We present acoustic velocity data that demonstrate a high-pressure solid-solid phase transition in molybdenum at about 2.1 Mbar and 4100 K. This observation is supported here by first-principles theoretical calculations which predict a zero-temperature bcc→ hcp transition within our experimental pressure range. In addition, our results show the melting transition for shock-compressed Mo at about 3.9 Mbar and 10000 K.

Scanning tunneling microscopy of a "5×5"-Cu/Si(111) ordered layer reveals an unusual type of quasiperiodic phase. Topographic and spectroscopic images together with their Fourier analysis reveal at least three electronically and structurally distinct local phases which arrange to form a quasiperiodically ordered structure. The nature and origin of this novel structure is discussed.

Vibrations of amorphous Si are represented by the network model of Wooten, Winer, and Weaire and the forces of Stillinger and Weber. It is shown how the Kubo formula for thermal conductivity can be evaluated "exactly" assuming harmonic vibrations, making only finite-system-size errors. The results support a "shunt-resistor model" which fits data for glasses very well.

We have studied the optical properties of GaInAs/GaAs strained-layer superlattices grown along the [100] and [111] crystallographic axes. Absorption and luminescence spectra for the two superlattices show qualitatively different behavior. These differences are attributed to the presence of straingenerated electric fields in the (111) strained-layer superlattice, which have been predicted theoretically but have not been previously observed experimentally.

We demonstrate the growth of epitaxially ordered Co-Au superlattices by ultrahigh-vacuum deposition techniques. The superlattice structure consists of hcp Co(0001) and fcc Au(111) layers with the in-plane Co [$11\overline{2}0$] crystallographic axis parallel to GaAs [001]. X-ray scattering, high-resolution transmission-electron microscopy, and measurements of the magnetic anisotropy show that the Co-Au interfaces are abrupt even in the presence of a large lattice mismatch.

We report the first successful measurements of the magnetic fluctuation, or paramagnon, spectrum of an exchange-enhanced paramagnet. The spectrum measured in pure specimens of ${\mathrm{Ni}}_3$Ga by inelastic neutron scattering can be described by a spin-density relaxation rate $\Gamma \left(\mathrm{q}\right)$ which vanishes at small wave vectors and is strongly dispersive. The experimental findings are analyzed in terms of the theory of a Fermi liquid in the collisionless quasiparticle regime.

A supercell procedure for exact evaluation of the one-electron Kubo-Greenwood formula is applied to the resistivity $\rho$ of ${\mathrm{V}}_{1-x}{\mathrm{Al}}_x$ alloys and compared with a Korringa-Kohn-Rostoker coherent-potential approximation calculation. The results of these calculations agree well, consistent with the observation of delocalized eigenstates, in spite of the very high resistivity, $\rho \approx 200$ μΩ cm.

We have compressed xenon in a diamond-anvil cell to approximately 200 GPa. The metallization of xenon by band-gap closure was investigated by obtaining optical data in both the metallic and insulating states. In the metallic state, we determined the pressure dependence of the plasma frequency from absorption data fitted with a free-electron model. In the insulating state, we determined the pressure dependence of the band gap from absorption data fitted with an indirect-band-gap model. Our optical data indicate that the insulator-to-metal transition in xenon occurs at 132(5) GPa.

The equation of state and optical absorption of condensed Xe have been measured in a diamond-anvil cell up to 172 GPa. Pressures were determined by use of the ruby-fluorescence technique or from the equation of state of the Re gasket used to confine the sample. Xe transformed to an hcp structure between 70 and 90 GPa that remained stable to 172 GPa. At 150 GPa we observed sudden changes in absorption and reflection spectra of Xe that we attribute, on the basis of electron band calculations, to the onset of metallization.

We model a two-dimensional periodic array of Josephson junctions in a transverse magnetic field by the uniformly frustrated $\mathrm{XY}$ model. We report the results of extensive nonequilibrium simulations of the array within the resistively-shunted-junction model. $I-V$ curves are computed for the unfrustrated and fully frustrated cases, and consistency with recent experiments is found. For the fully frustrated case, new theoretical ideas are presented which show the Ising degrees of freedom to dominate nonequilibrium behavior near $T_c$.

The dissipation observed at the superconducting transition of the high-$T_c$ superconductor ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$Ca${\mathrm{Cu}}_2$${\mathrm{O}}_8$ is explained quantitatively by the Kosterlitz-Thouless theory of vortex-antivortex pair excitations within the Cu${\mathrm{O}}_2$ planes. This conclusion is drawn from the observation of an exponential square-root singularity in the resistivity and a power-law dependence of the resistivity on magnetic field. The Kosterlitz-Thouless phase transition ($T_c=84.7\mathrm{}$ K) is at the $\rho =0\mathrm{}$ point, and the mean-field Ginzburg-Landau transition ($T_{c0\mathrm{}}=86.8\mathrm{}$ K) at ∼25% of the normal-state resistivity.

We report on memory effects observed in the time dependence of the magnetization $M(T, H)$ of a superconducting Y-Ba-Cu-O single crystal. As in spin-glasses, the decay rate $S=\frac{\mathrm{dM}}{d\ln t}$ is not uniquely determined. It exhibits a characteristic change at time $t$ equal to the waiting time $t_w$ during which the specimen was kept at a given temperature $T<\mathrm{}{T}_c$ and magnetic field $H$ before a field perturbation $\Delta H$ is applied at time $t=0\mathrm{}$. This phenomenon, seen for the first time in a superconductor, reveals the glassy nature of the superconducting state in the high-$T_c$ oxides.

The desorption of C${\mathrm{H}}_4$ physisorbed on Ni(111) is observed to be induced by collision with Ar atoms incident with energies less than 2 eV. The absolute cross section for collision-induced desorption is measured as a function of the kinetic energy and incident angle of the Ar beam. The mechanism for desorption is shown to involve a direct and impulsive, bimolecular collision between Ar and C${\mathrm{H}}_4$. Molecular-dynamics simulations show that the energy and incident-angle dependence of the desorption cross section are the consequence of two competing dynamical effects.

It is proposed to set stringent limits on possible nonlinear corrections to ordinary quantum mechanics by searching for the detuning of resonant transitions. A suggested nonlinear generalization of quantum mechanics is used to show that such detuning would be expected in the rf transition in ${}_{}{}^9{}_{}{}^{}\mathrm{Be}_{}^{+}{}_{}{}^{}$ ions that is used to set frequency standards. Measurements at the National Bureau of Standards already set limits of order ${10}^{-21\mathrm{}}$ on the fraction of the energy of the ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ nucleus that could be due to nonlinear corrections to quantum mechanics, with good prospects of improving this by 2-3 orders of magnitude.

We interpret the exchange-correlation potential of Kohn-Sham density-functional theory to be the work required to move an electron against the electric field of its Fermi-Coulomb hole charge distribution. We substantiate this interpretation in the exchange-only approximation by proving the satisfaction of a necessary condition as well as by application to few-electron atomic and many-electron metallic surface systems. This interpretation also provides insights into the exact Slater exchange potential.

In the weak noise limit ($D\to 0$), the escape rate over a one-dimensional potential barrier is $\Gamma \sim \exp (-\frac{S}{D})$. Here $S$ is calculated exactly by identifying appropriate instanton solutions in a path-integral formulation of the Langevin equation. For the "colored noise" problem, defined by the noise correlator $〈\xi \mathrm{}\left(t\right)\mathrm{}\xi \left(t^{\prime }\right)〉=\left(\frac{D}{\tau }\right)\exp \{-\frac{|t-t^{\prime }|}{\tau }\}$, the instanton satisfies a fourth-order, nonlinear, ordinary differential equation which can be readily solved numerically. Analytical results are derived for small and large $\tau$.

In this paper two representation integrals of elastic wave field, corresponding to the Rayleigh I and II integrals in the scalar wave case, are derived for the first time. The Green's functions needed for these integrals are constructed by using auxiliary functions which are singular at the "ghost-image point" of the singular point of the original Green's functions (Stoke's solutions).

We show that conformally invariant gravity in three dimensions is equivalent to the Yang-Mills gauge theory of the conformal group in three dimensions, with a Chern-Simons action. This means that conformal gravity is finite and exactly soluble.

Data from the Gamma-Ray Spectrometer on the Solar Maximum Mission satellite has been searched for excess $\gamma$-ray emission coincident with the ∼ 10-s neutrino burst from supernova 1987A. The absence of a detectable signal allows us to state that the energy in MeV $\gamma$ rays during the burst is < ${10}^{-10\mathrm{}}$ of that in supernova neutrinos above 9 MeV. Direct interpretation limits the lifetime of any radiatively unstable massive neutrinos to $\frac{{\tau }_{{\nu }_i}}{m_{{\nu }_i}}\ge 6.3\times {10}^{15}\left(B_{\gamma i}{f}_{\gamma i}\right)$ s/eV for $m_{{\nu }_i}<20\mathrm{}$ eV, where $B_{\gamma i}$ and $f_{\gamma i}$ are the branching ratio for $\gamma$ decay of species $i$ and the fraction of decays giving detectable $\gamma$ rays, respectively.

Using $\gamma$-ray observations made by the Solar Maximum Mission Gamma Ray Spectrometer in the energy range 4.1-6.4 MeV we limit the possible radiative decays of neutrinos and axions emitted by SN 1987A. For neutrinos less massive than about 20 eV our constraint is $\frac{{\tau }_{\nu }}{m_{\nu }}\gtrsim 1.7\times {10}^{15}{B}_{\gamma }$ sec/eV; for masses between 20 and 100 eV, ${\tau }_{\nu }\gtrsim 3.4\times {10}^{16}{B}_{\gamma }{\gamma }_{\mathrm{GRS}}^{-\frac{1}{2}}$ sec; and for masses between 100 eV and a few MeV, ${\tau }_{\nu }{m}_{\nu }\gtrsim 8.4\times {10}^{17}{B}_{\gamma }$ sec eV. Here $B_{\gamma }$ is the branching ratio for the radiative decay mode and ${\gamma }_{\mathrm{GRS}}$ is yet to be determined from the data, but should be of order unity. The axion mass limit is $m_a\lesssim 55{\gamma }_{\mathrm{GRS}}^{\frac{11}{94}}$ eV.