A density-functional theory for the transitions from isotropic to plastic crystal and to nematic fluid in a fluid of prolate hard ellipsoids is presented. Direct correlations are computed with a variational technique based on the Percus-Yevick equation for the correlation functions. As anisotropy is increased from the case of hard spheres, the isotropic-plastic transition density increases, as seen in computer simulations. For large anisotropies, the direct correlations are modified from previously assumed forms, bringing the isotropic-nematic transition density into agreement with simulations.

We have measured the angular profile of electrons inelastically scattered from oriented molecular ${\mathrm{O}}_2$, physisorbed on graphite at 20 K. Electrons ejected after vibrational energy loss from a temporary negative-ion resonance observed near 9 eV have an angular profile which peaks at 15°±5° from the normal to the crystal surface, independent of the incident beam angle. From an analysis of these angular distributions we are able both to identify the ${\mathrm{O}}_2$ negative-ion state (as ${}_{}{}^2{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}$) and to determine the orientation of the ${\mathrm{O}}_2$ molecular axis (as 25°±5° from the vertical) in what we believe to be the monolayer $\zeta 2$ phase.

To establish the nature of the early-stage structures that occur during phase separation, we studied solutions of a nonionic surfactant micellar solution (${\mathrm{C}}_{12}$${\mathrm{E}}_6$ in ${\mathrm{D}}_2$O). We report the existence of long-lived nonequilibrium structures during shallow quenches along the critical isochore. The power-law scattering profile of these structures is consistent with ramified clusters with a fractal dimension $D=1.6\mathrm{}\pm 0.1$.

Recombinative desorption of atomic hydrogen on the walls of a gas cell has been observed to populate vibrational levels up to $v=9\mathrm{}$. The vibrational populations follow a Boltzmann distribution near 3000 K up to $v=3\mathrm{}$ and for higher levels the populations are well in excess of this temperature. These observations bring to light a new mechanism for vibrational excitation of ${\mathrm{H}}_2$ in volume ${\mathrm{H}}^{-}$-ion sources.

Relative densities of H atoms and ${\mathrm{H}}_2\left(v^{\prime \prime }\right)$ molecules in vibrational levels up to $v^{\prime \prime }=5\mathrm{}$, effusing from a metal box containing heated tungsten filaments, were detected by a resonance-enhanced multiphoton ionization technique. The atom production is compatible with dissociation of molecules at the filament. The formation of ${\mathrm{H}}_2\left(v^{\prime \prime }\right)$ is attributed to an Eley-Rideal-type reaction, in which a free atom recombines with an adsorbed atom at the cold wall, followed by desorption. Above 2800 K we observe an abrupt drop in the atom and the ${\mathrm{H}}_2\left(v^{\prime \prime }\right)$ densities. It is ascribed to the effects of annealing on the filament surface.

Time-dependent fluctuations of conductance exist in disordered metals. A constant electric field applied on the sample will be modulated by the conductance fluctuations to generate noise which in turn can suppress quantum mechanical interference, leading to an enhanced conductivity in a higher electric field.

The enhancement of the optical transition rate in (111)-oriented GaAs-AlGaAs quantum wells (QW's) compared with (100) QW's has been confirmed by photoluminescence. It is experimentally determined by absorption measurements of QW's that the anisotropy of the heavy-hole band in GaAs is extremely large; $\frac{m_{\mathrm{hh}}^{*}\mathrm{}\left[111\right]\mathrm{}}{m_{\mathrm{hh}}^{*}\mathrm{}\left[100\right]\mathrm{}}=2.65\mathrm{}$. This large anisotropy in the host GaAs is considered to result in the higher optical transition rate in (111) QW's compared with (100) QW's.

The coupling of quasiparticle excitations in an electron gas to the LO phonons gives rise to novel phonon modes with energies below $\lesssim q{v}_{\mathrm{F}}$. We propose that these low-energy "phonon" modes, in addition to the usual plasmon-phonon coupled modes, are responsible for a large (by orders of magnitude) enhancement of the hot-electron energy-loss rate to the lattice at low electron temperatures, as observed in several recent experiments. Experimental conditions are outlined for an unambiguous observation of this predicted many-body effect.

The number of surface atoms on Ge(100)-(2×1) and $\mathrm{Ge}\mathrm{}\left(111\right)\mathrm{}-c(2\mathrm{}\times 8)$ which contribute to the $\mathrm{Ge} 3d$ core shift is determined by a combination of synchrotron photoemission, high-energy electron diffraction, and molecular-beam epitaxy techniques. The result is 0.87 ± 0.09 monolayer for Ge(100)-(2×1), revealing that the buckled-dimer bond is primarily covalent in nature. The $\mathrm{Ge}\mathrm{}\left(111\right)\mathrm{}-c(2\mathrm{}\times 8)$ surface shows "adatom" emission corresponding to 0.33 ± 0.04 monolayer; the departure from the ideal value of 0.5 monolayer is attributed to a large number of defects on the surface.

We observe in Si-doped GaAs, by capacitance transient spectroscopy, electronic occupation of a highly localized state of the donor-related $\mathrm{DX}$ center. The emission and capture kinetics are those of a metastable state which lies above the conduction-band edge. The state is so spatially localized that its emission kinetics are not measurably perturbed by neighboring Si atoms (donors or acceptors) at an average distance ≅3.5 nm. Occupation of this state is therefore a previously unsuspected mechanism which can limit the free-carrier density in very heavily doped $n-\mathrm{GaAs}$.

The proximity-induced Josephson effect is studied microscopically by analysis of the quasiclassical transport equations in a sandwich geometry. The main Shapiro step is found at $V=\frac{h\nu }{4e}$. It is thus concluded that the proximity-induced Josephson effect is not seen in a recent experiment by Han et al.

We have studied the low-temperature tunneling behavior of point-contact junctions with adjustable capacitances in the ${10}^{-18\mathrm{}}$-F range. The tunneling characteristics show clear evidence of single-electron tunneling, induced by a Coulomb blockade, and are in quantitative agreement with theoretical predictions.

We have studied the dynamics of the phase separation between a dilute lyotropic lamellar phase (smectic) and an isotropic phase in a quaternary (water, sodium dodecyl sulfate, pentanol, dodecane) system. Observation of the phase separation with small-angle light scattering on an oriented sample allows us to demonstrate the anisotropic growth of the isotropic phase inside the lamellar phase. Measurement of the variation of the largest characteristic distance as a function of time indicates that the kinetics of the phase separation follows a power law with a characteristic exponent ⅓.

We show that the geometrical phases recently discovered in quantum mechanics also occur naturally in the theory of any classical complex multicomponent field satisfying nonlinear equations derived from a Lagrangean which is invariant under gauge transformations of the first kind. Some examples are the paraxial wave equation for nonlinear optics, and Ginzburg-Landau equations for complex order parameters in condensed-matter physics.

We have constructed a prototype transmission positron microscope (TPM) and taken magnified pictures of various objects with it. Information gained from the prototype TPM has allowed us to predict resolutions achievable in the near future with an upgraded TPM. Applications are discussed.

The serendipitous observation of neutrinos and light from the recent supernova in the Large Magellanic Cloud has provided a wealth of new information about stellar collapse and about neutrinos. It is pointed out that, in addition, the nearly simultaneous arrival of the photons and neutrinos after a journey of some 160 000 yr provides a new, stringent test of the Einstein equivalence principle on intergalactic distance scales. The relativistic time delays of the photons and neutrinos from SN1987A caused by the gravitational field of our Galaxy are compared, and it is shown that they are equal, to an accuracy of approximately 0.2% of the predicted delay.

The observation of a neutrino burst within 3 h of the associated optical burst from supernova 1987A in the Large Magellanic Cloud provides a new test of the weak equivalence principle, by demonstrating that neutrinos and photons follow the same trajectories in the gravitational field of the galaxy. The accuracy of the test depends on the poorly known mass distribution in the outer parts of the galaxy, but is at least 0.5% and probably much better. This result provides direct evidence that the Shapiro geodesic time delay is identical, to this accuracy, for different elementary particles, independent of spin and internal quantum numbers.

Finite-temperature transitions in lattice QCD are studied for various numbers of flavors in the range $1\le N_f\le 18$ on an $8^3$×4 lattice by the Langevin simulation technique. It is found that the weakening of the transition at intermediate quark mass is a general feature for $N_f\ge 2$ but that the smoothing out of the transition observed for $N_f=2\mathrm{}-4$ does not occur for large numbers of flavors ($N_f\ge 10$). For $N_f=1\mathrm{}$ the transition weakens toward small quark mass $m_q$ but remains first order down to $m_{q}a=0.05\mathrm{}$.

Limits on charged-scalar Yukawa couplings from $\tau$ and $B$ decays are discussed. They (and other existing limits) are consistent with "strong" couplings (∼1 for the third generation) even if the lightest scalar mass is in the range $20 \mathrm{GeV}\lesssim M_{\phi }\lesssim 100 \mathrm{GeV}$ but saturated in this case. $B(B\to \tau \nu X)$ and (for $M_{\phi }>m_t$) $B(T\to \tau \nu X)$ may be then as large as 30% and 70%, respectively. Both for $M_{\phi }<m_t$ and $M_{\phi }>m_t$ the potentially possible top-quark signature in $p\overline{p}$ collisions is $\tau +2\mathrm{}$ jets final state. The upper limit for $\tau \to \nu \eta \pi \mathrm{is}\simeq 0.003\%$.

The masses of the $K^{-}$ and ${\Sigma }^{-}$ have been measured by use of exotic-atom x-ray transitions in Pb and W. The transitions used were free from contaminant nuclear $\gamma$ rays and strong-interaction effects. Two $K^{-}$ transitions and three ${\Sigma }^{-}$ transitions for each target satisfied these criteria. The results are $M_{K^{-}}=493.636\mathrm{}\pm 0.011$ MeV and $M_{{\Sigma }^{-}}=1197.532\mathrm{}\pm 0.057$ MeV. The precision of these measurements is greater than that of any previous experiment. When combined with previous measurements they form a consistent set.