It is shown that the orbital response of a two-dimensional electron gas in a periodic potential to a weak magnetic field is always paramagnetic when the Fermi level is sufficiently close to a saddle point of the band structure. Another instance of orbital paramagnetism is found when the Fermi level is close to a narrow indirect gap.

Systems whose dynamics are described by a quasilogarithmic or stretched-exponential time dependence are usually fitted by models which use disorder to create a distribution of relaxation times. Here we describe a model which decays slowly towards equilibrium but does not require disorder to provide the slow dynamics. The model consists of a spin system with the spins interacting via the dipole-dipole interaction. The model is able to replicate the more pronounced features observed in the magnetization decay of magnetic systems and high-temperature superconductors.

A recently developed formalism is used to calculate the saturation magnetization and temperature-dependent, exchange-enhanced Pauli susceptibility of paramagnetic and weakly ferromagnetic Pd-Ni alloys. The effects of quantum and thermal spin fluctuations are included in alloy calculations for the first time, through the use of a semiphenomenological theory consistent with the results of renormalization-group theory for the single-impurity problem. Two different cluster methods are used, both of which yield results superior to those of all previous theories and in good agreement with experiment.

We consider the possibility of an odd-parity singlet pairing in the positive-U Hubbard model, which naturally incorporates short-ranged antiferromagnetic correlations in the ground state. In a quantum Monte Carlo simulation of a nearly half-filled Hubbard model, we find that the repulsive U enhances the tendency for this type of pairing more strongly than for other pairing channels considered earlier. We propose a BCS-type wave function for this pairing where the gap has no nodes over the Fermi surface, and discuss various experimental consequences in connection to high-${\mathit{T}}_{\mathit{c}}$ superconductivity.

A short laser pulse breaks Cooper pairs and photoexcites electrons in a superconducting film of niobium. As the quasiparticles recombine and rejoin the condensate, changes in the electrodynamic response of the sample are detected by a fast oscilloscope. The observed biexponential decay is consistent with a previous measurement. The fast recovery time is used to obtain the pair recombination time, which is in good agreement with theory.

The system of superconducting layers with Josephson coupling J is studied. The ordering temperature ${\mathit{T}}_{\mathit{f}}$ for the fluxon loops parallel to the layers is shown to determine the transition temperature ${\mathit{T}}_{\mathit{c}}$ of the system, except when J is exponentially small; in the latter case ${\mathit{T}}_{\mathit{c}}$ drops towards the J=0 vortex transition, accounting for data on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ superlattices. A magnetic field parallel to the layers produces a sequence of phases in which the induced fluxon lines are l layers apart. For l>8 nd J not too small, phases with two-dimensional fluxon correlations occur near ${\mathit{T}}_{\mathit{c}}$; this accounts for the observed V∼${\mathit{l}}^{\mathit{a}\left(\mathit{T}\right)}$ current-voltage relation in high-${\mathit{T}}_{\mathit{c}}$ superconductors.

We have performed a high-precision measurement (5×${10}^8$ coincidence counts) of the basal-plane electron-positron momentum density in well oxygenated, twin-free, single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The raw, processed, and K-space reduced spectra unambiguously show a clear image of a major Fermi surface sheet. The form and profile of that image are in substantial quantitative agreement with theoretical predictions of a Γ-X electron ridge section associated with states in the CuO chains.

We present a new theoretical approach that permits the rf surface impedance ${\mathit{Z}}_{\mathit{s}}$ of type-II superconductors to be calculated over a wide range of angular frequencies ω, magnetic inductions B, and temperatures T. Our approach, based on a self-consistent treatment of vortex dynamics, includes the influence of both vortex pinning and flux creep upon ${\mathit{Z}}_{\mathit{s}}$ in a unified manner, by making use of continued-fraction-expansion results for the dynamic mobility of a particle undergoing Brownian motion in a periodic potential.

The intrinsic pinning in twinned and untwinned single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is observed using a novel crossed-magnetic-field technique for obtaining angular resolutions of Δθ<0.005°. The pinning occurs for magnetic-field directions within a critical angle ${\mathrm{\theta }}^{\mathrm{*}}$<1° of the Cu-O planes. The data are discussed in the context of a lock-in transition of vortices in the a-b plane of the crystal.

We present a microscopic model of a granular pile submitted to vibration, emphasizing the competing roles of collective and single-particle excitations which lead to specific characteristics of the resulting state. The effect of vibration on a powder is next investigated by a novel computer-simulation approach in three dimensions, which enables us to probe cooperative in the bulk; satisfying agreement with the predictions of the model is obtained.

The induced electrorheological solid has thick columns in the field direction (z axis), spreading between two electrodes. The ground state is proposed to be a body-centered tetragonal lattice with ${\mathbf{a}}_1$= √6 ax^, ${\mathbf{a}}_2$= √6 ay^, and ${\mathbf{a}}_3$=2az^, where a is the radius of dielectric spheres. This bct lattice can be regarded as a compound of chains of class A and B, where chains B are obtained from chains A by shifting a distance a in the z direction.

One-dimensional critical quantum sytems have a universal, intensive ‘‘ground-state degeneracy,’’ g, which depends on the universality class of the boundary conditions, and is in general noninteger. This is calculated, using the conjectured boundary conditions corresponding to a multichannel Kondo impurity and shown to agree with Bethe-ansatz results. g is argued to decrease under renormalization from a less stable to a more stable critical point and plays a role in boundary critical phenomena quite analogous to that played by c, the conformal anomaly, in the bulk case.

We study properties of interfaces between stationary phases of the two-dimensional discrete-time Toom model (north-east-center majority vote with small noise): phases not described by equilibrium Gibbs ensembles. Fluctuations in the interface maintained by mixed boundary conditions grow with distance much slower than in equilibrium systems; they have exponents close to 1/4 or 1/3, depending on symmetry, rather than 1/2, and have long-range correlations reminescent of self-organized critical behavior. Approximate theories reproduce this behavior qualitatively and lead to novel nonlinear partial differential equations for the asymptotic profile.

Models with many technifermion doublets have enhanced interactions. The one-family technicolor model, which has four doublets, may well have a scalar bound state that resembles a Higgs boson. Using a bubble summation, which is justified in the limit of many isodoublets of technifermions, the binding energy and the decay width of the state can be calculated as a function of the technipion mass. Its partial width into WW and ZZ is of order 10–40 GeV, while the partial width into gluon-gluon is about 0.4 GeV. As a result, this surrogate Higgs boson would be narrow, but copiously produced at very-high-energy hadron colliders.

The tensor polarization ${\mathit{t}}_{20}$ of the recoil deuteron in elastic e-d scattering has been measured for three values of four-momentum transfer, q=3.78, 4.22, and 4.62 ${\mathrm{fm}}^{\mathrm{-}1}$. The data have been used to locate the first node in the charge monopole form factor of the deuteron at q=4.39±0.16 ${\mathrm{fm}}^{\mathrm{-}1}$. The results for ${\mathit{t}}_{20}$ are in reasonable agreement with expectations based on the nucleon-meson description of nuclear dynamics.

A calcium atomic beam excited in an optical Ramsey geometry was rotated about an axis perpendicular to the plane defined by the laser beams and the atomic beam. A frequency shift of the Ramsey fringes of several kHz has been measured which is proportional to the rotation frequency of the apparatus and to the distance between the laser beams. The results can be interpreted in three equivalent ways as the Sagnac effect in a calcium-atomic-beam interferometer: in the rotating frame of the laser beams either along straight paths or along the curved trajectories of the atoms, or in the inertial atomic frame.

The mechanical effects of stimulated Raman transitions on atoms have been used to demonstrate a matter-wave interferometer with laser-cooled sodium atoms. Interference has been observed for wave packets that have been separated by as much as 2.4 mm. Using the interferometer as an inertial sensor, the acceleration of a sodium atom due to gravity has been measured with a resolution of 3×${10}^{\mathrm{-}6}$ after 1000 sec of integration time.

Distorted-wave calculations of the triply differential cross sections for electron-impact ionization of H and He targets are presented for final-state electrons sharing 4-eV excess energy and leaving in opposite directions. The experimentally observed target dependence of the angular distributions is shown to stem essentially from short-range effects on the s-wave phase shifts of both incident and final-state continuum electrons.

We have developed an exact second-order S-matrix code for the relativistic numerical calculation of cross sections for Compton scattering of photons by bound electrons within the independent-particle approximation. We find good agreement with less exact treatments in regions where such theories are valid, recovering the impulse approximation and the soft-photon infrared divergence. We do not find agreement with an earlier calculation of Whittingham. We compare with recent scattering experiments in the regime Zα(ħ${\mathrm{\omega }}_{\mathit{i}}$)∼${\mathit{E}}_{\mathit{B}}$, where we find that the impulse approximation is not adequate.

We report experimental evidence that the scattering of sound waves by a fluid may be used, as a nonintrusive and nonlocal method, to characterize the space-time structure of the flow. The experiment has been performed using, as a test flow, the von Kármán vortex street behind a cylinder at a low Reynolds number (Re=50). The results are in good qualitative agreement with earlier experiments on the von Kármán vortex street and with recent theoretical developments on sound-vorticity interaction.