We have observed an instability of the uniform orbital texture of a slab of ${}_{}{}^3{}_{}{}^{}\mathrm{He}-A$ in a perpendicular magnetic field (Fréedericksz transition) in two different experiments with slab thicknesses of 105 μm and 2 mm. The theoretically predicted threshold fields for the two experiments are slightly larger than the observed values but have the same temperature dependence. Behavior characteristic of the uniform texture was obtained only when the $A$ phase was entered by cooling from the normal phase. Very different behavior was observed when the $A$ phase was entered by warming from the $B$ phase.

The supercooling and freezing of a restricted liquid has been studied by the probing of its molecular dynamics with picosecond optical techniques. A newly developed transparent porous host material makes it possible to study the liquid viscosity as a function of the confining pore radius and temperature. The observed behavior is remarkably different from that of an ordinary liquid, and can be interpreted in terms of a simple model.

The formation of In-As pairs and clustering of As atoms in Si was studied via their electric field gradients by use of the perturbed-angular-correlation technique and ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ as probe atoms. Clustering of As atoms strongly depends on As concentration and temperature treatment. The observed pairing between In and As atoms seems to be typical for the interaction between acceptor and donor atoms in Si and it explains the earlier observation that the presence of donor atoms influences the lattice site of acceptor atoms.

A conductance formula for a sample of arbitrary shape with four terminals is derived to describe transport in the limit where carriers can traverse the sample without suffering phase-destroying events. The Onsager-Casimir symmetry relations are deduced. Experiments measure an off-diagonal Onsager coefficient and the magnetoconductance of such a sample is asymmetric even in the presence of an Aharonov-Bohm flux only. Symmetry relations between conductance measurements which exchange the role of current and voltage leads are predicted.

Universal conductance fluctuations in wires and Aharonov-Bohm oscillations in loops are not symmetric about $H=0\mathrm{}$. The observation of asymmetry in the periodic oscillations is possible when the phase-coherence length of the wave function is comparable to the separation of the voltage probes. In both cases, four-probe measurements yield resistances which depend on lead configuration. The asymmetries appear like Hall voltages, and are consistent with Onsager's relations.

We present results on the transverse magnetoconductance in the range 0.3 to 8 T of a narrow, variable-width channel in a GaAs:AlGaAs heterojunction. As the width of the channel is decreased below 0.25 μm the structure in the magnetoconductance changes from Shubnikov-de Haas oscillations to the magnetic depopulation of one-dimensional subbands. A semiclassical WKB calculation is presented which gives good agreement with experiment.

The quantum theory of local modes in molecules is extended to few-dimensional solids such as polyacetylene. Local modes are found to be present below the phonon band no matter how weak the anharmonicity. The problem of the local mode interacting with a phonon background is reduced to a Bose variety of the Kondo problem. Based on this analogy, a theory is constructed of thermal dissociation of local modes and the generation of high local-mode quantum numbers through intense radiation.

The density of states $\rho \mathrm{}\left(E\right)\mathrm{}$ in the tail for an electron in a correlated Gaussian random potential in three dimensions is constructed from first principles by means of a simple physical argument. This yields a linear exponential dependence of $\rho$ on $E$ which, for reasonable values of the rms potential fluctuation and correlation length, spans many decades, and occupies most of the experimentally observable energy range. This is suggested as the origin of the fundamental Urbach optical-absorption edge.

Measurements of the time history of ground-state neutral-lithium desorption from LiF during pulsed electron irradiation have resulted in a new model for the lithium desorption mechanism. We show that the slow diffusion to the surface of bulk $F$ centers created by the electron beam is the rate-controlling factor responsible for most of the time history of the lithium desorption rather than the thermal evaporation of the metal from the surface. Comparison between theoretical and experimental time dependences yields values for the $F$-center diffusion constant and its activation energy for diffusion.

Longitudinal ($\parallel c$) and transverse ($\perp c$) ac susceptibilities have been measured on single crystals of the Ising-type spin-glass ZnMn and the $\mathrm{XY}$-type spin-glass CdMn. A clear cusp is observed in each direction at different temperatures for ZnMn with 270, 390, and 600 ppm Mn. These behaviors indicate that two successive transitions occur in these systems as was suggested by mean-field theories for anisotropic spin-glasses. The phase diagram obtained is in accordance with the theoretical one.

We have measured the energy distribution of positrons reemitter from the surfaces of solids after being implanted at low energy. It is shown that this provides a unique possibility to study energy losses of a charged particle down to near thermal energy. Such measurements are used to estimate the positron thermalization time in Al. A dramatic change in this energy distribution was observed after oxidation of the Al surface. We attribute this to the band gap of the oxide. Trapping of epithermal positrons with a remarkably high cross section was observed for both Al and Cu.

We report here measurements of excitation-profile spectra for Raman scattering from the central-carbon breathing mode of crystal violet adsorbed on a smooth gold surface. For polarization configurations that involve the $xx(or yy)$ component of the Raman tensor, the spectra exhibit a resonance peak corresponding to the first singlet $\pi -{\pi }^{*}$ electronic excitation of adsorbed crystal-violet ions. The spectrum for the $\mathrm{zz}$ component does not exhibit this feature, but rather exhibits a steadily increasing scattering intensity with wavelength, that is attributed to an "intermolecular" charge-transfer electronic excitation.

The phase coherence and orientational relaxation of excitons resonantly excited in optically thin GaAs layers by a picosecond light pulse is explored by means of time-resolved degenerate-four-wave mixing. At low excitation intensities where contributions due to exciton-exciton scattering can be neglected, a rapid loss of the phase coherence as well as of the optical alignment of the excitonic dipoles within 7 ps is observed. This implies a weak coherent coupling between excitons and photons and raises the question about the range of validity of the commonly employed polariton concept for GaAs.

Neutrino ($\nu$) excitation of nuclear levels (NUEX) via neutral currents is shown to be crucially important to the solar-$\nu$ problem as a method of detecting $\nu \text{'}\mathrm{s}$ regardless of flavor. We examine ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$, and ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ as NUEX targets. The ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ system offers means for concurrent solar-$\nu$ spectroscopy by NUEX and by charged-current ${\nu }_e$ capture with a set of differing thresholds. It promises a self-contained solution to the solar-$\nu$ problem as well as key probes for revealing the structure of resonant flavor and nonflavor $\nu$ oscillations in the sun. This approach may be feasible in an ICARUS-type underground experiment.

The phenomena of neutrinos changing flavor as they propagate through matter is examined with three neutrino species. The three-neutrino problem separates into two resonances when the mixing angles are small and/or when the mass scales are well separated. We find the general solution to the solar-neutrino puzzle for the three-flavor case in the small-angle limit.

From the spectrum of dimensions of a fractal invariant measure of a dynamical system one can extract information about the dynamical process that gave rise to the measure. This is equivalent to finding the class of Hamiltonians of an Ising model with a given thermodynamics.

We present and discuss a three-parameter class of exact solutions to the localized-induction-approximation equations. These are one-soliton excitations (Bäcklund transforms) of the circular vortex motion. The corresponding generic vortex filament (of infinite or finite length) remains in the interior of the sphere (or torus) moving with a constant velocity. The above solutions generate a new class of exact solutions to the classical one-dimensional continuous Heisenberg ferromagnet model.

We demonstrate the existence of new nonrenormalizable ultraviolet divergences in bosonic nonlinear sigma models with the target-space metric corresponding to weak transverse gravitational waves. These infinities arise when the loop expansion is summed to all orders. For string theories they correspond to couplings of gravitons to massive fields.

The measurement of a nonvanishing asymmetry $\alpha \equiv \frac{\left[\Gamma \right(s\overline{b})-\Gamma (\overline{s}b\left)\right]}{\left[\Gamma \right(s\overline{b})+\Gamma (\overline{s}b\left)\right]}$ would signal $\mathrm{CP}$ nonconservation in $Z^0$ decays. We study here this effect within the standard model. In the three-generation case, the $\alpha$ value comes out small because of the effective degeneracy of $u$ and $c$ quarks at these high energies. In the four-generation case, results are encouraging for the CERN $e^{+}{e}^{-}$ collider LEP: One could have a branching ratio of this flavor-changing decay to the flavor-conserving one of ∼${10}^{-6\mathrm{}}$ and reach $\alpha$ values near unity.

We describe distinctive signatures of a possible fourth-generation charge - frac13; quark $v$ (assumed lighter than the $t$ quark) arising from $e^{+}{e}^{-}\to v\overline{v}$ production, especially at the $Z^0$ resonance. We emphasize the special charm and topological characteristics expected from $v$-quark cascade decays and the possibility of $v\to b+\mathrm{gluon}$ decays by an effective flavor-changing neutral current.