The low-temperature behavior of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ crystallization waves has been studied from 0.03 to 0.5 K. From the damping of these waves we have determined the interfacial growth resistance, (Km${)}^{\mathrm{-}1}$. At the lowest temperatures, (Km${)}^{\mathrm{-}1}$ is consistent with the assumption that the growth velocity is limted by the scattering of ballistic phonons by the moving interface. Above T≊0.25 K, (Km${)}^{\mathrm{-}1}$ increases more slowly with temperature, indicating a decrease in the phonon contribution from that observed at lower temperatures.

We have performed NMR studies on an aligned, multicrystalline ${\mathrm{NbSe}}_3$ sample at various temperatures. We find conclusive evidence of field-induced Fermi-surface changes at low temperatures, and associate these changes with charge-density-wave (CDW) enhancement mainly localized on the yellow crystallographic site, contrary to expectations, since the low-temperature CDW is mainly localized on the orange site.

The spin-orbit scattering of 4sp and 5sp impurities in a Mg host is investigated theoretically by self-consistent local-density-functional theory. The calculated spin-orbit scattering cross sections ${\mathrm{\sigma }}_{\mathrm{SO}}$ agree with the available experimental results for low valent impurities. For higher valent impurities we predict a p resonance behavior. For Cu and Ag impurities our results point to errors of density-functional theory in estimating the d contribution to ${\mathrm{\sigma }}_{\mathrm{SO}}$. In total a consistent interpretation of the trends is given.

Near the boundary, a 2D electron gas can phase separate into fractional quantum Hall fluid regions of successively lower filling factors (ν), in the case of a slowly increasing boundary potential. We present experimental evidence in the ν=1/3, 2/3, and 1 quantum Hall regimes on a four-lead Hall junction showing that by side gating the junction boundary, the Hall resistance can be made to approach the quantized value of the lower ν fluids in the top-gated leads, rather than the higher-ν fluid at the junction center. This implies the presence of the lower-ν fluids near the junction boundary.

We study singularities in the differential resistance of metal point contacts containing structural disorder. We find zero-bias features which are well described by theories in which two-level tunneling systems couple strongly to the conduction electrons to produce a Kondo resonance whose origin is not magnetic impurities. Away from zero bias, we observe abrupt resistance steps which indicate transitions in the multichannel Kondo condensate. We suggest that two-level systems are the explanation for many of the zero-bias anomalies seen in tunnel junctions and point contacts.

Measurements are reported of surface plasmon energy and dispersion carried out by electron energy loss spectroscopy on Ag(110) along both high-symmetry crystallographic directions. For small ${\mathit{q}}_{\mathrm{\parallel }}$ the data show a linear and positive dispersion with a strong azimuthal dependence that is twice as large along the 〈001〉 direction as along 〈11¯0〉.

We show that the wave functions of the fluid ground states of fractional quantum Hall systems, in the thermodynamic limit, are universal at long distances and that they have a generalized Laughlin form. This universality is a consequence of the analytic properties of the equal-time density correlation functions at long distances. The correlation functions calculated from the field theoretic approach to the fractional quantum Hall effect have the correct analytic properties and the wave function calculated in the Gaussian approximation becomes exact in the asymptotic limit.

We present the first direct measurements of ${\mathit{T}}_{\mathit{c}}$ as a function of uniaxial stress ${\mathit{p}}_{\mathit{i}}$ (i=a,b,c) along the crystal axes for an untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystal. ${\mathit{T}}_{\mathit{c}}$ is linear in ${\mathit{p}}_{\mathit{i}}$ with slopes ${\mathit{dT}}_{\mathit{c}}$/${\mathit{dp}}_{\mathit{a}}$=-2.0±0.2 K/GPa, ${\mathit{dT}}_{\mathit{c}}$/${\mathit{dp}}_{\mathit{b}}$=+1.9±0.2 K/GPa, and ${\mathit{dT}}_{\mathit{c}}$/${\mathit{dp}}_{\mathit{c}}$=-0.3±0.1 K/GPa. These results confirm predictions based on thermal expansion measurements [C. Meingast et al., Phys. Rev. Lett. 67, 1634 (1991)] and show that the small hydrostatic pressure dependence of ${\mathit{T}}_{\mathit{c}}$ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is due to a cancellation of large and opposite effects in the a-b plane.

Thermodynamic properties such as the specific-heat discontinuity at the transition temperature (${\mathit{T}}_{\mathit{c}}$), ΔC/${\mathit{T}}_{\mathit{c}}$, and the zero-temperature critical field, ${\mathit{H}}_{\mathit{c}}$(0), of oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ are analyzed to show that, within the framework of the BCS formalism, the density of states (DOS) at the Fermi level is peaked at y=0. The energy dependence of the DOS is considered for several models and found to be consistent with the Fermi level lying close to a two-dimensional van Hove singularity.

The tilted vortex lattice on the surface of 2H-${\mathrm{NbSe}}_2$ is investigated with a scanning-tunneling microscope at magnetic fields, such that the vortex separation is comparable to or smaller than the magnetic penetration depth. The basis vectors show a distortion roughly consistent with bulk anisotropic London theory, however, their angular orientation with respect to the tilt direction differs by 30° from that predicted. At field inclinations greater than 80° from the c axis, we find two instabilities into ordered, buckled rows of vortices, followed by a disordering transition.

A novel approach to S=1/2 antiferromagnets with strong quantum fluctuations based on the representation of spin-1/2 operators as bilinear forms of real (Majorana) fermions is suggested. This representation does not contain unphysical states and thus does not require the imposition of constraints on the fermionic Hilbert space. This property allows one to construct a simple and effective mean field theory of the spin liquid state. As an example illustrating the basic properties of this state I consider a model of Kondo lattice. It is shown that this model has a singlet ground state; elementary excitations have a spectral gap and are S=1 real (Majorana) fermions.

Swendsen-Wang-type cluster dynamics is used in order to reduce relaxation time in spin-glass simulations. The shapes and sizes of the clusters are related to the local environment of the spins using dynamical information. For a Gaussian spin glass on a square lattice, the relaxation times are fitted by the Arrhenius form, exp(A/kT). The constant A for the cluster dynamics is found to be 5 times smaller than the one for conventional dynamics. We also obtain ν=2.0±0.2.

It is demonstrated that it is possible to reverse the time evolution of polarization in a dipole-coupled nuclear spin system (by refocusing the ‘‘spin diffusion’’ process), leading to the appearance of a new type of echo, called polarization echoes. The polarization echoes for abundant I spins can be measured conveniently by a two-step polarization-transfer process from and to rare S spins that allows a local perturbation and local detection of the I-spin polarization. Polarization echoes have been recorded for a single crystal sample of ferrocene.

The behavior of a q-deformed Heisenberg-Weyl algebra nearby q=1 is investigated. We calculate the q-deformed symplectic structure and * product up to the first-order corrections by using the q-analog of the boson coherent state. Within this order, the Poisson bracket associated with the symplectic structure coincides with the one extracted from the * product and gives the phase space realization of the original operator commutation relations.

We have evaluated the density-density static response of a many-body system by calculating with the quantum Monte Carlo method the energy and density change caused by an external potential. Our results for the linear response function of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at zero pressure and temperature are in excellent agreement with the available experimental data. The results for the response function of 2D electrons also at zero temperature, obtained within the fixed-node approximation, constitute the most accurate information available to date for this system.

On simple cubic lattices, we compute the low-temperature expansion for the energy of the Ising model through 50 excited bonds in three dimensions and 44 excited bonds in four dimensions. We also give the magnetization through 42 excited bonds. Our method is a recursive enumeration of states with given energies on a set of finite lattices with generalized helical boundary conditions. A linear combination of such lattices cancels finite volume effects.

Progress in numerical relativity has been hindered for 30 years because of the difficulties of avoiding spacetime singularities in numerical evolution. We propose a scheme which excises a region inside an apparent horizon containing the singularity. Two major ingredients of the scheme are the use of a horizon-locking coordinate and a finite differencing which respects the causal structure of the spacetime. Encouraging results of the scheme in the spherical collapse case are given.

The standard Einstein-Maxwell equations in 2+1 spacetime dimensions, with a negative cosmological constant, admit a black hole solution. The 2+1 black hole—characterized by mass, angular momentum, and charge, defined by flux integrals at infinity—is quite similar to its 3+1 counterpart. Anti–de Sitter space appears as a negative energy state separated by a mass gap from the continuous black hole spectrum. Evaluation of the partition function yields that the entropy is equal to twice the perimeter length of the horizon.

A formula is derived for the Euclidean action of a black hole surrounded by Yang-Mills, scalar, and fermion fields. This is used to prove that the entropy of a black hole with classical hair is given by the usual formula, S=1/4A, where A is the area of the event horizon, and the modifications for quantum hair are discussed.

Inflation creates both scalar (density) and tensor (gravity wave) metric perturbations. We find that the tensor-mode contribution to the cosmic microwave background anisotropy on large-angular scales can only exceed that of the scalar mode in models where the spectrum of perturbations deviates significantly from scale invariance (e.g., extended and power-law inflation models and extreme versions of chaotic inflation). If the tensor mode dominates at large-angular scales, then the value of ΔT/T predicted on 1° is less than if the scalar mode dominates, and, for cold-dark-matter models, bias factors b>1 can be made consistent with Cosmic Background Explorer Differential Microwave Radiometer results.