We present a novel interpretation of the traditional Landau-Feynman picture of excitations in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Detailed consideration of neutron-scattering data on the temperature dependence of S(Q,ω) in the region 0.4≲Q≲2 A${\mathrm{̊}}^{\mathrm{-}1}$ strongly suggests that while the photon is a collective zero-sound mode, the maxon roton is a strongly renormalized single-particle excitation. The condensate-induced hybridization of these two modes explains the smooth photon-maxon-roton dispersion curve as well as the vanishing intensity of the maxon-roton peak in S(Q,ω) above ${\mathit{T}}_{\lambda }$.

Using the boundary-integral formulation, we show that the fully isotropic model of eutectic growth in directional solidification supports solutions with a broken parity symmetry. These correspond to the tilted ‘‘waves’’ observed recently by Faivre et al. Our results are in good agreement with these experiments. For a fixed wavelength λ, we predict a collapse to zero of the tilt angle φ at a critical velocity V=${\mathit{V}}_{\mathit{c}}$ which scales as ${\mathit{V}}_{\mathit{c}}$∼${\lambda }^{\mathrm{-}2}$. For not too small V, we find that φ≃const along the line ${\lambda }^2$V=const. We make predictions which call for new experimental investigations.

Systems undergoing spinodal decomposition often exhibit an interconnected morphology similar to a fractal in the early stage. It has been speculated that this early-stage structure is in fact a fractal. In this Letter I show that the linear theory of spinodal decomposition does indeed predict that the early-stage morphology is fractal and, in addition, has what one might call a multifractal structure. In contrast, there is no fractal or multifractal structure in continuous ordering. In addition, I construct percolation cluster growth models isomorphic to spinodal decomposition and continuous ordering so that precise tests of these predictions can be performed.

In situ x-ray specular reflectivity and glancing-incidence-angle x-ray-diffraction measurements have been performed at the Au(001) surface in a 0.01M ${\mathrm{HClO}}_4$ solution under potential control in an electrochemical cell. At -0.4 V versus an Ag/AgCl electrode, the gold surface exhibits a hexagonal reconstructed layer with a mass density 21% greater than the underlying bulk layers. The reconstruction disappears above 0.5 V, and the excess atoms form a new atomic layer with a density corresponding to 22% of a bulk layer. The reconstruction fully recovers below -0.3 V.

The Lieb-Mattis theorem on the absence of one-dimensional ferromagnetism is extended here from ground states to T>0 by proving, inter alia, that M(β,h), the magnetization of a quantum system in a field h>0, is always less than the pure paramagnetic value ${\mathit{M}}_0$(β,h)=tanh(βh), with β==1/kT. Our proof rests on a new formulation in terms of path integrals that holds in any dimension; another of its applications is that the Nagaoka-Thouless theorem on the Hubbard model also extends to T>0 in the sense that M(β,h) exceeds ${\mathit{M}}_0$(β,h).

The wave-number-dependent static dielectric function of the superionic conductor α-AgI is determined by use of structural-analysis data. The dieectric function indicates the existence of a static charge-density wave, which reflects the iodine sublattice. The dielectric functions of the β phase and the molten phase are also determined for comparison. We have found that the dielectric function in the molten phase resembles that of the strongly coupled one-component plasma near its fluid-solid transition.

The electron pair-correlation function for diamond is calculated using a variational quantum Monte Carlo approach. The full pair-correlation function reveals striking anisotropy and spatial-dependent structures that result from the crystal structure and chemical bonding of the real material. The effects of correlation on the occupation of single-particle states have also been calculated for diamond and silicon, as well as quasihole energies within a single-mode approximation.

We calculate the collective-mode spectrum for a layered superconductor structure. We find that for wave vectors directed close to the direction of the superlattice axis, the plasmon mode remains below the superconducting gap edge (i.e., the Anderson-Higgs mechanism does not give rise to a massive collective mode as it does in the bulk three-dimensional system). We also find a clear signature of superconductivity in the form of a unique line splitting in the collective-mode spectrum as the plasmon crosses the superconducting gap. Experimental implications are discussed.

Far-infrared magnetotransmission and magnetophotoconductivity measurements on selectively doped GaAs-GaAlAs multiple-quantum-well structures reveal photoionization transitions from two-dimensional ${\mathit{D}}^{\mathrm{-}}$ centers to successive Landau levels. Selective doping of the quantum-well structure favors the formation of ${\mathit{D}}^{\mathrm{-}}$ centers in the well. The two-dimensional nature of this center results in a dramatic enhancement of its binding energy with respect to the three-dimensional case.

We explore the connection between Mott-insulating behavior and the breakdown of lattice-translation symmetry. We find that if a system of hard-core particles in d=2 is given to be a Mott insulator, then the charge must form a commensurate charge-density wave. This is true for particles of any statistics. Our result is based on duality arguments and does not seem to depend on the details of the Hamiltonian. For average site-occupation number n<1, this implies a breakdown of lattice-translation symmetry.

We have studied weak localization and conductance fluctuations in 2D mesoscopic GaAs/AlGaAs heterojunctions for T=60 mK to 7 K. The weak-localization data show that the spin-orbit scattering rate exceeds the phase-breaking rate below ∼2 K. At the same time, the conductance-fluctuation amplitude is reduced significantly below ∼2 K, as compared to that extrapolated from the high-temperature region. Our data agree well with calculations for the effect of spin-orbit scattering on the conductance-fluctuation amplitude. A related effect on the magnetic correlation field is also discussed.

We present an exact formulation of the self-induced effective gauge fields both in the Hubbard model and in the true continuum description of the Cu-O planes of the high-${\mathit{T}}_{\mathit{c}}$ perovskites. We bypass the solution of the ground state in favor of the chirality order parameter as the intrinsic variable. The constraint of gauge symmetry greatly narrows the admissible forms for the gauge fields. We find that these fields can produce broken-symmetry chiral ground states on a lattice, but the intrinsic frustration in the continuum form of the gauge field seems to rule out such a state in the real Cu-O plane.

The hierarchy schemes for the fractional quantum Hall effect are reexamined and it is shown that different schemes all give the same lattice of excitations whose statistics is determined by the norm of the corresponding vector, and hence have equivalent Ginzburg-Landau theories. Similar ideas apply to the anyon liquid. The schemes can be generalized by using different lattices; many inequivalent states can be obtained at any filling factor (or value of the statistics parameter).

We consider the low-lying states of a class of tight-binding models related to the parity- and time-reversal-breaking chiral-spin liquid, which describes a magnetically frustrated Mott insulator. In precise analogy with the soliton excitations of polyacetylene, the presence of a midgap state bound to topological disruptions of the chiral order leads to neutral, spin-1/2 spinors and charge-e, spinless holons.

It is shown that, for a system at the mobility edge, the conductance distribution ${\mathit{P}}_{\mathit{L}}$(g) approaches an entirely universal function ${\mathit{P}}^{\mathrm{*}}$(g) when the size L of the system increases. The function ${\mathit{P}}^{\mathrm{*}}$(g) depends on dimensionality d alone and does not depend on any details of the model. In obtaining ${\mathit{P}}^{\mathrm{*}}$(g) use was made of the conductance cumulants calculated by Altshuler, Kravtosv, and Lerner [Phys. Lett. A 134, 488 (1989)]. The calculations are limited to spatial dimension d=2+ε, where ε is a small number.

A new reentrant behavior of the ac susceptibility in proximity-induced superconductivity of silver has been discovered. This is observed when the induced coherence length becomes comparable to the size of the specimen. The effect is destroyed by applying a magnetic field or by raising the temperature. This phenomenon points to the existence of a new form of quantum coherence effect in clean proximity-induced superconductors.

We consider the 1D Schrödinger equation with a potential proportional to energy. When the spatial part of the potential is twice continuously differentiable, is less than 1 everywhere, and satisfies a certain integrability condition, we compute the scattering matrix. For the same class of potentials, under the further assumption that the potential is non-negative, we obtain the potential from one of the reflection coefficients.

Scattering connected with symmetry is studied. Explicit expressions for the S operator are provided for the scattering related to any symmetric space of the noncompact type.

A data sample of 1040 days from the Kamiokande II detector, consisting of subsamples of 450 days at electron-energy threshold ${\mathit{E}}_{\mathit{e}}$≥9.3 MeV and 590 days at ${\mathit{E}}_{\mathit{e}}$≥7.5 MeV, yields a clear directional correlation of the solar-neutrino-induced electron events with respect to the Sun and a measurement of the differential electron-energy distribution. These provide unequivocal evidence for the production of ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ by fusion in the Sun. The measured flux of ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ solar neutrinos from the two subsamples relative to a prediction of the standard solar model is 0.46±0.05(stat)±0.06(syst). The total data sample is tested for short-term time variation; within the statistical error, no significant variation is observed.

An analysis of the Mikheyev-Smirnov-Wolfenstein effect using 1040 days of Kamiokande-II data is reported, which provides constraints on neutrino-oscillation parameters. The measured recoil-electron energy spectrum alone leads to the conclusion that the adiabatic region, 7.2×${10}^{\mathrm{-}4}$<${\sin }^2$2θ<6.3×${10}^{\mathrm{-}3}$, Δ${\mathit{m}}^2$∼1.3×${10}^{\mathrm{-}4}$ (eV${)}^2$, is disfavored at the 90% confidence level.