As part of a program to investigate light-atom vibrations associated with defects in nonmetallic solids, we have developed a general, dimensionless procedure to solve for the energy levels, wave functions, and transition-matrix elements for Morse oscillators. We can thus readily investigate transition-strength anomalies associated with electrical anharmonicities such as those recently observed in certain ${\mathrm{OH}}^{\mathrm{-}}$ and ${\mathrm{OD}}^{\mathrm{-}}$ systems in alkali halides. A center-of-mass correction in adapting the ${\mathrm{OH}}^{\mathrm{-}}$ dipole moment function to ${\mathrm{OD}}^{\mathrm{-}}$ is of central importance in interpreting isotope effects on fundamental and overtone transition strengths.

The nonlinear quadrupolar susceptibility of the microscopic model for quadrupolar glass freezing of axial quadrupoles in solid hydrogen is derived in the mean-field approximation. It is found that both linear and nonlinear responses exhibit nonsingular behavior as a function of temperature.

The noncutoff-kink theory in nonlinear lattices proposed by Kivshar [Phys. Rev. B 46, 8652 (1992)] is generalized to include damping and a parametric drive, which should be considered in order to describe the experimental results for the pendulum lattice more reasonably. The reduced equations of motion for lattice displacements are solved exactly and eight possible kinks of symmetric and antisymmetric kink solutions are obtained.

The optical conductivity in the high- and low-temperature phases of the semiconducting charge-transfer salt NPrQ(TCNQ${)}_2$ has been analyzed using a one-dimensional twofold-degenerate commensurate charge-density-wave model. The agreement with the model is good in the high-temperature state; it is somewhat poorer at temperatures below the structural phase transition, an indication of the increased complexity of the electronic structure. Above ${\mathit{T}}_{\mathit{c}}$, the semiconducting energy gap is found to have an almost equal contribution from the external cation potential and the intrinsic chain distortion whereas below ${\mathit{T}}_{\mathit{c}}$ it is primarily due to the intrinsic chain distortion.

The electric quadrupole interaction of ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$ nuclei was studied in the weak ferromagnetic system MnSi using muon level-crossing resonance (μLCR). The temperature dependence of the electric-field gradient (EFG) shows a critical behavior near the ferromagnetic transition temperature, indicating that the EFG due to conduction electrons is strongly correlated with the magnetic susceptibility in this itinerant electron helimagnet. Using this μLCR result, the previous data were reanalyzed to obtain a more accurate spin-lattice relaxation time for both muons (${\mathit{T}}_1^{\mathrm{\mu }}$) and ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$ nuclei (${\mathit{T}}_1^{\mathrm{Mn}}$). The temperature dependences of the EFG, ${\mathit{T}}_1^{\mathrm{Mn}}$, and ${\mathit{T}}_1^{\mathrm{\mu }}$ are in reasonable agreement with the self-consistent renormalization theory developed by Moriya and co-workers.

We examine the nature of the singlet excitations introduced recently to describe the spin dynamics of the antiferromagnetic Heisenberg model with anisotropic coupling. It is shown that they excite spin waves polarized linearly, as opposed to the usual magnons, that are triplet excitations and correspond to circularly polarized waves. The approach yielding the singlet excitations give more accuracy than ordinary spin-wave theory, which shows that the absence of linearly polarized modes is an important flaw of the latter.

We examine the effect of Co-cluster morphology on giant magnetoresistance (MR) in phase-separated Co-Cu films. The Co clusters were characterized through grazing incidence, anomalous, small-angle x-ray scattering. With thermal annealing the Co cluster diameter increases from 21 to ∼250 Å with a concomitant drop from ∼35% to 1% in the 4.2 K MR. The MR scales approximately as the inverse cluster size. Comparison with theory indicates that interfacial spin-dependent electron scattering is the dominant scattering mechanism underlying giant MR for cluster diameters up to at least 250 Å.

We investigate the effect of the marginal operator on the critical exponents of the one-dimensional S=1/2 Heisenberg model, using the relation between the critical exponents and the energy gaps of the finite system. The energy gaps behave as ΔE∝1/L with the logarithmic corrections from the marginally irrelevant operator. The numerical results obtained with the Bethe ansatz are well explained by the two-loop renormalization of the marginal coupling. Thus the discrepancies between the one-loop renormalization prediction and numerical results are resolved, though it is found that the nonuniversal constant of the logarithmic correction in the ground state does not agree with the asymptotic expansion of the Bethe ansatz.

Flux motion in anisotropic type-II superconductors is studied in the framework of the time-dependent Ginzburg-Landau theory. Expressions for the flux-flow resistivity tensor (including all the longitudinal and Hall elements) are obtained for the case that the applied magnetic field H is parallel to one of the principal axes of the sample and H is near the upper critical field ${\mathit{H}}_{\mathit{c}2}$. A simple method is proposed for obtaining the anisotropy ratios from the H dependences of the longitudinal resistivities.

The angular dependences of the transverse magnetization and torque in an anisotropic type-II superconductor in the mixed state are studied. Two simple formulas for obtaining the anisotropy ratio from the transverse magnetization measurements are derived by using the usual anisotropic Ginzburg-Landau theory. Possible generalization of the theory to the case of quasi-two-dimensional layered superconductors is also considered.

Nonlinear current-voltage (I-V) curves are measured in laser-ablated ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films deposited onto ${\mathrm{SrTiO}}_3$. The measurements are performed near the glass phase transition in a magnetic field of 5 T at various angles from the c axis. From a critical scaling analysis, the angular dependencies of the glass transition temperature and the critical glass exponents are extracted. At small angles, these results distinguish between a vortex glass, caused by random pointlike disorder, and a Bose glass, caused by linelike disorder. The results can be understood in terms of the vortex-glass model only. No evidence is found for the existence of a Bose-glass phase.

In this paper, we report positron lifetimes for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$${\mathrm{Sn}}_{\mathit{x}}$${\mathrm{O}}_{\mathit{y}}$ (x=0–1.5). From these results, we calculate the local electron density ${\mathit{n}}_{\mathit{e}}$ and vacancy concentration ${\mathit{C}}_{\mathit{V}}$ as a function of Sn content. The results show a significant Sn-substitution dependence. At low x (≤0.4), ${\mathit{n}}_{\mathit{e}}$ decreases as x increases while at high x (≥0.8) it reaches a constant value. For ${\mathit{C}}_{\mathit{V}}$, a large change occurs at x between 0.8 and 1.0 and corresponds to the known O-T phase transition. Positron experiments show that the structure of the Y 1:2:3 oxygen-deficient system is orthorhombic.

We examine the hypothesis that fluctuating local superconducting order exists well above the critical temperature in high-${\mathit{T}}_{\mathit{c}}$ materials. This might explain anomalies in the magnetic susceptibility of some of these systems. It could also create a gap or pseudogap in the density of states, which could be measured by photoemission or tunneling experiments. We compute the density of states for a two-dimensional tight-binding model with such local order in the coherent-potential approximation. The order parameter is taken to have a spatially random phase and constant magnitude. We calculate this magnitude by a Ginzburg-Landau method. The conclusion is that the disorder smooths out the gap in the density of states unless the local order parameter is larger than the zero-temperature gap.

(111)-oriented metallic overlayers of Cu, Au, Pd, and Pt with thicknesses up to 3 atomic layers (AL) on Fe(110) or Co(0001) show evidence for d-like quantum-well (QW) states in spin-resolved photoemission spectroscopy. The noble-metal d-like QW states are not spin polarized and are well below the Fermi level. In contrast, the d-like QW states of Pd and Pt approach the Fermi level for about 3 AL, but are spin polarized only for the first AL. Thus both types of QW states do not mediate the antiferromagnetic interlayer exchange coupling in corresponding (111)-oriented magnetic multilayers.

We study the evolution with pressure P and band filling y of the heat capacity, Hall coefficient, and resistivity at the approach to the T→0 Mott-Hubbard metal-insulator transition (MIT) in highly correlated ${\mathrm{V}}_{2\mathrm{-}\mathit{y}}$${\mathrm{O}}_3$. Under P, the electronic effective mass ${\mathit{m}}^{\mathrm{*}}$ diverges at the MIT with a negligible change in carrier concentration n away from half-filling. Conversely, in the doped system ${\mathit{m}}^{\mathrm{*}}$ actually decreases as the MIT is approached, while n increases linearly with y. The low-T magnetic order in the metal helps us deconvolute contributions from charge correlations and spin fluctuations.

Carbon-coated gadolinium carbide nanocrystallites are generated by a process based on the Kratschmer-Huffman carbon-arc method of preparing fullerenes, and a novel magnetic-field-gradient separation technique is used to separate them. This separation of nanocrystalline byproducts of the carbon arc process is a prerequisite for many of the proposed applications of these materials. While the data presented pertain to ${\mathrm{Gd}}_2$${\mathrm{C}}_3$, this method is generally applicable to any paramagnetic or ferromagnetic compound. Structural characterization by x-ray and electron diffraction and high-resolution transmission electron microscopy reveal the presence of a single gadolinium-containing (${\mathrm{Gd}}_2$${\mathrm{C}}_3$) phase and excess carbon. The carbide phase exists as 10–50 nm spherical particles. SQUID magnetometry shows paramagnetic response attributed to ${\mathrm{Gd}}^{3+}$ ions.

We have calculated the electron-electron scattering contribution to the electrical resistivity (${\mathrm{\rho }}_{\mathrm{\perp }}$) in the two transverse directions for an anisotropic tight-binding cosine band. This type of band structure is thought to apply to organic metals such as the Bechgaard salts. Simple expressions for ${\mathrm{\rho }}_{\mathrm{\perp }}$ were obtained using the standard variational method. In all cases we found ${\mathrm{\rho }}_{\mathrm{\perp }}$=αT+β${\mathit{T}}^2$. For the b direction α∼${\mathit{t}}_{\mathit{b}}^{\mathrm{-}2}$ and β∼${\mathit{t}}_{\mathit{b}}^{\mathrm{-}3}$, and for the c axis α∼${\mathit{t}}_{\mathit{c}}^{\mathrm{-}2}$ while the expression for β is more complicated. The above formulas are compared with experimental data and the effective Coulomb interaction for the Bechgaard salts is found to be U≃0.64 eV.

We have measured a thermomagnetic effect in the Josephson SNS junction Ta/Cu/Nb caused by the thermal diffusion of coreless Josephson vortices. We believe this effect is similar to the Nernst effect in the mixed state of type-II superconductors. The transport entropy per unit length of the coreless Josephson vortex has been introduced in a way similar to that for the mixed state of type-II superconductors. From our experimental results the transport entropy has been obtained. Its value is ${\mathit{S}}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$≊${10}^{\mathrm{-}12}$ J/K m in agreement with the theoretical estimation.

The low-energy physics of the Emery model, representing the high-${\mathit{T}}_{\mathit{c}}$ cuprates, can be understood by separating intracell interference within ${\mathrm{CuO}}_4$ cells from intercell correlations described by an effective single-band Hubbard model. This decomposition explains the anomalous transfer of spectral intensity from the high- to the low-energy scale, the asymmetry between electron and hole doping, and the breakdown of low-energy sum rules. These features are related to the existence of an antibound charge transfer exciton band, are are very sensitive to the strength of the Coulomb repulsion ${\mathit{U}}_{\mathit{d}}$.

We present a model for the c-axis resistivity ${\mathrm{\rho }}_{\mathit{c}}$ in the cuprates which incorporates interplanar disorder. Higher-order perturbative calculations demonstrate that this special disorder stabilizes a low-T metallic state and in the dynamical limit leads to temperature-dependent slopes d${\mathrm{\rho }}_{\mathit{c}}$/dT which are negative (positive) for low (high) hole concentrations. Predictions are presented for correlations which associate a nonlinear planar resistivity with the magnitude of negative d${\mathrm{\rho }}_{\mathit{c}}$/dT.