The broadening of resistivity curves is measured for a c-axis-in-plane-aligned a-axis-oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ epitaxial thin film in the vicinity of the superconducting transition in magnetic fields up to 12 T with various current and magnetic-direction configurations. The direction of the magnetic field with respect to the crystallographic axis is the most important factor governing the broadening of the resistive transition. The direction of the current with respect to the crystallographic axis and to the magnetic-field direction is of less importance. Based on the results, flux-flow resistivity is extracted for the film in the 12-T magnetic field running parallel to the c-axis direction.

Results are for finite-temperature optical conductivity σ(ω) and dc resistivity ρ(T), obtained within the t-J model on a square lattice by a method based on the Lanczos diagonalization. Studying systems with N=16 or 18 sites and hole concentrations ${\mathit{c}}_{\mathit{h}}$≤0.25, a thermodynamic behavior is reached for quite low temperatures. σ(ω) spectra reveal a slow non-Drude falloff at large frequencies and the emergence of a broad midinfrared peak at low doping. At optimum doping ρ(T) shows nearly linear variation with T. Both quantities agree well with experimental results in cuprates.

Small-angle multiple scattering is considered in the case when the single scattering is a refraction on an inhomogeneity. A system of spherical particles or pores with radii R randomly distributed over the sample volume V is studied in the diluted case when V≫δV, where δV is the volume occupied by the particles (pores). It is shown that there are two regions of the sample thickness L. In the first one the multiple scattering is a result of slightly modified random walks when the characteristic transferred momentum is proportional to [(L/l)ln(L/l)${]}^{1/2}$, where l is the mean free path. For larger L (the second region) the characteristic momentum transfer is equal to (L/2lR). For lower q the intensity is q independent and for large q it is determined by single scattering and decreases as ${\mathit{q}}^{\mathrm{-}3}$.

Structural, electrical, and magnetic studies of [(TMTSF${)}_{1\mathrm{-}\mathit{x}}$(TMTTF${)}_{\mathit{x}}$${]}_2$${\mathrm{ReO}}_4$ alloys are reported. They exhibit around x∼0.5 an enhanced charge localization and a strong decrease of the (1/2,1/2,1/2) anion-ordering (AO) transition temperature. The localization enhancement is attributed to a 4${\mathit{k}}_{\mathit{F}}$ order of charge on the sites due to the alternate order of TMTSF and TMTTF molecules. This heteropolar charge distribution is suggested to weaken the 2${\mathit{k}}_{\mathit{F}}$ bond response and thus to depress the AO transition. These properties contrast with those exhibited by the hybrid salt (TMDTDSF${)}_2$${\mathrm{ReO}}_4$.

We derive a chiral XY model with right chirality on a linear chain as a simple model for the liquid-crystalline ${\mathit{D}}_{\mathit{h}\mathit{d}}$ phase of hexahexylthiotriphenylene. We explain a liquidlike short-range intracolumn order in the ${\mathit{D}}_{\mathit{h}\mathit{d}}$ phase by calculating the distribution function of the difference between angles and the correlation functions for the angles for radicals on different molecules in a column.

The octahedrally coordinated hydrogen (${\mathrm{H}}_{\mathit{o}}$) of the superstoichiometric rare-earth dihydrides β-${\mathrm{TbH}}_{2+\mathit{x}}$ (0.03<x<0.25) has been probed by incoherent inelastic neutron scattering. The ${\mathrm{H}}_{\mathit{o}}$ sublattice arrangements and associated optical vibrations are sensitive to the value of x. For x=0.03 at low temperature, the majority of the low-concentration o-site hydrogens are isolated in a local cubic environment and exhibit a relatively sharp vibrational density of states at 80.8 meV, in accord with the presence of triply degenerate normal modes. In contrast, for x=0.25, a broad bimodal band peaking at ∼76.7 and 83.1 meV is evident, consistent with the type of long-range order that is known to occur in the ${\mathrm{H}}_{\mathit{o}}$ sublattice at this higher concentration. Increasing the temperature above the long-range ordering transition leads to a more disordered ${\mathrm{H}}_{\mathit{o}}$ sublattice and a density of states similar to that at low values of x but broadened, most likely by the persistence of some short-range order.

The phonon dispersion in the ${\mathrm{Ni}}_{0.55}$${\mathrm{Pd}}_{0.45}$ and ${\mathrm{Ni}}_{0.5}$${\mathrm{Fe}}_{0.5}$ alloys has been studied within the framework of the transition-metal model-potential approach of Animalu and the mean-crystal model. A short-range three-body interaction has also been incorporated in the study. The theoretically calculated phonon-dispersion curves of these alloys are found to be in good agreement with the experimental data.

A generalization of the p-spin-interaction quantum multicomponent Mattis model in a transverse field is investigated by use of the Suzuki-Trotter approach with the thermodynamic perturbation theory. The phase diagram is obtained in the limit p→∞, and the effects of quantum fluctuations on the first-order phase transitions separating the disordered phase from the ordered phases are discussed. We find that paramagnetic phase is characterized by transverse ordering, whereas the ordered phases are not affected by the quantum fluctuations.

Blatter, Blazey, and Portis [Phys. Rev. B 44, 2800 (1991)] interpreted observed electron-spin-resonance spectra in terms of small Na-Cs alloy clusters residing within the pores of zeolite Y. It is the purpose of this paper to reinterpret these important results in the light of recent developments.

We performed magnetic circular x-ray dichroism (MCXD) measurements on thin epitaxially grown layers of Ni on bcc Fe(100). These measurements show that at room temperature the layers are magnetically ordered. Values for the orbital, spin, and total magnetic moments are estimated and compared to calculations.

The zero-field splitting (ZFS) parameters D and F-a of ${\mathrm{Fe}}^{2+}$ ions in ${\mathrm{GeFe}}_2$${\mathrm{O}}_4$ are considered by diagonalizing the crystal-field and spin-orbit interaction Hamiltonian matrix. Results show that the spin-triplet contributions to ZFS are important, and the stronger the trigonal crystal field and the covalency effect assumed, the larger are these contributions.

We examine a quantum-spin-glass model with a transverse field. The network is based on a modified version of the Hopfield spin glass. A set of equations for the order parameters describing the ordered phases are obtained, and the effects of the quantum fluctuations and the stored patterns on the phase transitions between the ordered phases are examined. We see that reentry occurring in the mixed-phase transitions can be observed in the present system.

In this paper, we report ac and dc susceptibility, resistivity, and heat capacity studies of the antiferromagnet ${\mathrm{Ho}}_5$${\mathrm{Os}}_4$${\mathrm{Ge}}_{10}$. We find that this compound undergoes an antiferromagnetic transition at 2.1 K. The effective magnetic moment is found to be 10.85${\mathrm{\mu }}_{\mathit{B}}$, which is slightly higher than the free-ion value for the holmium atom. The resistivity [ρ(T)] shows unusual linear temperature dependence from 10 to 50 K. Further, the heat-capacity data show an anomaly around 6 K, which is well above the antiferromagnetic ordering temperature. Absence of an anomaly in the magnetic susceptibility and a change of slope of ρ(T) around this temperature suggest a possible structural transformation in this compound although confirmation of this transformation requires further study.

Electronic structures of 13-atom Rh clusters with three possible high-symmetry geometries are studied using the discrete variational local-spin-density-functional method. An anomalous symmetry dependence of the cluster magnetism is found: the total magnetic moment of the icosahedral ${\mathrm{Rh}}_{13}$ cluster is smaller than that of the other two lower-symmetry clusters in a wide range of interatomic spacings. An energy difference is identified to explain this anomalous relationship, which is found to be also useful for judging whether some techniques can or must be used in the local-spin-density-functional calculations. The calculated results are compared and discussed with those of previous theory and a recent experiment. The real geometry of the ${\mathrm{Rh}}_{13}$ cluster is suggested to be a distorted icosahedron.

We study the variation of the phonon structure in the conductance curves of superconducting variable-barrier junctions formed using a scanning tunneling microscope. The observed vanishing of this structure as the tunneling barrier decreases is explained as a result of the mixing of electronic states in the Andreev reflection process, whose importance increases as the tunneling barrier decreases.

We present an analysis of the measured excess conductivity which results from the fluctuation of the superconducting order parameter of a ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{y}}$ single crystal and found it was fit best by the two-dimensional Aslamazov-Larkin theory. The magnetoconductivity and magnetization studies are in good agreement with the two-dimensional (2D) scaling behavior in the fremework of Lawrence-Doniach model revealing the 2D nature of the fluctuation near the mean-field transition temperature in the presence of a field.

The Luttinger-Tisza method has been applied, taking into account both the dipole-dipole and exchange interactions, to calculate the low-temperature ordered states of the magnetic rare-earth ions in the various R${\mathrm{Ba}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ (R=rare earth) compounds. Dipolar interaction alone has been found to explain the observed orderings in ${\mathrm{ErBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ and ${\mathrm{DyBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$, the only materials whose low-temperature magnetic orderings have been determined by neutron diffraction. The expected low-temperature orderings for the other R${\mathrm{Ba}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ compounds based on purely dipolar interactions have also been calculated.

We present evidence for the crossover between creep of individual vortices and collective creep of vortex bundles from measurements of the relaxation of the persistent currents in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ single crystals over long periods of time. In the single-vortex regime at low fields the relaxation is fast and is characterized by an exponent μ≃0.16, as predicted in the collective-creep scenario. With increasing field, collective bundling of vortices induces a slowdown of the relaxation, with a maximum μ≃1.4. At even higher fields, μ decreases again to about 1 as the size of the bundles grows.

Photothermal measurements of diffusivity in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ have been made in regions only 10 μm in extent. The measured diffusivity within a single crystallite is much larger than that across a twin boundary. The results obtained are shown to be consistent with the microwave penetration depth measurements of normal electron density over a wide temperature range below the critical temperature. The results are not in good agreement with the BCS theory. It is shown that phonons are reflected at the twin boundaries, and thermal resistance of the boundary is approximately a linear function of temperature in the range 20<T<300 K.

The influence of librational motion (orientational disorder) on the electronic structure of molecular solid hydrogen is investigated by means of a microcrystal formalism. A cluster consisting of 64 molecules in the hexagonal close-packed structure with periodic boundary conditions is diagonalized exactly in a parametrized tight-binding approximation. The influence of pressure and libration are examined for a variety of orientational structures. It is found that the stable orientational structure varies with pressure, and that orientational phase transitions occur just before the insulator-to-metal transition (closing of the gap). The theoretical results seem to explain satisfactorily the anomalies observed in dielectric measurements of solid hydrogen at high pressure.