We report the first determination of the formation energy of excess-As-atom–related defects in Te-doped GaAs. The photocapacitance method in the constant-capacitance condition is applied to GaAs:Te prepared by 67-h annealing at 850–1100 °C under various As vapor pressures followed by rapid cooling. From an Arrhenius plot of the saturating deep-level density at quasi thermal equilibrium under high As vapor pressure, the formation energy of the defect is determined to be 1.16 eV in Te-doped horizontal-Bridgeman-grown GaAs crystals.

Using lattice sums we show that the dense, monatomic, face-centered, icosahedral quasicrystals can be stable at zero temperature. We compare the energy to other periodic crystals and find it minimal. We create phason strain in this lattice, calculate the energy, and show that this lattice is at least locally stable against such strains. We calculate the two elastic constants associated with this strain. Our calculations indicate that it is also globally stable against phason strains. We argue that it is also stable under elastic strains. We show that the energy as a function of the strain is not described by a simple quadratic expansion as implied by simple phason elastic theory.

We present an analysis technique for understanding coverage- and temperature-dependent desorption phenomena. Using a quasiequilibrium model, we derive an expression for the desorption rate in terms of a lattice-gas virial expansion and apply the results to CO desorption from Cu(001) and Cu(011). The interplay between short-range repulsion and long-range attraction leads to the vanishing of virial coefficients at some temperature and thus to a nearly coverage-independent desorption rate. The origin of kinetic compensation is shown in a simple statistical thermodynamic treatment of the adsorbed layer.

Thermodynamic studies of the liquid-vapor transition of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ confined in silica aerogel with a porosity of 95% is reported. In spite of the random nature of the pores, sharp heat-capacity signatures are seen. The positions of the sharp peaks are used to map out the liquid-vapor coexistence boundary, which is describable by a power law with an exponent β=0.28±0.05.

While amorphization by solid-state reaction is limited to multilayers of A-B metal couples with large negative heat of mixing Δ${\mathit{H}}_{\mathrm{mix}}$≪0, mechanical alloying allows amorphization of mixtures of some pure metals with Δ${\mathit{H}}_{\mathrm{mix}}$≊0. We have recently shown that for an amorphous phase in a sharp concentration gradient there is critical thickness ${\mathit{x}}_{\mathit{c}}$ below which crystallization is thermodynamically disallowed. We show here that ${\mathit{x}}_{\mathit{c}}$ is very small when Δ${\mathit{H}}_{\mathrm{mix}}$≊0 but that in mechanical alloying the simultaneous growth and deformation of the amorphous interlayers allows them to grow substantially in volume while remaining under the critical thickness ${\mathit{x}}_{\mathit{c}}$.

Using first-principles calculations, we demonstrate an explicit reaction pathway for carrier-induced changes in hydrogenated Si through dissociation of paired-H complexes. The complex dissociates through the capture of charge carriers with calculated kinetic barriers for electron- and hole-induced dissociations of 1.1 and 0.9 eV, respectively, in crystalline Si, in general agreement with the experimentally observed activation energies for single-carrier defect creation in hydrogenated a-Si. Exciton lowering of the barrier accounts for the low activation energy of light-induced defect formation.

Evidence for weak-localization corrections to the electronic thermal conductivity of a quasi-two-dimensional electron system is reported for the first time. The validity of the Wiedemann-Franz relation theoretically predicted for such disordered systems is also experimentally verified.

Resistivity and I-V measurements have been performed on thin amorphous ${\mathrm{Nb}}_3$Ge films in perpendicular fields. In the vicinity of the mean-field-transition line, pure flux-flow behavior occurs due to melting of the vortex lattice. The dependence of the melting field on temperature and thickness is well described by the melting theory for a two-dimensional lattice. In the narrow crossover regime between melting and weak pinning, dislocation-mediated flux creep occurs prior to melting.

We study the velocity versus applied-field relation of a moving Bloch domain wall, using the ‘‘collective-coordinate’’ method employed in the theory of soliton motion. For a sufficiently large field, the wall emits spin waves, thereby self-limiting its velocity. That velocity is equal to the common value of the phase and group velocities of a particular spin wave, and may be above or below the Walker limit ${\mathit{V}}_{\mathit{W}}$, depending on the intrinsic damping.

The nuclei of gaseous ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$ atoms are polarized by spin exchange with optically pumped Rb vapor. The Xe gas is subsequently frozen. The nuclear polraizations of up to 25% are largely unaffected by freezing. The spin-lattice relaxation time ${\mathit{T}}_1$ was measured as a function of temperature, magnetic field, and isotopic composition. At 77 K and 1800 G, ${\mathit{T}}_1$ is limited to about 3 h by a new relaxation mechanism. The potential for accumulating large samples with even higher polarizations makes laser-polarized Xe ice promising for a wide variety of new experiments.

We carry out equilibrium and steady-state simulations of a periodic square Josephson-junction array in the presence of a transverse magnetic field giving a uniform frustration of f=2/5. We find a first-order transition to the low-temperature superconducting phase. Linear resistivity above ${\mathit{T}}_{\mathit{c}}$ and nonlinear resistivity below ${\mathit{T}}_{\mathit{c}}$ are interpreted in terms of vortex fluctuations.

We show that the two-dimensional Heisenberg monolayer with perpendicular ground-state spontaneous magnetization may have a temperature ${\mathit{T}}_{\mathit{R}}$ at which the spontaneous magnetization turns into the plane of the monolayer. The reorientation of the magnetization occurs via a phase transition.

We use Brillouin scattering to measure the longitudinal-phonon dispersion curves for a suspension of hard-sphere colloids. Two distinct propagating acoustic excitations are observed when the wavelength of the sound is comparable to, or smaller than, the size of the spheres. One excitation has a velocity intermediate between that of the fluid and the solid phases and is interpreted as an acoustic wave propagating through the composite medium of fluid and solid spheres. The second has a velocity slower than both that of the solid and that of the fluid phase and is interpreted as a coupled interfacial, or Stoneley, wave.

We derive the exact distribution of times spent in the classically forbidden region by a particle as it tunnels through a square barrier potential. We find that the integrated distribution has a simple interpretation. If one restricts the time spent in the barrier to be ⩽τ, then an effective uncertainty ΔV is introduced in the barrier height, such thatΔVτ≃1. This uncertainty leads to unusual interference effects in the distribution. These interference effects explain why the average speed of a particle traversing a very opaque barrier may exceed the speed of light. © 1990 The American Physical Society

Dyson’s Brownian-motion model is recovered as a rigorous description of the dependence of Hamiltonians H=${\mathit{H}}_0$+λV on the control parameter λ when ${\mathit{H}}_0$ and V belong to different universality classes. For instance, time-reversal invariance may hold for ${\mathit{H}}_0$ but not for V. The clue to our result lies in (i) a certain rescaling of the energy, H→ √f(λ) (${\mathit{H}}_0$+λV), and (ii) not individual Hamiltonians but suitable families.

The thermodynamics of a gas of order two (Q=2) parabosons or parafermions is shown to be equivalent to that of a mixture of two gases of ordinary bosons or fermions, which either contain equal numbers of particles or the particle numbers of which differ by one.

A new mechanism through which a net baryon asymmetry could be generated at the electroweak phase transition is discussed. It works efficiently in ‘‘nonminimal’’ extensions of the standard model such as occur in supersymmetric theories, where the Higgs sector involves several fields. In these theories it appears capable of producing the observed asymmetry. Conversely, with more detailed calculations this mechanism could be used to put important constraints on nonminimal extensions of the standard model.

Recent observation of Υ(4S)→ψ+X with the ψ too energetic to come from the decay of a B meson suggests that there could be a substantial rate of non-BB¯ decay of Υ(4S). We attempt to explain this by conventional quarkonium spectroscopy and suggest that Υ(4S)→${\mathit{h}}_{\mathit{b}}$(1P)+η and Υ(4S)→${\mathrm{\eta }}_{\mathit{b}}$+${\mathit{h}}_1$(1170) with subsequent decays into ψ’s are possible candidates. A crude estimate of the corresponding rates show them to be of the correct order of magnitude. The model implies that ${\mathit{h}}_{\mathit{b}}$ decays to DD¯ are more frequent than into the ψ, causing a potentially serious background to the sample used for determination of ‖${\mathit{V}}_{\mathit{u}\mathit{b}}$/${\mathit{V}}_{\mathit{c}\mathit{b}}$‖.

Gluon-initiated contributions to deep-inelastic-scattering processes, such as charm production, can be comparable in magnitude to the ‘‘leading-order’’ sea-quark processes. A proper next-to-leading-order calculation in QCD confirms this and yields distinct dependences of these two contributions on the kinematic variables and on the charm-quark mass. These results imply that previous analyses of charm-production data to extract the strange and charm content of the nucleon, as well as the precise determination of standard-model parameters based on these analyses, need to be reassessed.

We propose a quantum description of color transparency in terms of light-cone operator matrix elements. The fundamental quantity needed is the distribution amplitude ${\mathrm{\phi }}_{\mathit{A}}$(x,${\mathit{Q}}^2$) for each hadron that participates in the elastic scattering in the nuclear target. Two large scales, ${\mathit{Q}}^2$ and a mass scale going like ${\mathit{A}}^{1/3}$ for A≫1, occur in a perturbatively consistent description. We find that the limit of large A and moderate ${\mathit{Q}}^2$ may shed light on controversies over the applicability of perturbative QCD to exclusive processes.