The resonant tunneling through an impurity level between the edges of a Hall bar is studied based on the theory of edge excitations. We find that the peak tunneling current scales with the potential difference between the edges ${\mathit{V}}_{\mathit{R}}$-${\mathit{V}}_{\mathit{L}}$=2Δ as ${\mathit{I}}_{\mathit{t}}$∼${\mathrm{\Delta }}^{\mathit{g}\mathrm{-}1}$ at low temperatures (T≪${\mathit{e}}^{\mathrm{*}}$Δ/2) and as ${\mathit{I}}_{\mathit{t}}$∼Δ${\mathit{T}}^{\mathit{g}\mathrm{-}2}$ at high temperatures (T≫${\mathit{e}}^{\mathrm{*}}$Δ/2), with the exponent g being intimately related to the propagator of the charge carriers. These results can be used to experimentally probe internal structures of the fractional quantum Hall states by measuring g.

A new direct experimental technique to study the form of the hot electron distribution function is presented. Far infrared emission induced by a weak periodic potential is observed in high-mobility GaAs/AlGaAs heterostructures with extremely low electron density. The electric-field-dependent spectra give for the first time evidence of a nonequilibrium shape of the electronic distribution in the electric field direction. The onset of LO-phonon emission is clearly observed. Perpendicular to the applied electric field, a smooth form of the distribution is found. The results agree well with theoretical distribution functions obtained from the Boltzmann transport equation.

We determine the phase diagram of a quasi-one-dimensional superconductor (weakly coupled chains system with an open Fermi surface) in a magnetic field. The usual Ginzburg-Landau regime is followed, when the field is increased, by a cascade of superconducting phases separated by first-order transitions, which ends in a strong reentrance of the superconducting phase. These new phases show a novel kind of symmetry of a laminar type. The Zeeman splitting does not completely suppress the reentrance in very strong field, the ground state being in this case a Larkin-Ovchinnikov-Fulde-Ferrell state.

Flow behavior of a flux line in the layered superconductor 2H-${\mathrm{NbSe}}_2$ is studied with a magnetic field parallel to the layers in the vicinity of a pronounced peak effect. A striking crossover in the current-voltage characteristics is observed as the system enters the peak regime. The results yield a nonequilibrium phase diagram where conventional depinning of an elastic medium occurs for a rigid lattice. A defective (plastic) flow instability occurs as the lattice softens, but heals at large drives. This defective flow dominates the dynamics for a very soft lattice.

In the new high-${\mathit{T}}_{\mathit{c}}$ superconductors both thermal as well as quantum fluctuations are large. The latter smear the vortex cores over a distance of the order of the coherence length and thereby strongly affect the melting transition. Taking account of quantum fluctuations, the shape of the melting line is given by a universal function which is different from a simple power law in 1-T/${\mathit{T}}_{\mathit{c}}$ and is in excellent agreement with recent experiments on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals. Above a crossover field of the order of a few tesla the resulting high-temperature phase is a quantum liquid.

The anomalous midinfrared absorption band in the optical conductivity of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ and ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4+\mathrm{\delta }}$ is interpreted in terms of photon-assisted hopping of small polarons. An analysis of the normal-state single crystal a-b plane data indicates that polarons in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4+\mathrm{\delta }}$ are small, whereas in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ the polarons are larger. The absence of bulk superconductivity in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4+\mathrm{\delta }}$ may be linked with smaller polaron size.

We report far-infrared transmission measurements on single crystal samples derived from ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$. The impurity scattering rate of the samples was varied by electron-beam irradiation, 50 MeV ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}_{}^{+6\mathrm{}}{}_{}{}^{}$ ion irradiation, heat treatment in vacuum, and Y doping. Although substantial changes in the infrared spectra were produced, in no case was a feature observed that could be associated with the superconducting energy gap. These results all but rule out ‘‘clean limit’’ explanations for the absence of the spectroscopic gap in this material, and provide evidence that the superconductivity in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ is gapless.

Controlled variations of the in-plane Cu-O bond length and interlayer proximity coupling have been produced in laser-ablated superlattices containing superconducting ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$ (LSCO). In LSCO/${\mathrm{Sm}}_2$${\mathrm{CuO}}_4$ strained superlattices, the in-plane lattice constant a of the LSCO layers is increased and the transition temperature (${\mathit{T}}_{\mathit{c}}$) is decreased. We estimate the effective pressure to be -8 GPa by comparing with LSCO/${\mathrm{La}}_2$${\mathrm{CuO}}_4$ unstrained superlattices. We conclude the in-plane Cu-O bond length is an important factor controlling ${\mathit{T}}_{\mathit{c}}$. For LSCO/overdoped-${\mathrm{La}}_{1.65}$${\mathrm{Sr}}_{0.35}$${\mathrm{CuO}}_4$ (metallic) superlattices, a proximity-induced coherence length of 50 Å is calculated for the overdoped material using the de Gennes–Werthamer theory, from the ${\mathit{T}}_{\mathit{c}}$ dependence on individual layer thickness.

We present the exact ground-state wave function and energy of a general Hubbard model with kinetic energy t and on-site, nearest-neighbor, and bond-charge interaction U,V,X, respectively, for a wide, physically relevant range of parameters with X=t on lattices with arbitrary coordination numbers Z at half filling.

The frustrated quantum spin-one-half Heisenberg model on the triangular and kagomé lattices is mapped onto a single species of fermion carrying statistical flux θ=π. The corresponding Chern-Simons gauge theory is analyzed at the Gaussian level and found to be massive. This provides a new motivation for the spin-liquid Kalmeyer-Laughlin wave function. Good overlap of this wave function with the numerical ground state is found for small clusters.

A microscopic calculation of the optical properties of the Si(100)2×1 surface for both asymmetric and symmetric dimers is presented. The comparison with experiments allows us to discard a substantial presence of symmetric dimers from 40 to 310 K. An upper bound of 10% is obtained for their instantaneous population. Suggestions are made for future experiments.

Strongly trajectory-dependent neutralization is observed for low energy ${\mathrm{Li}}^{+}$ scattered from Cs adsorbed on Ni(111). For near glancing-exit scattering, the ion yield as a function of Cs coverage shows a pronounced peak and this behavior can be described well by a resonant charge-exchange model using the average work function. However, the yield is much smaller for near normal-exit scattering and the neutralization is mainly determined by the local electrostatic potential. Trajectory-independent, complete neutralization is observed at high coverages.

High-resolution angle-resolved photoemission measurements have been performed on Pt impurities on an Ag(110) surface. The linewidth of the Pt 5${\mathit{d}}_{5/2}$ spin-orbit-split component near the Fermi level is observed to be twice as large as the 5${\mathit{d}}_{3/2}$ component at ∼1 eV higher binding energy. This result is contrary to the usual expectation based on final-state lifetimes for Fermi liquids. Calculations performed within the Anderson single-impurity model clearly suggest that this effect is mainly caused by an energy-dependent coupling between the Pt 5d state and the Ag conduction band states.

We report the first observation of interference effects in the light scattered from two trapped atoms. The visibility of the fringes can be explained in the framework of Bragg scattering by a harmonic crystal and simple ‘‘which path’’ considerations of the scattered photons. If the light scattered by the atoms is detected in a polarization-sensitive way, then it is possible to selectively demonstrate either the particle nature or the wave nature of the scattered light.

We consider particle oscillations and their damping in second-quantized form. We find that the damping or ‘‘decoherence’’ may be described by a Boltzmann-like collision integral with ‘‘non-Abelian blocking factors’’ (fermions). Earlier results are generalized in that the momentum degrees of freedom are included and that the mixing equations become intrinsically nonlinear at high densities.

We propose a new type of resonant-mass gravitational wave detector, a truncated icosahedral gravitational wave antenna. It will be omnidirectional, and able to measure the direction and polarization of a detected wave. We solve a model for this system, calculate the strain noise spectrum, and conclude that its angle-averaged energy sensitivity will be 56 times better than the equivalent bar-type antenna with the same noise temperature.

Cosmological perturbations generated quantum mechanically (as a particular case, during inflation) possess statistical properties of squeezed quantum states. The power spectra of the perturbations are modulated and the angular distribution of the produced temperature fluctuations of the cosmic microwave background radiation is quite specific. An exact formula is derived for the angular correlation function of the temperature fluctuations caused by squeezed gravitational waves. The predicted angular pattern can, in principle, be revealed by observations like those by the Cosmic Background Explorer.

We have identified a class of high-lying doubly excited resonance in helium which are directly linked to a classical orbit first proposed by Langmuir for the quantization of helium. The projection of the wave function onto three mutually orthogonal planes is shown to trace the Langmuir orbit. This class of resonances forms the top of the intrashell manifold, and are long-lived compared to other members of the same manifold. In the limit Z→∞, the wave function traces the modified, asymmetric Langmuir orbit generated by bifurcation of the Langmuir orbit.

Stark spectra for atomic barium in the autoionizing region below the 5${\mathit{d}}_{3/2}$ ionization threshold are studied by photoionization of the laser-excited 5d6p ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_1^0$ bound level, in a combined experimental and theoretical study. The electric field dramatically increases the complexity of the resonance pattern beyond that of the already complex zero-field spectrum. A quantum defect and R-matrix calculation describes 86 relevant zero-field channels. A frame transformation couples these channels together nonperturbatively in the presence of the field, reproducing most of the observations.

A new way of measuring atomic parity nonconservation is proposed that utilizes the remarkable sensitivity inherent in experiments with a single trapped atomic ion. The accuracy may be sufficient to provide a valuable test of electroweak theory. As an illustration, the prospects are analyzed for carrying out such a measurement with ${\mathrm{Ba}}^{+}$ by observing the ground state spin rotation induced by an intense laser beam. This technique might also be used to search for T-odd interactions in atomic ions.