Simultaneous replacement of Sr by Ba and Y by Eu in the Pb-2213 system was found to yield single-phase ${\mathrm{Pb}}_2$${\mathrm{Ba}}_2$${\mathrm{EuCu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ samples, suitable for ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ Mössbauer measurements. The samples were synthesized by a solid-state reaction of metal oxides and carbonates under an inert atmosphere. An oxygen-rich sample corresponding to δ=1.79 was obtained by annealing the as-synthesized material (δ=0.16) in oxygen. The oxygen annealing increased the size of the lattice constants and led to an orthorhombic-to-tetragonal phase transition of the structure. The samples were also oriented in an 11.7-T magnetic field. The x-ray-diffraction spectra showed that the c axes tended to orient perpendicular to the applied field. The measured Mössbauer spectra exhibited an electric quadrupole interaction typical of the Eu site in high-${\mathit{T}}_{\mathit{c}}$ cuprates. The average orientation angles obtained from fittings of the Mössbauer spectra were in accordance with the results from the x-ray-diffraction measurements.

We consider an extended one-dimensional Hubbard model in which the magnitude of the hopping between two sites for particles with given spin depends on the occupation of the states with opposite spin at both sites. Diagonalizing exactly finite-size chains, and using known results of conformal field theory we delimit the regions of parameters for which two particles bind and the pair superconducting correlation functions are the dominant ones at large distances. For Coulomb repulsion U smaller than a critical density-dependent value ${\mathit{U}}_{\mathit{c}}$ and any density, there are ranges of the ratios of the hopping parameters for which the superconducting fluctuations dominate. At half filling, for parameters within this range, a transition from a regime with dominant superconducting correlations to an insulating state takes place as a function of U. We also study the model for parameters near a recently found exactly solvable limit in which the number of doubly occupied sites is conserved.

We use a one-component description to analyze the current experimental situation for the low-frequency magnetic properties of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$ as determined by NMR and neutron-scattering experiments. We show that the measured ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ spin-lattice relaxation rate is in sharp conflict with the incommensurate-magnetic-structure interpretation of neutron-scattering experiments, but is quantitatively explained if the local-spin-fluctuation spectrum (measured by NMR) possesses a commensurate peak. We conclude that the formation of domains, as suggested by Slichter and Phillips, represents the best (and, quite possibly, only) way of reconciling NMR and neutron-scattering experiments on ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$.

A center in thermochemically reduced CaO crystal associated with nitrogen has been characterized with electron paramagnetic resonance. The electron paramagnetic resonance spectrum of this center displays axial symmetry with a threefold hyperfine splitting. The spin-Hamiltonian parameters for this center are S=1/2, I=1, ${\mathit{g}}_{\mathrm{\perp }}$=1.9702(2), ${\mathit{g}}_{\mathrm{\parallel }}$=1.9436(2), ‖${\mathit{A}}_{\mathrm{\perp }}$‖=5.4(1) MHz, and ‖${\mathit{A}}_{\mathrm{\parallel }}$‖=3.0(1) MHz, with symmetry axis along the 〈100〉 direction. A plausible candidate for this center is the [${\mathit{F}}_{\mathit{N}}$${]}^0$ center—an ${\mathit{F}}^{+}$ center next to a nitrogen substitutional impurity. This center is stable from 13 to 140 K.

The effect of frustration on magnetic systems has been largely discussed in the case of localized moments but only recently its consequences close to a magnetic instability have been addressed. Particularly interesting are the ordered mixed phases, with coexistence of magnetic and nonmagnetic sites observed in R${\mathrm{Mn}}_2$ (R=Dy, Th, Tb), CeSb, and ${\mathrm{PrCo}}_2$${\mathrm{Si}}_2$ compounds. A model derived from the Hubbard Hamiltonian has been shown to describe these systems. Here, we discuss the crucial role of the anisotropy for the stabilization of these mixed phases. We show that they can also be induced by the application of a magnetic field.

In the Ruderman-Kittel-Kasuya-Yosida (RKKY) approach to oscillatory exchange coupling in magnetic multilayers, the oscillation period may be determined from the bulk spacer Fermi surface by inspection. In the quantum-well theory of exchange coupling this is only possible for simple cases and no such recipe exists for fcc (111) multilayers, for example. The correct procedure for this fcc (111) case, which is relevant to the interesting Co/Cu (111) system, is established. The subtle manner in which the RKKY period is recovered in this case is explained. The equality of the RKKY and quantum-well periods is proved for any single-band tight-binding model with hopping normal to the layers restricted to nearest-neighbor planes.

Fe/Si multilayers with thin Si layers have given evidence of both structural coherence over many bilayer distances and magnetic coupling between the Fe layers. The presence of Fe silicides, inferred from several different properties, has been suggested as the origin of these effects. We have done transmission electron-diffraction measurements on Fe/Si samples exhibiting structural coherence and magnetic coupling in an attempt to directly observe these silicides. No evidence of known Fe silicides was found.

The temperature dependence of magnetic anisotropy in cobalt films grown on Cu(1 1 17) has been studied by means of the magneto-optic Kerr effect. An in-plane uniaxial anisotropy is found in the as-grown films. At elevated temperatures the films exhibit changes in anisotropy. At temperatures around 100 °C a transition to nearly biaxial behavior is found which transforms again into uniaxial behavior at higher temperatures. Both transitions generate pronounced secondary maxima in the temperature-dependent susceptibility.

Recent neutron-scattering experiments on ${\mathrm{La}}_{1.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_4$ find an isotropic but incomplete suppression of the scattering intensity below ${\mathit{T}}_{\mathit{c}}$. Here we show that for a ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$-wave superconductor with a strong enhancement of the spin-fluctuation scattering and a proper choice of the model parameters, this is the type of behavior that is expected.

We report measurements of the third-sound frequency and attenuation in a capacitive resonator as a function of temperature for various submonolayer densities of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ (0.007≤${\mathit{n}}_3$≤0.049 A${\mathrm{̊}}^{\mathrm{-}2}$) in a thin ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ film (${\mathit{n}}_4$=0.389 A${\mathrm{̊}}^{\mathrm{-}2}$), which result in a determination of the ground-state and first-excited-state energies for the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ which are in good agreement with the energetics determined by NMR. Interesting features are seen in the temperature dependence of the frequency and phase of the third sound for 100<T<200 mK.

In this paper, we discuss the physical meaning of pseudospin symmetry, which is actually a kind of particle-hole rotation invariance. In an isotropic state with the total pseudospin S̃=0, by using the invariance of the pseudospin rotation in space, we shall establish a relation between off-diagonal long-range order (ODLRO) and charge-density waves (CDW). This result indicates that ODLRO and CDW must coexist in such an isotropic state, which is usually called a supersolid. Finally, we present two examples to show that a supersolid exists.

In previous work estimates showed that observed static deformations of their crystal lattices into soft modes can explain observed high values of ${\mathit{T}}_{\mathit{c}}$ in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. In a different calculation dealing with conventional superconductivity an attempt was made to derive the Ginzburg-Landau (GL) equations from microscopic BCS wave functions. Combining these developments, the origin of the anomalously high values of the GL coefficients α and β characterizing high-${\mathit{T}}_{\mathit{c}}$ materials is indicated here. Good agreement with experiments for GL quantities ξ and ${\lambda }_{\mathit{L}}$ is obtained for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$.

Exact diagonalization calculations show a continuous transition from delocalized to polaron behavior as a function of intersite electron-lattice coupling. A transition, found previously at the Hartree-Fock level between a magnetic and a nonmagnetic state, does not subsist when fluctuations are included. Local phonon modes become softer close to the polaron and by comparison with optical measurements of doped cuprates we conclude that they are close to the transition region between polaronic and nonpolaronic behavior. The barrier to adiabatically move a hole vanishes in that region suggesting large mobilities.

In situ Raman scattering, ultraviolet-photoemission-spectroscopy (UPS), and electron-energy-loss-spectroscopy (EELS) measurements have been performed on crystalline films of ${\mathrm{RbC}}_{60}$ through the fcc-to-orthorhombic structural phase transition. The Raman spectra exhibit large changes in intensity of modes not active for isolated icosahedra, including ungerade vibrations. This is attributed to a substantial increase in interfullerene coupling in the presence of ${\mathrm{C}}_{60}$ merohedral disorder. UPS measurements indicate a finite density of states at the Fermi energy for both phases with small changes in the ${\mathrm{C}}_{60}$ derived states through the transition. Low-energy metalliclike excitations are observed in EELS.

Diffusion in an interacting adsorbate is analyzed with the use of a master equation for the lattice-gas model. The coefficient of chemical diffusion for the diluted, interacting adsorbate is calculated using concepts of diffusion theory in a disordered medium in which the random potential is related to the local field that affects each particle due to the particle interactions. We use the random-local-field method to evaluate the distribution of local fields in the equilibrium state. Results obtained for the simplest, interacting lattice gas are successfully compared with Monte Carlo simulations.

An ab initio theory of interface-interface interactions in metallic magnetic multilayers is discussed based on a surface-Green’s-function formalism that reflects the underlying two-dimensional translational symmetry. Due to the short-range character of the interlayer interactions in the tight-binding linear muffin-tin orbital method, the problem can be conveniently formulated in real space. In particular, a model consisting of two magnetic Co(001) slabs of varying thickness in a nonmagnetic fcc Cu spacer is considered, for which a pronounced suppression of the long-period oscillations was found with increasing thickness of the magnetic slabs.

An L×∞ system of an odd number of coupled Heisenberg spin chains is studied using a degenerate perturbation theory, where L is the number of coupled chains. An effective chain Hamiltonian is derived explicitly in terms of two spin-1/2 degrees of freedom of a closed chain of L sites, valid in the regime where the interchain coupling is stronger than the intrachain coupling. The spin gap has been calculated numerically using the effective Hamiltonian for L=3, 5, 7, and 9 for a finite chain up to ten sites. It is suggested that the ground state of the effective Hamiltonian is correlated, by examining various variational trial states for the effective-spin chain Hamiltonian.

Extensive histogram Monte Carlo simulations of the XY antiferromagnet on a stacked triangular lattice reveal exponent estimates that strongly favor a scenario of mean-field tricritical behavior for the spin-order transition. The corresponding chiral-order transition occurs at the same temperature but appears to be decoupled from the spin order. These results are relevant to a wide class of frustrated systems with planar-type order and serve to resolve a long-standing controversy regarding their criticality.

We discuss measurements of the ac complex conductance of a-MoGe films in the presence of a perpendicular magnetic field. The ac response of vortices in samples with weak pinning reveals an anomaly that is found to be consistent with a dislocation mediated melting in the two-dimensional vortex lattice on short length scales. However, increasing the effects of disorder on the vortex lattice by reducing the sample thickness leads to dominance of thermal-activated creep over melting. With increasing disorder, the length scale associated with the correlations in the vortex lattice is reduced and eventually the melting phenomenon disappears.

The excess low-frequency (1/f) noise of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin-film microbridges has been studied in the normal state. The normalized noise-power spectral density, ${\mathit{S}}_{\mathit{V}}$/${\mathit{V}}^2$, shows quantitatively different behavior depending on the film-substrate lattice mismatch and the oxygen content. The noise behavior is largely attributable to the hopping of basal-plane oxygen vacancies subject to strain-dependent activation barriers. Both electromigration biases and oxygen plasma annealing can be effective techniques for lowering these noise levels, but in different ways that point to the origin of the noise. The low-temperature, ∼100 K, noise behavior, indicates that even films with high critical current densities have a substantial density of oxygen-defect states with very low activation energies for hopping.