Motivated by widespread experimental observations of the ‘‘banded structure’’ in rapidly solidified alloys, we have studied numerically by Green’s-function method the dynamics of the planar interface under rapid directional solidification condition. We find that the formation of this structure can be explained and partially characterized quantitatively in terms of highly nonlinear ‘‘relaxation oscillations’’ of the interface whose spacings are controlled by latent heat diffusion but not by the thermal gradient. A comparison with experiment is presented for Al-Fe alloys.

We have measured the mass distribution of cluster ions formed from a supercritical expansion of helium gas. We find two distinct cluster groups which we identify as condensation and fragmentation clusters. The latter first appear when the expansion conditions approach the critical isentrope. The measurements also suggest that the neutral fragmentation cluster mass distribution is a universal function of the source entropy.

We have studied dissipative phase-slip events in the oscillatory superflow of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ through two different orifices of submicron size. The dissipation events are found to be caused by the phase difference across the orifice changing by 2πn, where the slip integer n depends on the orifice geometry, flow direction, and temperature. The temperature dependence of the phase-slip critical velocity indicates that surface microstructure determines the magnitude of relevant energy barriers.

The average liquid-vapor density profiles 〈ρ(z)〉 of thick ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed onto a silicon substrate were measured using x-ray reflectivity. The results are well represented by a 90%-10% interfacial width of 9.2±1 Å at 1.13 K which extrapolates to a T=0 K, 90%-10% interfacial width of 7.6+1,-2 Å. The sensitivity of the measurement to the width, shape, and asymmetry of the density profile is discussed.

Analysis of low-energy electron diffraction spectra for Be(0001) shows that the interplanar spacing between the first and second planes is expanded by 5.8% or 0.10 Å. The size of this expansion is large compared to that of close-packed fcc (111) surfaces, which show little or no expansion. This large expansion is discussed in terms of the unique bonding of Be. The second and third interplanar spacings show almost no relaxation.

The first low-temperature scanning-tunneling-microscope (STM) images of Si(001) are presented. It is observed that on cooling to 120 K the number of buckled dimers increases, confirming that dimers have an asymmetric character. Buckled-dimer domains of c(4×2) order are bounded by p(2×2) regions. Defects pin nearby dimers into a buckled configuration and act to smear out the transition to order. At room temperature dimers rapidly switch orientation leading to an averaged symmetric appearance in STM images.

We have studied the dereconstruction of Si(111)2×1 upon hydrogen adsorption by means of first-principles total energy and molecular-dynamics calculations. We find that the (1×1) bulk-terminated surface structure becomes energetically favored over the (2×1), starting from a coverage θ∼0.3. This is consistent with LEED results showing that the dereconstruction is completed at θ∼0.5. A path for this conversion has been determined via direct simulation at θ=0.5. This has a rather low energy barrier, ∼0.10–0.15 eV/(surface atom), and should therefore occur with significant probability during hydrogenation of the surface.

The temperature dependence of Rayleigh waves in Fe (110), measured by helium atom scattering in the range of 150 to 905 K (0.91 ${\mathit{T}}_{\mathit{c}}$), is shown to provide new information on surface magnetization. The magnetic shifts of the surface phonon frequencies exhibit a large anisotropy, with the [11¯0] direction showing the expected decrease of frequency for increasing temperature, while the [001] direction shows an anomalous increase with temperature. The results are interested in terms of a simple model.

Exact solutions of the Hubbard model and the periodic Anderson model in the limit of infinite interaction strength are presented. Both models are studied on a D-dimensional decorated hypercubic lattice with periodic boundary conditions for any dimension D≥2 and arbitrary size. The lattice is very similar to the perovskite lattice. In addition to the ground-state energy, a corresponding eigenstate is constructed. This ground state contains at least two particles per unit cell. For the Anderson model, the exact solution is restricted to a surface in the (${\mathit{E}}_{\mathit{f}}$,V) parameter space; however, the resulting relation V(${\mathit{E}}_{\mathit{f}}$) does not lead to unphysical parameters.

We have obtained a phase diagram of the α→γ transition (moment formation) as a function of the U and J Kanamori Hubbard parameters. Overlap and hybridization terms are obtained by a tight-binding fit to first-principles band calculations. At the one-electron level we obtain no fundamental change in the interaction energies in going through the α→γ transition region. This is the first model to obtain self-consistent values for the Hubbard parameters in the solid state and they are in reasonable agreement with the experimentally derived values.

Inverse photoemission spectra have been measured as a function of temperature in ${\mathrm{CePd}}_3$. Although the experimental energy resolution is 1 order of magnitude larger than the characteristic thermodynamic energy scale of this system, a temperature dependence of the spectral function is observed. This result proves that high-energy spectroscopies are sensitive to the breakdown of the Kondo state as the low-lying magnetic states become thermally populated.

We have developed an experimental approach which permits evaluation of the entire transmission matrix of a mesoscopic conductor. Results are presented from two new investigations enabled by this technique: (a) We study ballistic multiprobe conductors in the limit of weak probe coupling, and (b) we image modal features in the distribution function of electrons emerging from a quantum point contact.

Magnetotransport properties of p-type (In,Mn)As, a new diluted magnetic semiconductor based on a III-V semiconductor, are studied. The interaction between the holes and the Mn 3d spins is manifested in the anomalous Hall effect, which dominates the Hall resistivity from low temperature (0.4 K) to nearly room temperature, and in the formation of partial ferromagnetic order below 7.5 K, which is a cooperative phenomenon related to carrier localization. The coexistence of remanent magnetization and unsaturated spins as well as the large negative magnetoresistance at low temperatures is explained by the formation of large bound magnetic polarons.

The emission of light from metal-oxide-metal tunnel junctions has been studied as a probe of the interaction of the tunneling current with the surface plasmon polaritons of the tunneling junction structure. The slow mode, with large fields in the oxide layer, is most directly coupled to the tunneling current. In this experiment, diffraction gratings with periods between 70 and 100 nm provide the momentum for the slow mode to radiate. We report the unambiguous measurement of the direct emission from the slow mode showing that it does contribute to the light emission.

Using a SQUID voltmeter we have measured current-voltage curves in untwinned crystals of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ as a function of temperature and magnetic field. Our data show a clear crossover from high-temperature Arrhenius behavior to a critical region associated with the low-temperature three-dimensional vortex-glass phase transition. The critical exponents ν(z-1)=7±1 in this system are in accord with theoretical models and previous measurements in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. The width of the critical region collapses below 2 T, reflecting the changing role of dimensionality with field.

We consider the effect of interplane coupling J’ on a pair of layers in the Kalmeyer-Laughlin model of anyon superconductivity. We show by exact diagonalization of an effective continuum model that this coupling has little effect on the chiral-spin-liquid ground state for small J’. For J’ larger than twice the gap, one sees a transition away from the chiral spin liquid. We argue that the J’ term should introduce short-range attractive interactions between holons in different layers. A model of anyons with such an interaction yields a phase diagram in qualitative agreement with what one expects when working directly with the spin system.

The nonlinear susceptibility is discussed as a probe of multispin correlation functions. In the heavy-fermion compound ${\mathrm{URu}}_2$${\mathrm{Si}}_2$, measurements of the leading nonlinear susceptibility (${\mathrm{\chi }}_3$) reveal a sharp anomaly at 17.3 K closely matching that of the specific heat at the same temperature. We attribute this result to the development of anisotropic triplet pairing in the particle-hole channel of the heavy electron fluid.

Spin-orbit coupling in doped ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ can result in (1) a novel electron-phonon coupling involving soft oxygen ‘‘tilting’’ phonons, and (2) stabilization of a commensurate antiferromagnetic state over a spiral state in the presence of a sufficiently large tilt distortion. This second effect may be responsible for the unusual electronic properties of ${\mathrm{La}}_{1.88}$${\mathrm{Ba}}_{0.12}$${\mathrm{CuO}}_4$.

A structure-dependent exchange coupling across wedge-shaped Cu layers in fcc (100) Co/Cu/Co and bcc (100) Fe/Cu/Fe epitaxial sandwiches has been determined from analysis of longitudinal magneto-optical Kerr effect hysteresis loops. In the fcc Co/Cu/Co system, the variation of the coupling with Cu thickness can be described as a superposition of two oscillatory terms, with periods of 2.6 and 8.0 monolayers (ML). The bcc Fe/Cu/Fe system shows oscillations with a period of 2 ML. These periodicities are related to the Fermi surface dimensions and topology of fcc and hypothetical bcc bulk Cu.

A scaling theory and simulation results are presented for fragmentation of percolation clusters by random bond dilution. At the percolation threshold, scaling forms describe the average number of fragmenting bonds and the distribution of cluster masses produced by fragmentation. A relationship between the scaling exponents and standard percolation exponents is verified in one dimension, on the Bethe lattice, and for Monte Carlo simulations on a square lattice. These results further describe the structure of percolation clusters and provide kernels relevant to rate equations for fragmentation.