Nanosecond-pulsed-laser melting of amorphous ($a$) Si on crystalline ($c$) Si substrates produces a highly undercooled liquid ($l$) that solidifies into a complex morphology. Recently developed techniques for including undercooling, interface kinetics, and nucleation in heat-flow calculations are used to analyze the experimental results. It is shown how explosive crystallization, involving the difference in the $a$- and $c$-phase latent heats, can produce a nearly self-sustaining crystallization front mediated by a thin $l$ layer.

A consistent Fermi-liquid theory of heavy-electron compounds at low temperatures is microscopically derived from the Kondo-lattice model. Interactions between the heavy quasiparticles are mediated by fluctuations of effective valence-conduction hybridization parameters. The computed values of the Wilson ratio, the $T^3\ln T$ term in the specific heat, and the $T^2$ coefficient of the low-temperature resistivity are in agreement with systematic experimental trends.

Surface electrons were selectively photoexcited into the normally unoccupied antibonding surface state on the cleaved Si(111) (2×1) surface by 2.8-μ m infrared laser radiation. The time decay of the antibonding-state population was then followed in real time by picosecond time-resolved uv photoemission spectroscopy. The relaxation dynamics was found to be cleavage dependent, and appears to be controlled by defects, which give rise to a unique signature in the photoemission spectra.

An explanation is proposed for the recent observation that in ultrathin Sn films there is apparently a universal normal-state sheet resistance above which superconductivity cannot be established. It is demonstrated that when the sheet resistance is larger than the quantum of resistance, $R_Q=\frac{h}{4e^2}$, the order parameter suffers phase slips due to quantum tunneling driving the film normal at low temperature. This value of the resistance agrees quantitatively with experiment.

At the anomalously high density of Å9 g/${\mathrm{cm}}^2$, expanded liquid mercury is found to undergo a metal-nonmetal transition which is associated with the formation of a gap in the density of states at the Fermi energy. Although previous calculations have all yielded values of the density for the metal-nonmetal transition that are too low (Å5.5 g/${\mathrm{cm}}^3$), it is shown that a simple model emphasizing fluctuations in the local coordination number produces the correct value.

Far-infrared measurements of intersubband absorption spectra of $n$-inversion layers in (100) Si metal-oxide-semiconductor field-effect transistors with mobile positive ions in the oxide have been carried out between 4.2 and 70 K. Results provide direct evidence for screening of localized states in this quasi two-dimensional electronic system as well as the existence of long band tails and impurity bands at low electron densities associated with subbands due to both the inequivalent conduction-band valleys.

High-resolution temperature-dependent measurements of the infrared spectra associated with the acceptor complexes Be-H and Be-D in Si together with direct far-infrared studies of the low-lying transitions provide an unambiguous identification of motional tunneling at electronic defects in elemental semiconductors. This novel system consists of a proton or deuteron tunneling around the substitutional Be double acceptor.

Good-quality tunnel junctions have been fabricated on ultrathin films of Nb whose thicknesses ranged from 9 to 900 Å. As the film thickness is reduced below 50 Å, $T_c$ decreases, $\frac{2\Delta }{k{T}_c}$ increases, and mode softening is observed through $\frac{d^{2}V}{d{I}^2}$ measurements. These results imply that as the thickness decreases below 50 Å the electron-phonon constant, $\lambda$, becomes larger than expected from the drop in $T_c$.

Using a novel magnetometer technique probing a ${(15-\mathrm{\mu }\mathrm{m})}^3$ volume of a sample directly on top of a dc-SQUID chip with integrated pickup loops and field coils, we measure simultaneously the magnetic fluctuation noise and the ac susceptibility of a spin-glass, ${\mathrm{Eu}}_{0.4}$${\mathrm{Sr}}_{0.6}$S, as a function of temperature. The data demonstrate directly the validity of the fluctuation-dissipation theorem both above and below the freezing temperature. New results on low-frequency dynamics are presented for this sample.

The band-tail density of states for the tight-binding model with Gaussian distribution of site energies is calculated numerically in one, two, and three dimensions by use of the recursive transformation method. Our results confirm the band-tail scaling properties as predicted by the Halperin-Lax theory but indicate that in 3D, the near-tail behavior displayed by the model is qualitatively distinct from the simple exponential as deduced from experimental data. Implications are discussed.

A general theory of neural networks with nonlinear synapses is developed. To this end a meanfield model of a novel type is introduced and solved exactly. For suitable nonlinearity, synaptic sign changes may be eliminated altogether without affecting the efficiency of the network. Static noise is easily included.

At a critical point the golden-mean Kolmogrov-Arnol'd-Moser trajectory of Chirikov's standard map breaks up into a fractal orbit called a cantorus. The transition describes a pinning of the incommensurate phase of the Frenkel-Kontorowa model. We find that the fractal dimension of the cantorus is $D=0\mathrm{}$ and that the transition from the Kolmogorov-Arnol'd-Moser trajectory with dimension $D=1\mathrm{}$ to the cantorus is governed by an exponent $\overline{\nu }=0.98\mathrm{}\dots$ and a universal scaling function. It is argued that the exponent is equal to that of the Lyapunov exponent.

An experiment to search for the production of neutral penetrating particles decaying into electron-positron pairs was performed with a 2.5-GeV electron beam. A total of 0.027 C was injected into a tungsten target. No such particle is found. Constraints on coupling constants ${\alpha }_e$ and ${\alpha }_{\gamma }$ are given.

Certain superstring theories contain new exotic particles and couplings active at low energies. We show that they offer a solution to a long-standing problem of simultaneously having naturally small Dirac neutrino masses and vanishing Majorana masses. It is therefore consistent with (a) lack of observation of neutrinoless double $\beta$ decay, (b) possible evidence for nonvanishing neutrino mass, and (c) a recently proposed solution to the solar neutrino problem.

A measurement of lifetimes in ${}_{}{}^{172}{}_{}{}^{}\mathrm{W}$ by the Doppler-shift recoil-distance technique indicates a reduction of the transition quadrupole moments by about 15% for spins ≥ 12. This can be understood if the rotation-induced structure change is due not only to $i_{\frac{13}{2}}$ neutrons, which is the standard theory for this region of nuclei, but also to $h_{\frac{9}{2}}$ protons. The resulting three-band interpretation of the "upbend" in the yrast level sequence is supported by energy-level systematics suggesting that it is valid for a wide range of nuclei in this region.

Muonium atoms have been observed in vacuum after emission from a layer of finely divided silica powder. By extrapolation of the decay-positron track, both the time and position of muon decay are measured, confirming thermal emission at room temperature. The yields range up to (19 ± 6)% of muons stopping in the layer, depending on its thickness. The result is used to recalculate the upper limit for conversion of muonium to antimuonium.

Cross sections and probabilities for electron capture by protons scattered through 30° by Ne have been measured as a function of energy across the ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}\mathrm{}(p,p)\mathrm{}$ resonance at 1955 keV. A structure is seen near the resonance energy. The data are compared with theory.

The recent development of a circular-polarization analyzer for the vacuum-ultraviolet spectral region has enabled a Stokes parameter analysis to be carried out for the excitation of neon and argon by 80-eV incident electrons. The results show that the transfer of angular momentum to the atom is positive and is in fact surprisingly "heliumlike." Small deviations from total coherence were observed and are discussed.

The free decay of two-dimensional turbulence is simulated by direct numerical integration of the incompressible Navier-Stokes equations at resolutions ${512}^2$ and, for flows with symmetries, ${1024}^2$. At early times, quasirectilinear vorticity-gradient sheets develop, with a thickness decaying exponentially in time until dissipation becomes relevant. The energy spectrum then displays a $k^n$ range with $n\simeq -4\mathrm{}$. At later times, we observe a transition to $n\simeq -3\mathrm{}$. Vorticity gradients are then distributed on reconnecting, elongated, dissipative structures.

We report on the observation of photon antibunching in the resonance fluorescence field radiated by a many-atom source. The crucial feature of our experiment is the achievement of a phasematching configuration similar to that of four-wave mixing. The antibunching term results from a constructive interference of contributions from the $N$ atoms and thus scales as $N^2$.