The effects of quantum degeneracy due to the tunneling motion of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms in a two-dimensional solid isotopic solution have been observed through measurements of the nuclear magnetic susceptibility. This system constitutes a solid phase with the features of a heavy fermion fluid. A phase transition, identified as isotopic phase separation, was observed directly through the time variation of the magnetic susceptibility. The critical temperature was 50 mK with a time constant for phase separation of order 5 h at 10 mK.

Scanning tunneling microscopy studies have been performed on GaAs homoepitaxial films grown by molecular-beam epitaxy. Images show that in the earliest stages of deposition the morphology oscillates between one with two-dimensional islands and flat terraces. As growth proceeds there is a gradual coarsening of the surface features. Comparison with reflection high-energy electron diffraction (RHEED) leads us to propose that there is a direct correspondence between the surface step density and the RHEED specular intensity. As such, we associate the decay of the RHEED oscillation amplitude with a reduction in the temporal variation of the step density rather than the buildup of interface width.

We present the first experimental observation of the Rabi splitting of Mössbauer resonance lines. A radio-frequency magnetic field is tuned to induce transitions between hyperfine sublevels of the Zeeman-split excited or ground state of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nuclei. The line splitting corresponding to the Rabi flopping frequency is observed using a magnetically soft ${}_{}{}^{57}{}_{}{}^{}\mathrm{FeNi}$ absorber foil of composition with almost zero magnetostriction.

An all-electron formulation of density-functional linear-response theory is presented. It is based on representing the first-order corrections to the one-electron wave functions in terms of a muffin-tin basis set. This, for instance, makes ab initio calculations of lattice dynamics for transition-metal systems possible. The computability is demonstrated for phonon dispersions in Nb and the results are found to be in excellent agreement with frozen-phonon calculations and with experimental data.

We report on the impulsive excitation and phase-sensitive detection of terahertz phonon polaritons in the ferroelectric ${\mathrm{LiTaO}}_3$ using femtosecond pulses. The coherent propagation of the polaritons and their dephasing is investigated in detail. Polaritons with frequencies below 1.5 THz clearly show that there is a resonance at 0.95 THz that has not been found in previous Raman and IR reflectivity studies. This resonance allows the coherent excitation of two polaritons of different frequencies. This leads to the first observation of phonon-polariton beats.

Lebed predicted striking effects in both the metallic and field-induced spin-density-wave (FISDW) states of quasi-one-dimensional conductors when the orientation of the magnetic field causes commensurate electron motion in the least-conducting plane. We have observed sharp cusplike dips in the normal-state magnetoresistance of (TMTSF${)}_2$${\mathrm{PF}}_6$ suggesting a giant resonance at the commensurate angles. On entering the FISDW state some of the dips become peaks, others fade, and fractional states appear. Magic angle effects are also prominent in the phase boundary of the FISDW state and in the Hall resistance.

A simple quasiclassical model is presented to explain the large, highly anisotropic magnetoresistance and striking magnetoresistance dips at magic angles in the Bechgaard salts, (TMTSF${)}_2$X. In the presence of a magnetic field the electrons are swept along the open orbit sheets of the Fermi surface into ‘‘hot spots’’ where the scattering rate is high. At magic angles the commensurate motion allows some fraction of the electrons to avoid the hot spots. A striplike hot region seems appropriate for (TMTSF${)}_2$${\mathrm{PF}}_6$ and suggests a strong role for electron interactions.

We analyze the linear transport behavior of metallic multilayers via the Kubo formula for a Hamiltonian with zero-range spin-dependent potentials. We find a direct connection between the Boltzmann and the Kubo approaches. Our two-point transport theory validates the quasiclassical approach for multilayered structures and introduces a new treatment of interfaces via angle-dependent coherent transmission coefficients.

Experimental results on hole capture in germanium are reviewed; good agreement is found not only among various investigators for the same center, but among different acceptors regardless of charge state. For these acceptors, the capture cross section depends only on temperature (∝${\mathit{T}}^{\mathrm{-}1}$). Collision theory is used to transform temperature dependence to wave-vector dependence, σ=π/${\mathit{k}}^2$. This is equal to the s-wave component of the incident plane wave decomposed into polar coordinates by partial wave analysis. It is proposed that direct capture into the ground state exceeds cascade capture for T>3 K for capture at singly ionized acceptors and for T>11 K for capture at doubly ionized.

The eigenstates of interacting electrons in the fractional quantum Hall phase typically form fairly well-defined bands in the energy space. We study systems of six and eight electrons for filling factor 3/7>ν> 2) / 7 and show that composite fermion theory gives insight into the origin of these bands and provides an accurate and complete microscopic description of the strongly correlated many-body states in the low-energy bands. This implies that, somewhat like in Landau’s Fermi liquid theory, there is a one-to-one analogy between the low-energy Hilbert space of strongly interacting electrons in the fractional quantum Hall effect and that of weakly interacting electrons in the integer quantum Hall effect.

The differential conductance of InAs-AlSb quantum wells with superconducting Nb electrodes exhibits a sharp single peak at zero bias, followed by a drop below the normal conductance at voltages an order of magnitude higher than the gap voltage, before returning to the normal value at yet higher bias. The behavior is interpreted in terms of Andreev reflections, modified by multiple normal reflections of both electrons and Andreev holes between the Nb electrodes and the bottom barrier of the quantum well, in the presence of a nonuniform potential along the well.

The I-V curves of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ strips, with widths of 0.54 to 5.6 μm, were measured in magnetic fields. All the strips showed critical scaling behavior, and a clear decrease in the vortex-glass (VG) transition temperature with decreasing width was observed. This finding proves that the interaction range of vortices is relevant in characterizing the phase transition, and that softening of the vortex-matter elasticity occurs upon approaching this scale. Moreover, the results are not easily explained by the flux-creep picture and give strong support to the VG picture.

We report measurements of the Hall coefficient (${\mathit{R}}_{\mathit{H}}$) and a-b-plane resistivity ρ(T) of cobalt-doped ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${)}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals (0≤x≤0.096) from 20 to 400 K. Co doping gives rise to an unusual downward curvature in ρ(T) while the inverse Hall coefficient [1/${\mathit{R}}_{\mathit{H}}$(T)] also becomes nonlinear. Despite this, Anderson’s formula for the Hall angle ${\mathrm{\theta }}_{\mathit{H}}$, namely, cot${\mathrm{\theta }}_{\mathit{H}}$=${\mathit{AT}}^2$+B continues to remain valid. In contrast to Zn, doping with Co decreases A while B remains constant. A simple model involving a square Fermi surface with rounded corners that could also account for the ${\mathit{AT}}^2$+B law is proposed.

We have determined the structure of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$/${\mathrm{GdBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ high-${\mathit{T}}_{\mathit{c}}$ superlattices using a novel method to refine the structure from x-ray diffraction spectra. This technique allows quantitative determination of disorder in superlattices in good agreement with independent measurements where available. The results show that layer thickness fluctuations, interdiffusion, and strains are present in the high-${\mathit{T}}_{\mathit{c}}$ superlattices and cannot be neglected when studying the physical properties.

A generalization of the numerical renormalization-group procedure used first by Wilson for the Kondo problem is presented. It is shown that this formulation is optimal in a certain sense. As a demonstration of the effectiveness of this approach, results from numerical real-space renormalization-group calculations for Heisenberg chains are presented.

Results of calculations of the spin density at the Fermi energy for several illustrative examples—the clean Fe(110) surface and the O/Fe(110), Fe/Ni(111), and Ru/Ag(001) overlayer systems—using the full potential linearized augmented plane wave method demonstrate that this energy-sliced spin density differs substantially from the total spin density previously given in the literature. The difference of the density of states at ${\mathit{E}}_{\mathit{F}}$ between the two spin parts is found to play the key role. Its negative sign in the surface-vacuum region for Fe(110) and its sign reversal when oxygen is added as an overlayer explain the experimental results obtained by using spin-polarized metastable atom deexcitation spectroscopy.

Endohedral fullerites, composed of periodic lattices of endohedral complexes with polar molecules trapped inside the ${\mathrm{C}}_{60}$ cluster, constitute a new class of ferroelectric materials and the first practical realization of ideal electric dipolar lattices. In these substances, interactions between the partially screened and freely rotating dipole moments of the guest molelcules give rise to dipole-ordered low-temperature phases. The respective transition temperatures for the endohedral fullerites with LiF, LiCl, NaF, and NaCl as the guest molecules are estimated to be at 25, 36, 51, and 60 K, respectively.

Recently discovered connections between integrable evolution equations and the motion of curves are based on the following fact: The Serret-Frenet equations are equivalent to the Ablowitz-Kaup-Newell-Segur (AKNS) scattering problem at zero eigenvalue. This equivalence identifies those evolution equations, integrable or not, that can describe the motion of curves.

We consider a wavelet analysis of various time series for the Duffing oscillator, for which there is a potential maximum and a harmonic forcing term, and we focus on time series that return to the region of the potential maximum. When the dynamics is chaotic and the time series is highly nonstationary, there are many significant higher harmonics in a Fourier expansion and the usual Fourier analysis is problematic, especially for short total times. We show that the wavelet analysis is a robust tool that may be used to obtain qualitative information for highly nonstationary time series—specifically, that it may be used to detect a small-amplitude harmonic forcing term even when the dynamics is chaotic and even for short total times.

We derive the equations of motion for general strings, i.e., strings with arbitrary relation between tension τ and energy per unit length ε. The renormalization of τ and ε that results from averaging out small scale wiggles on the string is obtained in the general case to lowest order in the amount of wiggliness. For Nambu-Goto strings we find deviations from the equation of state ετ=const in higher orders. Finally we argue that wiggliness may radically modify the cosmic gauge string scenario.