We have used the Green's-function technique to carry out electronic-structure and total-energy calculations for the gallium vacancy in GaAs and for the nearest-neighbor (arsenic vacancy)-(arsenic antisite) pair which results when an adjacent arsenic atom hops over and fills the gallium vacancy. The results are interpolated with a tight-binding model for intermediate positions of the moving arsenic atom. The system shows bistability and metastability, and seems to describe many properties of the $\mathrm{EL}$ 2 center.

This Letter describes a novel concept that can lead to new quantum interference effects with potential applications in switching devices. The interference occurs between currents flowing in two parallel channels formed by contiguous GaAs quantum wells. Preliminary experiments with a simple structure show oscillations in the conductance as a function of the magnetic field with a period close to $\frac{h}{e}$ indicating an Aharonov-Bohm effect.

A quantitative evaluation of the lifetime of image-potential-induced surface states is presented for the first time. It is shown that Auger processes give contributions to those linewidths of a few thousandth of an electronvolt; these widths are much narrower than the values experimentally reported, which seem to be limited by the experimental resolution. Increased resolution could provide experimental information on inelastic interactions at surfaces.

The $b$-axis conductivity was measured as a function of the electric field $E$ for temperatures $T>\mathrm{}34\mathrm{}$ K in the one-dimensional organic conductor TTF-TCNQ (tetrathiafulvalene-tetracyano-quinodimethane). Deviations from Ohm's law, with a threshold field $E_T$, have been observed for $T<\mathrm{}60\mathrm{}$ K. $E_T$ reaches a minimum of 0.25 V/cm just below the onset of the 54-K Peierls transition before increasing to 6.4 V/cm at 34 K. The results are discussed in terms of the depinning of the charge-density-wave condensate at high fields; the interaction between the two types of chains in TTF-TCNQ is included.

We report on the metal-insulator transition of quench-condensed ${\mathrm{Al}}_x{\mathrm{Ge}}_{1-x}$. The transition is obtained either by variation of $x$ or by low-temperature annealing (amorphous-relaxation-like process). This last approach gives a straightforward illustration of the scaling theory of localization and information about the critical behavior. Tunneling measurements of the correlation gap $\Delta$ allow a comparison with other materials and show strong electron interactions in this highly disordered system.

We report experiments in which we determine the rate of LO-phonon emission by hot electrons. High-energy tails of the photoluminescence spectra are used to measure the distributions of hot electrons, which are heated both optically and electrically. Comparison of resultant temperatures shows that virtually all the excess kinetic energy of an optically excited electron-hole pair stays in the electron system. Both this result and an analysis of the spectra themselves show that electron scattering is much faster than LO-phonon emission. The measured LO-phonon emission rate is in agreement with theory. Hot phonons do not appear to play an important role in our experiments.

We predict the presence of a spectral feature in the isotropic cosmic gamma-ray background associated with gravitino decays at high red shifts. With a gravitino abundance that falls in the relatively narrow range expected for thermally regenerated gravitinos following an inflationary epoch in the very early universe, gravitinos of mass several gigaelectronvolts are found to yield an appreciable flux of 1—10-MeV diffuse gamma rays.

In contrast to previous treatments, a new analysis of two-dimensional many-electron systems subject to periodic boundary conditions in a magnetic field leads to a fully two-dimensional structure of the quantum numbers at rational Landau-level filling. The structure of the new symmetry analysis has an intrinsically many-particle character. Full agreement between numerical studies of quantized-Hall-effect systems in periodic and spherical geometries is achieved, and the problem of ground-state degeneracy is clarified.

The structure of the group corresponding to a Kac-Moody algebra is analyzed. The two-cocycle for the group is constructed in terms of local coordinates and the transition functions between different coordinate systems are computed. The results are extended to the group $\mathrm{Map}(S^3, \mathfrak{G})$ which arises in a (3+1)-dimensional Yang-Mills theory.

Certain first-order nonlinear ordinary differential equations exemplified by strongly damped, quasiperiodically driven pendula and Josephson junctions are isomorphic to Schrödinger equations with quasiperiodic potentials. The implications of this equivalence are discussed. In particular, it is shown that the transition to Anderson localization in the Schrödinger problem corresponds to the occurrence of a novel type of strange attractor in the pendulum problem. This transition should be experimentally observable in the frequency spectrum of the pendulum or Josephson junction.

It is shown that the one-loop infinities cancel between diagrams for $M=5\mathrm{}$ open external lines, and any $M$ for suitably constrained kinematics, in the O(32) superstring theory when one employs a principal-part prescription for regularization.

It is shown that the symmetry algebra of the Kadomtsev-Petviashvili equation can be related to an infinite-dimensional subalgebra of the loop algebra [$\mathrm{SL}(5, R)\otimes R(t, t^{-1\mathrm{}})\oplus \left[R\right(t, t^{-1\mathrm{}}\left)\frac{d}{\mathrm{dt}}\right]$. The algebra is used to generate new classes of solutions of the Kadomtsev-Petviashvili equation, depending on several arbitrary functions.

Lower partial-lifetime limits for the decay of the free proton are obtained in the range ${10}^{30}$ to ${10}^{32}$ yr, from a 417-d exposure of a water Cherenkov detector with a fiducial mass of 370 metric tons of hydrogen. In this time, 401 contained events were observed, at a rate consistent with atmospheric neutrino interactions. Eleven of these events are compatible with both neutrino interactions and the neutrinoless decay of the free proton. These results are of the same order of magnitude as previously published limits for bound plus free protons, but are independent of models for proton decay in oxygen.

Precise measurements are reported for the leptonic branching ratios of the $\tau$ lepton from the reaction $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$ observed with the MAC detector operating at the SLAC storage ring PEP. Results from samples in which one $\tau$ decays into three charged particles and the other to $e$ or $\mu$ are $B_e=0.180\mathrm{}\pm 0.009\pm 0.006$ and $B_{\mu }=0.183\mathrm{}\pm 0.009\pm 0.005$. A sample of $e\mu$ events gives $B_e{B}_{\mu }=0.0288\mathrm{}\pm 0.0017\pm 0.0019$. With the assumption of $e-\mu$ universality, these results are combined to give $B_e=0.178\mathrm{}\pm 0.005$, which implies a $\tau$ lifetime of (2.85±0.09)×${10}^{-13\mathrm{}}$ sec.

The one-loop renormalizability of both the two-component (LC2) and four-component (LC4) formulations of the light-cone gauge is demonstrated by construction of the complete one-loop counter Lagrangeans. The Mandelstam-Leibbrandt prescription is used to regularize the singular $\frac{1}{p^{+}}$ factor. In LC4, the one-loop self-energy and three-vertex both have anomalous, unrenormalizable ultraviolet divergences, but the counterterms associated with these divergences cancel exactly, rendering the total counter Lagrangeans for the two formulations identical, at least to $O\left(g^3\right)$.

We work out the general solution of the chiral Schwinger model in the path-integral formalism and show that there is no mass generation in this model for the vector boson and that the theory is inconsistent.

We have reinvestigated the ${\beta }^{+}$ decay of ${}_{}{}^{24}{}_{}{}^{}\mathrm{Al}$($4^{+}$) and find that the analog transition has $\mathcal{F}t=3106\mathrm{}\pm 38$ sec. This is consistent with the value $\mathcal{F}t=3081.7\mathrm{}\pm 1.9$ sec obtained from the $0^{+}$ → $0^{+}$ pure Fermi transitions but inconsistent with the value $\mathcal{F}t=5715\mathrm{}\pm 13$ sec claimed for the Fermi component of ${}_{}{}^{35}{}_{}{}^{}\mathrm{Ar}$ decay. We find no evidence for the speculation that Fermi matrix elements of high-$J$ states are anomalously large and conclude that the ${}_{}{}^{35}{}_{}{}^{}\mathrm{Ar}$ result is probably due to experimental error.

Significant intershell interaction effects are observed in the $5d$ branching ratio of the heavy non-transition metal Pb. Atomic calculations reveal their atomic character, but a distinct influence of the chemical environment on the branching-ratio's energy dependence is reported for the compounds Pb${\mathrm{I}}_2$ and Pb${\mathrm{O}}_2$.

Spontaneous radiation by an atom in a Rydberg state has been inhibited by use of parallel conducting planes to eliminate the vacuum modes at the transition frequency. Spontaneous emission is observed to "turn off" abruptly at the cutoff frequency of the waveguidelike structure and the natural lifetime is measured to increase by a factor of at least 20.

It is shown that substantial ionization during the rise of the pulse of a strong laser is inevitable. Thus only multiply charged ions are likely to be exposed to fields above ${10}^{14}$W/${\mathrm{cm}}^2$. The behavior of the $N$-photon generalized cross section for $N\gg 1$ and its connection to tunneling are discussed.