dependence of the magnetic structure of this compound was studied at 5.5 K as a function of an externally applied magnetic field both along the b and c axes of the tetragonal structure.

We have carried out powder-neutron-diffraction studies of the antiferromagnetic order that develops below ${\mathit{T}}_{\mathit{N}}$=6.8 K in the magnetic superconductor (${\mathit{T}}_{\mathit{c}}$=11 K) ${\mathrm{ErNi}}_2$${\mathrm{B}}_2$C. The antiferromagnetic structure is associated with spins on the Er atoms, which order in a transversely polarized planar sinusoidal structure propagating along the a or b axis in equal domains, with the Er moments parallel to the b or a axis, respectively. Third and fifth harmonics of the sine wave are observed at low temperatures, indicating a squared-up sine wave, with a low-temperature amplitude of 7.8${\mathrm{\mu }}_{\mathit{B}}$/Er atom. There is also evidence of an induced moment of -0.35${\mathrm{\mu }}_{\mathit{B}}$ on the Ni ions. A similar study of the isostructural compound ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C revealed no magnetic peaks (μ<0.13${\mathrm{\mu }}_{\mathit{B}}$) developing down to 0.3 K for Q values less than 1.6 A${\mathrm{̊}}^{\mathrm{-}1}$.

Superconducting tunneling measurements for amorphous ${\mathrm{Sn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Cu}}_{\mathit{x}}$ (0.08≤x≤0.41) have been inverted to determine the Eliashberg function ${\mathrm{\alpha }}^2$F(ω) as the electron-phonon coupling parameter λ and the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ decrease from 1.8 to 1.0 and 6.87 K to 3.91 K, respectively, with the Cu content. The best fit to the experimental tunneling data was given with a linear dependence for ${\mathrm{\alpha }}^2$F(ω) at low frequencies for all compositions. This linear dependence is consistent with the model for phonon damping proposed by Belitz and Wybourne (following paper) which properly incorporates electronic damping but also includes a strong nonelectronic contribution.

A connection is proposed between the anomalous thermal transport properties of amorphous solids and the low-frequency behavior of the Eliashberg function. By means of a model calculation we show that the size and frequency dependence of the phonon mean free path that has been extracted from measurements of the thermal conductivity in amorphous solids lead to a sizable linear region in the Eliashberg function at small frequencies. Quantitative comparison with recent experiments gives very good agreement.

The ${}_{}{}^{169}{}_{}{}^{}\mathrm{Tm}$ nuclear spin-lattice relaxation in oxygen-deficient ${\mathrm{TmBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ compounds, as quenched and room-temperature annealed, has been measured at low temperatures. The results are consistent with the existence of paramagnetic centers in the ${\mathrm{CuO}}_2$ double layer, which have a nonmagnetic (singlet) ground state separated from an excited magnetic state by an energy gap of the order of 1 K.

The "neutral lines" which were recently predicted to occur during flux creep in thin superconductors in a transverse magnetic field, are observed by magneto-optics in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ strips and rectangular platelets. These stationary neutral lines or loops separate regions where the flux density increases or decreases during flux creep due to the large demagnetization field. The observations are in good agreement with theory, which is extended to rectangular specimen shape.

We report detailed measurements of current-voltage characteristics for a ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$/Ag tape immersed in liquid neon (27 K) and in liquid oxygen (54–90 K) as a function of applied magnetic field up to 17 T. It is found that the electric-field-versus-current-density (E-J) curves can be well described by the relation E/J∼exp[-(${\mathit{J}}_0$/J${)}^{\mathrm{\mu }}$], where ${\mathit{J}}_0$ is a constant. The exponent μ at elevated temperature (T≥54 K) is less than 0.9, and is inconsistent with the vortex-glass description, which predicts μ<1. At low-temperature (T=27 K), a plateau in the value of μ(≊1.45) is found for 2≤H≤8 T, which may be described by the collective creep theory at this temperature and field range.

The magnetic phase diagram of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films was studied as a function of anisotropy by systematically changing the oxygen stoichiometry (δ value). For small δ values the vortex-solid phase transition boundary ${\mathit{H}}_{\mathit{g}}$(T) is observed over the full accessible field range (50 kOe) to follow the (1-T/${\mathit{T}}_{\mathit{c}}$${)}^{\mathrm{\sim }1.5}$ dependence that is expected for an anisotropic superconductor in the three-dimensional regime. As δ increases, a portion of the phase boundary no longer follows the (1-T/${\mathit{T}}_{\mathit{c}}$${)}^{\mathrm{\sim }1.5}$ dependence making it possible to identify the anisotropy-dependent dimensional crossover field ${\mathit{H}}_0$ at which the system begins to behave quasi-two-dimensionally. Plotting ${\mathit{H}}_{\mathit{g}}$(T) normalized by ${\mathit{H}}_0$ as a function of (1-T/${\mathit{T}}_{\mathit{c}}$) results in a collapse of the phase boundaries onto a single universal curve. Using published values of the anisotropy γ=8.5 and the upper critical field slope ${\mathit{dH}}_{\mathit{c}2}$/dT=-20 kOe/K it is possible to determine the specific relation ${\mathit{H}}_0$≊1.2${\mathrm{\phi }}_0$/(${\mathit{d}}^2$${\gamma }^2$), where d is the multilayer spacing, which can be used to determine the δ dependence of γ.

The response of a hyperpure Ge detector (volume ∼0.7 ${\mathrm{cm}}^3$) to ionizing radiation has been studied with regard to the radiation ionization energy ratio, ${\mathrm{\varepsilon }}_{\mathrm{ion}}$/${\mathrm{\varepsilon }}_0$, using radioactive (α,γ) sources and accelerated ion beams ${(}^1$H${,}^4$He). These data provide evidence that ${\mathrm{\varepsilon }}_{\mathrm{ion}}$ is not identically equal to ${\mathrm{\varepsilon }}_0$ for low-energy (e.g., few MeV) charged particles, in qualitative accord with data for Si detectors. The apparent thickness of the ion-implanted entrance window of the detector exhibits a marked dependence on temperature in the region 80–183 K. As well, channeling effects have been observed which can be attributed to residual crystalline structure in this inactive layer.

By means of a change of variable, it is possible to write the grand partition function of a system of interacting classical particles, in terms of a functional integral on the particle density. Minimizing the resulting effective free energy with respect to the particle density itself, a nonlinear integral equation for the equilibrium density is obtained, and the stability conditions are turned into a linear eigenvalue problem. Some special solutions are considered, such as the homogeneous liquid and vapor densities, with the conditions for their coexistence, the liquid-vapor interface, and, finally, small periodical (in space) density fluctuations, in the neighborhood of an instability point.