The out-of-plane orbital magnetic moment of Co in Co/X multilayers (X=Pd, Pt, and Ni) with perpendicular magnetic anisotropy has been investigated using x-ray magnetic-circular-dichroism spectroscopy. The orbital d moment per Co atom, ${\mathit{m}}_{\mathrm{orb}}^{\mathrm{\perp }}$, is considerably enhanced by (0.15±0.02)${\mathrm{\mu }}_{\mathit{B}}$ and (0.07±0.02)${\mathrm{\mu }}_{\mathit{B}}$ in Co/Pd and Co/Pt multilayers, respectively, compared with a single thick Co film. No respective enhancement is found in the case of Co/Ni. This enhancement does not scale with the spin-orbit coupling parameter ${\xi }_{\mathrm{SO}}$ of Ni-3d, Pd-4d, and Pt-5d bands despite strong d-d mixing which leads to the presence of ferromagnetic moments of Pd and Pt atoms. However, a roughly linear scaling relation of the out-of-plane orbital moment ${\mathit{m}}_{\mathrm{orb}}^{\mathrm{\perp }}$ with the intrinsic uniaxial anisotropy constant ${\mathit{K}}_{\mathit{u},1\mathrm{}}^{\mathrm{Co}}$ per Co volume is observed. This experimental finding is interpreted in conjunction with recent ab initio and tight-binding band-structure calculations as evidence for a preferential enhancement of the out-of-plane orbital moment of Co in Co/Pd and Co/Pt multilayers.

We introduce an xy generalization of the frustrated Ising model on a triangular lattice. The presence of continuous degrees of freedom stabilizes a finite-temperature spin state with power-law discrete spin correlations and an extensive zero-point entropy. In this phase, the unquenched degrees of freedom can be described by a fluctuating surface with logarithmic height correlations. Finite-size Monte Carlo simulations have been used to characterize the exponents of the transition and the dynamics of the low-temperature phase.

We compute high-temperature expansions of the three-dimensional Ising model using a recursive transfer-matrix algorithm and extend the expansion of the free energy to 24th order. Using inhomogeneous-differential Padé and ratio methods, we extract the critical exponent of the specific heat to be α=0.104(4).

A spin-field model in two space and one time dimensions is proposed and investigated using a bilinearization technique. This model, which can be considered as a modified version of the Ishimori system, allows a Hamiltonian formulation, a symmetry algebra of Kac-Moody type with a loop-algebra structure, and the conformal invariance property. Many nonlinear excitations are found, which turn out to be of the helical and roton type, meronlike configurations endowed with a fractional topological charge, radially symmetric solutions, and domain walls. Our results are compared with those of the study of the O(3) nonlinear σ model and the Ishimori system. Finally, the physical role of both our model and the Ishimori system is discussed in the light of certain reduced equations which find applications in the theory of vortex filaments in liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and in dealing with the two spin-correlation functions of the two-dimensional Ising model in the scaling limit.

We examine the sign reversal of the vortex Hall effect in MoGe/Ge superconductor-insulator multilayers. To specifically examine the relevance of thermally generated Josephson vortices between the layers, we compare our results with single-layer two-dimensional superconducting MoGe films. While the extent of the Hall sign reversal regime does depend upon the interlayer vortex coupling, we conclude that Josephson vortices are not the dominant contribution to the Hall sign reversal for MoGe/Ge multilayers. Our results differ from recent suggestions for Y-Ba-Cu-O single crystals.

We observe a sign reversal in the Hall resistivity ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$ of the conventional superconductor amorphous (a) ${\mathrm{Mo}}_3$Si. In the Ohmic regime, ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$ is qualitatively similar to that observed in the high-${\mathit{T}}_{\mathit{c}}$ superconductors. It changes sign near ${\mathit{T}}_{\mathit{c}}$, and the sign change persists until both ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$ and ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$ become immeasurably small at T∼0.8${\mathit{T}}_{\mathit{c}}$(H). At current densities above the depinning current density, the Hall anomaly persists at low temperatures T∼0.2${\mathit{T}}_{\mathit{c}}$(H). This is contrary to a theory by Ferrell which attributes the anomaly to the backflow of thermally excited quasiparticles. In addition a model proposed by Harris, Ong, and Yan explains the anomaly as an effect arising from the layered nature of the high-${\mathit{T}}_{\mathit{c}}$ cuprates. This model, however, does not explain the anomaly in a-${\mathrm{Mo}}_3$Si which is an isotropic unlayered material.

The interaction between an external microwave field and a system of weakly coupled superconducting grains is considered. It is demonstrated that intensive fields can substantially modify the intergrain Josephson coupling leading to additional residual resistance and absence of a coherence peak in granular superconductors.

We discuss the effects of a weak random external potential on the properties of the dilute Bose gas at zero temperature. The results recently obtained by Huang and Meng for the depletion of the condensate and of the superfluid density are recovered. Results for the shift of the velocity of sound as well as for its damping due to collisions with the external field are presented. The damping of phonons is calculated also for dense superfluids.

We have performed measurements on one-dimensional (1D) arrays of underdamped Josephson junctions connected in parallel by superconducting wires which in a magnetic field show resonances in the current-voltage characteristics. From analytical calculations, we find that the position of the steps is determined by the dispersion relation ω(k) of a 1D discrete, linear transmission line, where the Josephson junction has been replaced by the capacitor and the resistance of the junction. When nearest-neighbor inductances are accounted for, the model agrees well with the measured positions of the steps. Numerical calculations of the full dynamics with inclusion of inductances between all cells support our model. As an example of a system with a more complicated dispersion relation, we have measured 1D arrays with two different junctions per primitive cell. The measured dispersion relation shows an acoustic and an optical branch in agreement with numerical simulations of the full dynamics.

The low-energy quasiparticle scattering and recombination lifetimes for a proximity sandwich of two superconductors S and S’ with different bulk energy gaps, are calculated as a function of the spatial coordinate and temperature. The spatial dependence of the order parameter and density of states are calculated on the basis of a microscopic model of the proximity effect, based on the Usadel equations, for dirty superconductors in thermal equilibrium. A zero boundary resistance between S and S’ and a Boltzmann-like energy distribution of the excess quasiparticles are assumed. In the case of a small diffusion time constant an effective quasiparticle relaxation rate into and excitation rate out of the reduced gap region in the SS’ sandwich are obtained as a function of (finite, but low) temperature and strength of the proximity effect, determined by the parameter ${\gamma }_{\mathit{m}}$, by averaging over the energies and positions of the quasiparticles. In the same way effective tunneling times for electrons and holes tunneling out of the trap in the SS’ sandwich to the other electrode of an SS’IS’’S junction are determined as a function of temperature, voltage, and ${\gamma }_{\mathit{m}}$.

We report direct experimental observations of the Josephson properties of Nb/Cu multilayers achieved by measurements across layers. Multilayered sandwiches with a small cross section (20 μm in diameter) consisting of 10 Nb/Cu/Nb junctions in series were fabricated for such measurements. Applying RF power we observed Shapiro steps in the current-voltage characteristics. Changing the temperature we have seen the sequential change in the principal Shapiro-step voltage representing the change in the number of the phase-locked junctions. This phenomenon is direct evidence for the dimensional 3D-2D crossover responsible for the subdivision of the sample in the individual layers. Simultaneously with the change of the RF step voltage, the behavior of the I-V characteristics change. For T<${\mathit{T}}_{2\mathrm{D}}$ hysteresis appears in the I-V characteristics and the slope of the temperature dependence of the Josephson critical current, ${\mathit{I}}_{\mathit{c}}$(T) , changes.

The self-consistent equations, which have been derived recently as a microscopic model for the crossover between BCS superconductivity and Bose-Einstein condensation in a three-dimensional interacting Fermi system [R. Haussmann, Z. Phys. B 91, 291 (1993)], are solved numerically by repeated Fourier transformation. We find a superfluid transition temperature ${\mathit{T}}_{\mathit{c}}$ which increases monotonically with increasing attractive coupling strength. Furthermore, we determine the chemical potential μ, the fermion distribution function n(k), and the complex effective mass 2${\mathit{m}}^{\mathrm{*}}$ of the fermion pairs at T=${\mathit{T}}_{\mathit{c}}$. The bound fermion pairs cause a power-law tail ∼${\mathit{k}}^{\mathrm{-}4}$ in n(k) for large k and behave as short-living quasiparticles in the crossover region, which is indicated by a large imaginary part of 2${\mathit{m}}^{\mathrm{*}}$.

Magnetization curves of high-quality superconducting ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{y}}$ single crystals in fields oriented parallel to the c axis have been reported at various temperatures. The irreversibility line and magnetic relaxation of this single crystal have been measured in magnetic fields parallel to the c axis of the crystal. We have observed a low-field anomaly in the magnetization together with a sharp rise of the irreversibility line and a nonlogarithmic magnetic relaxation at 25 K. We argue that 25 K is a boundary temperature where the system undergoes a dimensional crossover from the three-dimensional (3D) to the 2D regime. The relaxation rate also predicts that although the flux creep above this crossover temperature follows the conventional Kim-Anderson thermally activated flux-creep model, below this crossover temperature it follows that predicted by the collective flux pinning or vortex-glass models.

The muon-spin-relaxation (μSR) technique has been used to study the internal field modulation dynamics of fluxons in crystals of κ-[BEDT-TTF${]}_2$Cu[NCS${]}_2$ (${\mathit{T}}_{\mathit{c}}$≊10.5 K). The data exhibit a strong field dependence for applied fields in the range 129 Oe≤‖${\mathbf{H}}_{\mathrm{ext}}$‖≤4 kOe, which is attributed to a combination of vortex motion, finite core size effects, and pinning. In low fields (‖${\mathbf{H}}_{\mathrm{ext}}$‖=129 Oe), the data exhibit a flux-pinning transition at ${\mathit{T}}_{\mathit{x}}$∼5 K, as evidenced by a peak in the second moment of the local field distribution at ∼5 K accompanied by an enhanced minimum in the first moment at the same temperature. In higher fields (‖${\mathbf{H}}_{\mathrm{ext}}$‖=750 Oe, 3 kOe, and 4 kOe), where the effects of motion and pinning are minimized, the symmetry of the underlying gap function is confirmed as s wave. Corrections for finite fluxon core, finite mean free path, and flux-lattice disorder were applied in determining the London penetration depth, ${\lambda }_{\mathit{L}}^{\left(\mathit{b}\mathit{c}\right)}$(0)=7680±700 Å.

A simple model for electron-vibron interactions on charged fullerenes ${\mathrm{C}}_{60}^{\mathit{n}\mathrm{-}}$, n=1,...,5, is solved both at weak and strong couplings. We consider a single ${\mathit{H}}_{\mathit{g}}$ vibrational multiplet interacting with ${\mathit{t}}_{1\mathit{u}}$ electrons. At strong coupling the semiclassical dynamical Jahn-Teller theory is valid. The Jahn-Teller distortions are unimodal for n=1,2,4,5 electrons, and bimodal for 3 electrons. The distortions are quantized as rigid-body pseudorotators which are subject to geometrical Berry phases. These impose ground-state degeneracies and dramatically change zero-point energies. Exact diagonalization shows that the semiclassical level degeneracies and ordering survive well into the weak-coupling regime. At weak coupling, we discover an enhancement factor of 5/2 for the pair binding energies over their classical values. This has potentially important implications for superconductivity in fullerides, and demonstrates the shortcoming of Migdal-Eliashberg theory for molecular crystals.

The ground-state energy shifts and excitation spectra of charged fullerenes ${\mathrm{C}}_{60}^{\mathit{n}\mathrm{-}}$, n=1,...,5 are calculated. The electron-vibron Hamiltonian of part I is extended to include all ${\mathit{A}}_{\mathit{g}}$ and ${\mathit{H}}_{\mathit{g}}$ modes with experimentally determined frequencies and theoretically estimated coupling constants. Complex splitting patterns of ${\mathit{H}}_{\mathit{g}}$ vibrational levels are found. Our results are relevant to EPR measurements of spin splittings in ${\mathrm{C}}_{60}^{2\mathrm{-}}$ and ${\mathrm{C}}_{60}^{3\mathrm{-}}$ in solution. Spectroscopic gas-phase experiments will be of interest for further testing of this theory. As found in part I, degeneracies in the electron and vibron Hamiltonians give rise to a dynamical Jahn-Teller effect, and to a considerable enhancement of the electronic pairing interaction. This helps to overcome repulsive Coulomb interactions and has important implications for superconductivity in ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and the insulating state in ${\mathrm{K}}_4$${\mathrm{C}}_{60}$.

The quasiparticle excitation spectrum of a type-II superconductor placed in high magnetic field near ${\mathit{H}}_{\mathit{c}2}$(T) is shown to be gapless. The gap turns to zero at the points in the magnetic Brillouin zone that are in correspondence with the vortex lattice in real space. When the field decreases below a certain critical value, branch crossings occur and gaps start opening up at the Fermi surface. The strong dispersion around the gapless points leads to an algebraic temperature dependence in the thermodynamic functions and the algebraic voltage dependence in the tunneling conductance between the microscope tip and superconductor in a scanning tunneling microscope experiment. The crossover from the localized midgap states at the core of an isolated vortex to the coherent quasiparticle band at high fields is reflected in a qualitative change of differential conductance.

A change of the I-V characteristics of a long Josephson junction, operating in the zero-field single-fluxon regime, under the action of a ‘‘hot spot’’ (e.g., created by a focused electron beam) is calculated analytically by means of the perturbation theory, and also investigated numerically. The change of the average voltage at a given value of the bias current is calculated as a function of the hot spot-position. The overlap Josephson junction geometry is considered in detail, while the inline one is briefly discussed. A good accord between analytical and numerical results is found. The results are relevant for the interpretation of the low-temperature scanning electron microscopy experiments on imaging the fluxon dynamic states in a long Josephson junction.

We study the competition, for a single flux line, between pinning by bulk point randomness and an extended defect, such as an ion track, dislocation line, or twin plane. In three dimensions, there is an unbinding transition for a linear (but not a planar) defect. This transition is analyzed using Flory, Migdal-Kadanoff, functional renormalization-group, and numerical methods. The localization length, describing the typical transverse separation between the vortex line and defect, diverges like ${\mathit{l}}_{\mathrm{\perp }}$∼‖T-${\mathit{T}}_{\mathit{c}}$${\mathrm{\Vert }}_{\mathrm{\perp }}^{\mathrm{-}\nu }$, with ${\nu }_{\mathrm{\perp }}$≊1.4. We predict the effects of this localization length on the I-V characteristics of the superconductor, and suggest experiments to observe the transition.

We analyze an effective action of bosons moving in a fluctuating gauge field, which describes the motion of the holons in the t-J model. The analysis is in terms of a dimensionless coupling constant g∼24πt/J. We use a path-integral approach, which is a convenient method in the strong-coupling limit g≫1. The effect of the gauge field is to suppress boson loops that enclose a large area, so that the main contribution comes from almost self-retracing paths. By taking a quenched average over the gauge field we are able to derive a renormalized density of states that has a shifted band edge, and that is enhanced near the band edge. We also find an expression for the diamagnetic susceptibility ${\mathrm{\chi }}_{\mathit{B}}$, that in the strong-coupling limit varies like ${\mathrm{\chi }}_{\mathit{B}}$∼(gT${)}^{\mathrm{-}1}$, and is suppressed by a factor 0.2g compared to the weak-coupling limit. The enhanced density of states and the suppression of ${\mathrm{\chi }}_{\mathit{B}}$ both imply that Bose condensation is suppressed by the fluctuating gauge field. At lower temperatures the retracing path approximation breaks down, and one has to do a self-consistent calculation to determine ${\mathrm{\chi }}_{\mathit{B}}$. A numerical calculation shows that below ${\mathit{T}}_{\mathit{B}\mathit{E}}$∼0.08${\mathit{T}}_{\mathit{B}\mathit{E}}^0$ the susceptibility ${\mathrm{\chi }}_{\mathit{B}}^{\mathrm{SC}}$ diverges exponentially.