We have measured the critical velocity of superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ flow through a submicron orifice from 1.2 K down to 5 mK. The velocity for the onset of phase-slip events is found to be independent of pressure, to be strongly altered by minute traces of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ impurities, and to vary with temperature as $1-\frac{T}{T_0}$ with $T_0=2.46\mathrm{}$ K. These results imply the existence at $T=0\mathrm{}$ of a hydrodynamic instability which is thermally activated at finite temperature and takes place in a very small volume.

We have utilized differential scanning calorimetry to monitor the solid-state reaction of crystalline metals to form an amorphous alloy in multilayered thin-film diffusion couples of elemental Ni and Zr. The heat of formation of amorphous ${\mathrm{Ni}}_{68}$${\mathrm{Zr}}_{32}$ alloy from the elemental metals is measured directly and found to be 35 ± 5 kJ/mole. The kinetics of these reactions has been examined. The activation energy for interdiffusion in the amorphous phase is determined to be $E=1.05\mathrm{}\pm 0.05$ eV.

We report the direct measurement of the excitation of electron-hole pairs at a single-crystal InP (100) surface, induced by the scattering of ground-state Xe atoms over an incident energy range $2 \mathrm{eV}<E_i<10\mathrm{} \mathrm{eV}$. Above $E_i\sim 3$ eV the yield is fitted well by an exponential dependence on $E_i^{}{}_{}{}^{-1\mathrm{}}$. The incident-energy and angle dependence suggest the creation of a local thermal hot spot with fast electronic equilibration.

We have observed a series of narrow peaks in the low-temperature photocurrent-voltage characteristic of multiple-quantum-well $p-i-n$ junctions grown by molecular-beam epitaxy. In addition, the forward dark current and capacitance exhibit a steplike decrease at the same voltages of the photocurrent peaks. These features provide the first direct evidence of electronic transport as well as of depletion of quantum wells by tunneling through deep levels in the barrier layers in semiconductor superlattices.

Photoelectrons emitted along a metal surface from atoms located at a small distance outside that surface have a high probability to ricochet with the excitation of a surface plasmon if the angle of emergence $\alpha$ is less than a critical angle ${\alpha }_M$. The line shape of the surface-plasmon satellite $I_1({\varepsilon }_k,\alpha )$ peaks strongly at $\alpha =0\mathrm{}$ and presents another peak at $\alpha ={\alpha }_M$. Experiments are suggested to locate the second peak and thereby determine ${\alpha }_M\left({\varepsilon }_k\right)$, a function which should yield direct information on the form and the parameters of the surface-plasmon dispersion relation.

Electron-capture spectroscopy has been used to investigate magnetic order at the topmost atomic layer of atomically clean and flat (100) surfaces of bulk paramagnetic vanadium. From low-energy electron-diffraction measurements, we find a $p(1\mathrm{}\times 1)$ surface structure. At 300 K, the long-ranged electron-spin polarization amounts to ∼34%, clearly demonstrating the existence of ferromagnetic order at the V(100) $p(1\mathrm{}\times 1)$ surface. With increasing temperature, the electron-spin polarization decreases almost linearly and disappears at a surface Curie temperature $T_{\mathrm{C}s}\approx 540$ K.

Evidence for a condensed phase of plasma in Si above the electron-hole-liquid (EHL) critical temperature (≅ 24 K) is presented. The condensed plasma has a density of about one-tenth that of the low-temperature EHL phase and is observed both above and below the EHL critical point. This new low-density liquid appears in the region expected for the metal-insulator transition. A new phase diagram of excitonic matter in Si is presented which is radically different from previous work.

Partial occupation ${\overline{n}}_a$ of a resonant adsorbate valence state due to chemisorption leads to ${\overline{n}}_a$-dependent phase space for adsorbate dipole electronic transitions. This fundamentally influences the dynamic polarizability and related physics of an adsorption complex. We outline a unified interpretation of such different observations as the induced dipole moment of adsorbed rare-gas atoms, the cross section of He scattering from adsorbed CO, and the change in effective deexcitation energy of metastable atoms in Penning spectroscopy of adsorbates, based on the ${\overline{n}}_a$ dependence of van der Waals forces at surfaces.

The results of a detailed calculation of the effects of resonant neutrino oscillations in the sun on the current and proposed solar-neutrino experiments are presented. Analytic results are used for the electron-neutrino survival probability so that a sophisticated model for both the production distribution of the solar neutrino sources and the solar electron-number density can be employed. Contour plots for the electron-neutrino capture rate, in the plane of the mass difference squared versus vacuum mixing angle, are given for the current ${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$ experiment and the proposed ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ga}$ detector.

Galactic halos seeded by loops of string in a universe dominated by dark matter fall into two categories: (1) Non-self-intersecting loops precipitate nearly spherically symmetric collapse and lead to a halo with low angular momentum $J$ and a density profile $\rho \sim r^{-\frac{9}{4}}$. (2) Progeny of a self-intersecting primary loop recollapse into a single object which acquires $J$ through tidal interactions. Its density profile, $\rho \sim r^{-2\mathrm{}}$, is consistent with "flat rotation profiles" observed in spiral galaxies. It is tempting to identify these two types of string-seeded objects with (1) elliptical and (2) spiral galaxies.

A phenomenological quantization scheme is carried out for the collision process of identical solitons of a nonlinear Schrödinger equation. An effective potential is determined from the classical expression for the time delay. The corresponding effective $S$ matrix reproduces the exact $S$ matrix of the quantized nonlinear field as well as the ground-state binding energy for large $N$, but it is not adequate to describe the scattering process in the low-energy region.

A search for neutral penetrating particles has been made by use of an 800-GeV proton beam incident on the magnetized beam dump of the spectrometer of Fermilab experiment 605. Limits on the mass and couplings of such particles are presented. A 1.8-MeV axion coupled only to $e^{+}{e}^{-}$ is ruled out.

We analyze nuclear internal-pair-transition experiments for evidence of a possible 1.7-MeV axion and find that the conventional axion and the new axions described by the theories of Krauss and Wilczek and of Peccei, Wu, and Yanagida are inconsistent with these data. The results are independent of nuclear models.

We examine the effect on neutrino counting in $e^{+}{e}^{-}\to \nu \overline{\nu }\gamma$ due to light right-handed neutrinos and extra gauge bosons that could occur in some superstring-inspired $E_6$ gauge models. The effect can be as high as $\Delta N_{\nu }\simeq 0.75$ at $\sqrt{s}=30\mathrm{}$ GeV, but is smaller at higher energies.

For the classical nonlinear $\sigma$ model on a general target manifold $M$, the progressive imposition of world-sheet supersymmetry (with torsion) from (1,0) to (2,2) is systematically analyzed and generalized; at each step, implications both for $M$ and for the background gauge connection are stated. Quantum conformal invariance is then imposed. Our analysis applies to superstrings without known space-time supersymmetry.

A fermion mass generated by spontaneous symmetry breaking is proportional to its Yukawa coupling $y$ to a Higgs field. Like the Higgs-field self-copuling, $y$ may well be trivial and diverge at a finite energy scale ${\Lambda }_f$, corresponding to an upper limit on fermion masses. We verify this by solving a simple model by means of a $\frac{1}{N}$ expansion. Applied to the standard model, this suggests that there is an upper limit to quark and lepton masses. These results have implications for the "decoupling" of heavy fermions and bear on the issue of whether apparently "anomalous" gauge theories can be consistently quantized.

We show that in order to explain the observed dependence of multiplicity distributions on energy and shifts of the centers of rapidity bins it is sufficient to assume the existence of two sources. One source is concentrated at small rapidities and has properties of a thermally equilibrated system as could be expected from a quark-gluon plasma. The other one, contributing to the whole rapidity region, displays characteristics of bremsstrahlung emission.

The mass shift of charmonium near the critical temperature of the deconfining transition is investigated by a $c-\overline{c}$ potential model through the change of the string tension. On the basis of the results, detection of the shift by lepton-pair production is discussed by the use of the hydrodynamical model of ultrarelativistic nucleus-nucleus collisions. It is shown that the mass shift is detectable if the critical temperature is not lower than 300 MeV.

A novel aspect of strangeness in the baryon structure is discussed. In particular, we propose a mechanism by which the strangeness flavor induces the baryon "size" (and bag radius) to be larger in hyperons than in nonstrange baryons. We also discuss the implications on the SU(3) chiral bag.

The SU(3) deconfinement temperature is studied on asymmetric lattices as a probe of scaling. For lattices with four sites in the temperature direction (${\beta }_c\approx 5.7$) we find precise agreement between the measured asymmetry dependence of the $\Lambda$ parameter and that predicted by one-loop perturbation theory. The agreement holds over a large range of asymmetry ($0.65<~\xi <~1.1$) and implies that violations of perturbative scaling above $\beta \approx 5.7$ are independent of asymmetry and therefore unlikely to be lattice artifacts. This provides evidence that the coupling range $5.7\lesssim \beta \lesssim 6.2$ is a regime of nonperturbative but universal scaling.