We have measured the optical reflectivity of single phase ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ compounds, and have evaluated the optical conductivity both below and above the superconducting transition temperature. Our results are in full agreement with a BCS singlet ground state, with the measured gap values consistent with the weak-coupling limit.

Pulsed laser irradiation is used to heat deeply buried damage layers in diamond. Over a small range of laser powers, damage annealing, formation of buried graphitic layers, and melting of diamond followed by its conversion upon cooling into graphite are observed. The diagnostics employed are channeling contrast microscopy, optical absorption, surface profilometry, and scanning and optical microscopies. The results are explained in terms of the behavior of carbon under high internal pressures close to the diamond-graphite-liquid triple point in the phase diagram.

Based on the subgraph expansions for phase transition models, we present a histogram Monte Carlo method to calculate the free energy and other physical quantities for such models as continuous functions of the system parameter. The method does not have any critical slowing down. We then use physical quantities thus obtained in a percolation renormalization group method to calculate critical point, critical exponents, and thermodynamic order parameter. Our method gives quite accurate results and may be applied to many phase transition models.

Bubbles in a first-order electroweak phase transition are nucleated with radii ${\mathit{R}}_0$ and expand with velocity v. If v is subsonic, a bubble becomes unstable to nonspherical perturbations when its radius is roughly ${10}^4$${\mathit{R}}_0$. These perturbations acclerate the transition, and the effective velocity of bubble growth rapidly becomes supersonic. The transition should subsequently proceed spherically via detonation. If for some reason the onset of detonation is postponed, the surface area of the bubbles may be enhanced by ${10}^5$. We discuss consequences for electroweak baryogenesis.

We have determined the phase diagram of the simplest version of a lattice model recently introduced by Kazakov and Migdal. Denoting by ${\mathit{m}}_0$ and λ the mass and the coupling of the scalar field in the model, we find a line of first-order phase transitions in the (${\mathit{m}}_0$,λ) plane ending in a critical point where λ is nonzero. Kazakov and Migdal speculate that their model of scalar field theory could induce QCD. Our work indicates that there is no continuum limit except at the critical end point. Whether or not a nontrivial continuum limit exists, there requires a careful study of the renormalization-group properties of the model.

It is shown that a technicolor theory containing a color-octet tecnipion, usually denoted by ${\mathrm{P}}_8^{0^{\prime }}$, will give rise to an enhancement oftt¯ production at the Fermilab Tevatron, the CERN Large Hadron Collider (LHC), and the Superconducting Super Colider (SSC), via the processgg→${\mathrm{P}}_8^{0^{\prime }}$→tt¯. The relevant cross sections are computed taking into account the large lower bound on the top mass coming from the “top-quark search” experiments at the CERN ${\mathrm{e}}^{+}$${\mathrm{e}}^{\mathrm{-}}$ collider LEP and the Collider Detector at Fermilab. At the LHC and SSC, the signal is found to be comparable to the QCD background, making the process quite accessible. © 1992 The American Physical Society

Production of tt¯ by gluon fusion fullowed by t→${\mathit{bW}}^{+}$, t¯→${\mathit{bW}}^{\mathrm{-}}$, and ${\mathit{W}}^{\pm }$→${\mathit{e}}^{\pm }$${\nu }_{\mathit{e}}$(ν${\mathrm{¯}}_{\mathit{e}}$) is studied to extract the CP-violating chromoelectric dipole moment form factor of the top quark. The fact that the spin of the top quark, in its rest frame, is in the direction of the ${\mathit{e}}^{+}$ momentum facilitates the analysis. Expressions for the differential cross section and decays are given. We find that the Superconducting Super Collider or Large Hadron Collider, it is possible to put a limit of ${10}^{\mathrm{-}20}$${\mathit{g}}_{\mathit{s}}$-to ${10}^{\mathrm{-}19}$${\mathit{g}}_{\mathit{s}}$ cm on these effective couplings through the use of ‘‘optimal observables.’’ Naive observables such as simple triple product correlations are typically an order of magnitude less effective.

The energy distribution of the ${\nu }_{\mathit{e}}$ from the decay of unpolarized ${\mathrm{\mu }}^{+}$ has been calculated on the basis of the most general four-fermion interaction. It depends on a parameter called ω which equals zero in the standard-model. It is shown that the recent measurement of the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\nu }_{\mathit{e}}$,${\mathit{e}}^{\mathrm{-}}$${)}^{12}$Ng.s. with ${\nu }_{\mathit{e}}$ from ${\mathrm{\mu }}^{+}$ decay at rest not only yields the absorption cross section, but also independently determines ω with a precision of Δω=0.026 for 100 measured events. This corresponds to ‖${\mathit{g}}_{\mathit{L}\mathit{L}}^{\mathit{S}}$‖≤0.37, where ${\mathit{g}}_{\mathit{L}\mathit{L}}^{\mathit{S}}$ describes a scalar interaction with left-handed μ and e and right-handed neutrinos.

The electroproduction of ${\mathrm{\pi }}^0$ on the proton was measured from 0 to 2.5 MeV above threshold for virtual-photon 4-momenta of -0.05 and -0.1 (GeV/c${)}^2$. The sum of the lowest-order contributing multipoles, ${\mathit{a}}_0$=‖${\mathit{E}}_{0+}$${\mathrm{\Vert }}^2$-${\mathrm{\epsilon }}_{\mathit{L}}$‖${\mathit{L}}_{0+}$${\mathrm{\Vert }}^2$, was determined with a precision an order of magnitude better than previously possible. Our results for ${\mathit{a}}_0$ are consistent with present calculations. Our extracted value for ‖${\mathit{L}}_{0+}$${\mathrm{\Vert }}^2$ at the ‘‘photon point’’ is in agreement with recent predictions.

We present a precision experimental test of atomic many-body theory that is currently applied to the interpretation of recent parity-nonconservation experiments in atomic cesium. We report the first measurement of the ${}_{}{}^{133}{}_{}{}^{}\mathrm{Cs}$ 6p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$ state lifetime using resonant diode-laser excitation of a fast atomic beam. The lifetime result of 30.55±0.27 ns determines the absorption oscillator strength for the 6s ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$–6p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$ transition in cesium to be 0.7133±0.0064, and establishes the accuracy of recent relativistic many-body calculations of the dipole transition matrix element to 0.5%.

We report the first measurements on radiative and dielectronic recombination (RR and DR) of very highly charged ions at energies ${\mathit{E}}_{\mathrm{cm}}$ as low as 0 to 50 eV. Novel techniques were employed at the heavy-ion storage ring ESR to obtain absolute recombination rates of ${\mathrm{Au}}^{76+}$(1${\mathit{s}}^2$2s). The increase of the RR rate for ${\mathit{E}}_{\mathrm{cm}}$→0 could be recorded and single DR resonances in ${\mathrm{Au}}^{75+}$(1${\mathit{s}}^2$2${\mathit{p}}_{3/2}$6${\mathit{l}}_{\mathit{j}}$) resolved. The experimental data allow sensitive judgement of calculations based on the Bethe-Salpeter approach to RR combined with results for DR from perturbative-relativistic, semirelativistic, and fully relativistic theories. Significant discrepancies are found only in the energy region of the 2${\mathit{p}}_{3/2}$6${\mathit{l}}_{3/2}$ resonances.

The current generation of photon scattering experiments requires a fast, accurate, general technique for calculation of transmission coefficients and of photonic band structure. A new methodology, which has significant advantages of speed and convenience, enables the band structure and, for the first time, transmission coefficients of complex dielectric materials to be calculated. Excellent agreement with experiment is found.

In previous works we have defined statistical equilibrium states for two-dimensional incompressible Euler equations. We establish here evolution equations governing the relaxation of the system towards these equilibrium states.

We analyze the smectic-liquid crystal phase realized by soft-core repulsive forces alone. Molecular dynamics simulations under hydrostatic pressure are performed for a system of soft, parallel spherocylinders. This system undergoes a clear first-order transition by heating from crystalline solid to the smectic B phase which has sixfold bond orientational order. In this smectic phase, the three-dimensional sixfold bond orientational order is induced by interacting layers of two-dimensional hexatic order. This is the first demonstration that models with soft-core repulsive intermolecular forces alone predict a hexatic B liquid crystal phase.

Line (string) defects at which the molecular orientation changes by 0, π/2, or π rad are reported in few-layer-thick freely suspended tilted smectic liquid-crystal films. These defects are ubiquitous, appearing in chiral and nonchiral smectic-C, hexatic, and antiferroelectric phases, either forming closed loops or terminating in fractionally charged vortices. The interaction between strings is complex, there being many ways in which they link to form multiarm (3 to 7) ‘‘star’’ and ‘‘tree’’ textures.

We have investigated a model of randomly placed defects with internal degrees of freedom which interact via elastic strain fields. Monte Carlo simulations and finite-size scaling indicate that two spin-glass phase transitions occur: one for the diagonal components of the defect stress tensor and the other for the off-diagonal components. The quenched ground state of the off-diagonal components exhibits antiferroelastic correlations while the diagonal components do not. We predict the fourth-order elastic susceptibilities associated with the defects diverge at the transition temperatures.

We report the first time-resolved x-ray scattering study of the homoepitaxial growth of GaAs by organometallic vapor-phase epitaxy. The growth mode was determined to be layer-by-layer by observing ≊1-Hz oscillations of the x-ray intensity from the 11l crystal truncation rod near the 110 position. We show that the spatial distribution of islands can be dynamically determined by measuring the x-ray diffuse scattering near the 110. Finally, we show that significant correlations exist between the locations of islands during layer-by-layer growth.

It is argued that the critical point of a nonequilibrium phase transition in the motion of dislocations results in the experimentally observed power-law creep rates in ${\mathrm{Ni}}_3$Al and related intermetallic alloys. A simple scaling argument is used to connect the critical exponents of the transition directly to the creep rate. A model of dislocation motion is constructed. The exponents predicted from the model are in agreement with those deduced from creep experiments.

We present a method to calculate the full phonon spectrum using the local-density approximation and Hellmann-Feynman forces. By a limited number of supercell calculations of the planar force constants, the interatomic force constant matrices are determined. One can then construct the dynamical matrix for any arbitrary wave vector in the Brillouin zone. We describe in detail the procedure for elements in the diamond structure and derive the phonon dispersion curves for Si. The anharmonic effects can also be studied by the present method.

The nature of the orientational phase transition in the three-dimensional Lebwohl-Lasher model of liquid crystals has been studied by computer simulation using reweighting techniques and finite-size scaling analysis. Unambiguous numerical evidence is found in favor of a weak first-order transition and the presence of pseudospinodal points, ${\mathit{T}}_{\pm }^{\mathrm{*}}$, which are extremely close to the equilibrium transition temperature, ‖${\mathit{T}}_{\mathit{c}}$-${\mathit{T}}_{\pm }^{\mathrm{*}}$‖/${\mathit{T}}_{\mathit{c}}$≲0.5×${10}^{\mathrm{-}3}$, in good agreement with experimental data for the nematic-isotropic transition.