Data are presented for the electrical resistivity ρ of fcc La as a function of T and p in the range 70 to 700 K and 0 to 1.25 GPa. The data are analyzed using the phenomenological ‘‘parallel resistivity’’ formula for resistivity saturation, which describes very well the T dependences of both ρ and its pressure coefficient. The pressure coefficients of the electron-phonon interaction parameter and ${\mathit{T}}_{\mathit{c}}$ are calculated from the p dependence of the electron-phonon component, and the p dependence of the parallel saturation resistivity is obtained from band-structure calculations using a simple Bloch-Boltzmann model. In both cases excellent agreement with experiment is found.

Photoluminescence in a-Si:H has been investigated using optical excitation energies ${\mathit{E}}_{\mathit{x}}$ varying from 1.27 to 2.0 eV. At low temperatures (T<100 K) a strong dependence of both the radiative quantum efficiency and the energy distributions of photoexcited carriers on ${\mathit{E}}_{\mathit{x}}$ has been observed for ${\mathit{E}}_{\mathit{x}}$<1.65 eV. At higher temperatures (T>200 K), the dependence of the energy distribution of carriers on ${\mathit{E}}_{\mathit{x}}$ disappears within a narrow interval of temperature. A model of phonon-assisted absorption is proposed to account for the experimental results.

Because of the complicated electronic and metallurgical properties of the metal-semiconductor interface, there is much controversy about the theoretical interpretation of experimental results on Schottky barrier heights. We present a new approch of barrier height measurements on a prototypical clean, abrupt and noninteracting system consisting of mercury contacts to hydrogen-passivated silicon surfaces. The resulting barrier to p-silicon is 0.9 V, totally at variance with all results presented for silicon Schottky barriers fabricated by standard metal deposition techniques. We believe this to be the first report in the limit of noninteracting metal contacts to silicon.

We show that the strong absorption structure observed in the a-b-plane optical conductivity of the high-${\mathit{T}}_{\mathit{c}}$ superconductors is correlated with c-axis longitudinal optical (${\mathrm{LO}}_{\mathit{c}}$) phonons. This suggests a resolution to the long-standing controversy over the origin of these features. The interaction with ${\mathrm{LO}}_{\mathit{c}}$ phonons is forbidden when the incident wave vector q is normal to the c axis which leads to the surprising result, confirmed with experiments, that the optical properties along a or b are different when measured on the a-b-plane face and a face containing the c axis.

The effects of charge-carrier density modulation on critical currents and vortex dynamics in ultrathin ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films were investigated through transport measurements in field-effect devices. Two distinct dissipation regions in resistive transitions in applied magnetic fields are identified by the electric-field effect. A Kosterlitz-Thouless-like transition is observed in zero magnetic field, which can be tuned by the electric field. The importance of thermal fluctuations for the dissipation in high-${\mathit{T}}_{\mathit{c}}$ superconductors is stressed.

Superlattices of one-unit-cell-thick ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, layers with (${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ were fabricated as a model system to study the effects of magnetic coupling on flux pinning in high-${\mathit{T}}_{\mathit{c}}$ superconductors. The sample can be transformed from an S/S to an S/N structure by increasing the temperature from below to above the ${\mathit{T}}_{\mathit{c}}$ of the intercalating (${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ layer. Dramatic changes in the characteristics of the magnetic field and angle dependence of the critical current density, ${\mathit{J}}_{\mathit{c}}$(B,Θ), caused by a 3D to 2D crossover were observed. The result demonstrates the significance of the dimensionality to the flux pinning in these materials with layered structures.

Nonlinear current-voltage characteristics are investigated for the high-magnetic-field state of ultrathin (down to ∼1 unit cell thick) ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films. The current density where nonlinearity sets in, ${\mathit{J}}_{\mathrm{nl}}$, exhibits a nontrivial power-law temperature dependence, ${\mathit{J}}_{\mathrm{nl}}$∝${\mathit{T}}^{3.0\pm 0.3}$, which quantitatively verifies a prediction from the vortex-glass theory for two dimensions (2D). Additionally, a critical scaling analysis of the I-V data clearly manifests the differences between a 2D and 3D vortex glass. The results suggest that a 2D vortex glass does not order at finite temperatures.

In the presence of thermal fluctuations the incommensurate NMR line shape is a convolution of a static inhomogeneous with a dynamic homogeneous line shape which can be determined separately by 2D NMR. The form of the dynamic line shape and its variation over the inhomogeneous NMR spectrum permit a separate determination of the relative sizes of the phason and amplitudon fluctuations, compared to the static part of the order parameter. ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ 2D spectra of ultrapure ${\mathrm{Rb}}_2$${\mathrm{ZnCl}}_4$ agree with the above theory and show the existence of a temperature range where the incommensurate splitting induced by the static part of the order parameter is averaged out by thermal fluctuations.

Four-wave-mixing signals from excitons in high-quality GaAs quantum wells show an unusually slow temporal evolution, in stark contrast to the behavior expected for a noninteracting two-level system. We show that Coulomb interaction effects, generally neglected in the analyses of four-wave-mixing experiments, dominate the signals by 2 orders of magnitude. Numerical calculations based on extended optical Bloch equations for semiconductors provide good qualitative agreement with the data.

In experiments on quasi-two-dimensional Turing structures, patterns form perpendicular to a concentration gradient imposed by the boundary conditions. Using linear stability analysis, with the ${\mathrm{ClO}}_2$-${\mathrm{I}}_2$-MA (malonic acid) reaction as an example, we obtain conditions on the position along the gradient direction and possible three dimensionality of the structures. Experiments on the effects of MA and starch concentrations on the position of the structures support the theory. Simulations taking into account the starch indicator yield Turing patterns even with equal diffusion coefficients for the activator and inhibitor species.

A particle that decays by the simplest interaction into two other particles is placed in a space-periodic external potential that acts only on the initial particle. For a strong potential, the decay rate is slowed drastically under most initial conditions. Comparing to the second-order perturbation treatment of a model where the decaying particle is imbedded in a medium of heavy, uncorrelated particles, noninteracting except for a potential interaction with the decaying particle, we find similar rate modifications. There exist possible applications in the supernova and early Universe environment, as well as possible laboratory demonstration of the effect, which bears some relationship to the ‘‘quantum Zeno’’ effect.

We propose and investigate a large class of models possessing resonance factorized S matrices. The associated Casimir energy describes a rich pattern of renormalization-group trajectories related to flows in the coset models based on the simply laced Lie algebras. From a simple resonance S matrix satisfying the ‘‘${\mathrm{\phi }}^3$ property’’ we predict new flows in nonunitary minimal models.

Our method uses topological properties of the large clusters in the single-cluster algorithm of Wolff for Monte Carlo simulations of spin systems. We have applied this method to the d=3 Ising model with film geometry near the special transition. As a test for the method we determined the bulk critical temperature and the critical indices in fair agreement with the results obtained by other methods. The new results refer to the critical surface coupling ${\mathit{J}}_{1\mathit{c}}$/J=1.5004(20) and the surface exponents, where φ=0.461(15) and ${\mathrm{\beta }}_{\mathit{i}}^{\mathit{m}}$=0.237(5).

The Tibet air shower array at an altitude of 4300 m has been in operation since January 1990. We have searched for continual emission of 10-TeV gamma rays from the Crab Nebula, Cygnus X-3 and Hercules X-1 during the period from June 1990 through January 1992. No dc excess is found from these three sources. Flux upper limits above 10 TeV are obtained to be 1.2×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{s}}^{\mathrm{-}1}$ for the Crab Nebula, 1.1×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^{\mathrm{-}2}$${\mathrm{s}}^{\mathrm{-}1}$ for Cygnus X-3, and 0.58×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^{\mathrm{-}2}$${\mathrm{s}}^{\mathrm{-}1}$ for Hercules X-1 at the 95% C.L. This is the first observation, with an air shower array, for these sources in the 10-TeV energy region.

We consider divergences of the perturbative expansions in large orders in quantum electro- and chromodynamics and concentrate on the dependence of large-order contributions on the choice of the normalization point of coupling constants. We find that for sign-alternating series the predictive power of perturbation theory as measured by the minimal term of an asymptotic expansion can be drastically improved by a proper choice of the normalization point. In particular, this allows the elimination of the leading uncertainty of perturbative expansions in quantum chromodynamics.

We show that in the minimal left-right symmetric model with triplet Higgs bosons the range of the muon neutrino mass for which ${\nu }_{\mathrm{\mu }}$ is required by cosmological considerations to be unstable can be probed by muonium to antimuonium conversion (M→M¯) and/or by the exotic muon decay ${\mathrm{\mu }}^{+}$→${\mathit{e}}^{+}$ ν${\mathrm{¯}}_{\mathit{e}}$${\nu }_{\mathrm{\mu }}$. We point out that if all the leptonic mixing matrices are hierarchical and the Dirac masses of the neutrinos are equal to the masses of the corresponding charged leptons, there is a lower bound in this range for the rates of both of these processes. We find ‖${\mathit{G}}_{\mathit{M}\mathit{M}\mathrm{¯}}$‖≳7×${10}^{\mathrm{-}5}$${\mathit{G}}_{\mathit{F}}$ and ‖${\mathit{G}}_{\mathrm{\mu }}^{\left(\mathit{e}\right)}$‖≳2×${10}^{\mathrm{-}4}$${\mathit{G}}_{\mathit{F}}$ for the strength of the M→M¯ and ${\mathrm{\mu }}^{+}$→${\mathit{e}}^{+}$ ν${\mathrm{¯}}_{\mathit{e}}$${\nu }_{\mathrm{\mu }}$ interactions.

Entry distributions leading to normal and superdeformed (SD) states in ${}_{}{}^{192}{}_{}{}^{}\mathrm{Hg}$ have been measured. A model, based on Monte Carlo simulations of γ cascades, successfully reproduces the entry distribution for SD states, as well as all other known observables connected with the population of SD states. Comparison of experimental and model results, together with the measured SD entry distribution, suggest that the SD band lies 3.3–4.3 MeV above the normal yrast line when it decays around spin 10 and that the SD well depth is 3.5–4.5 MeV at spin 40.

We demonstrate cooling and slowing of atoms in isotropic laser light. As the atoms slow, they compensate for their changing Doppler shift by preferentially absorbing photons at a varying angle to their direction of motion, resulting in a continuous beam of slow atoms unperturbed by an intense slowing laser beam. We point out several novel features of slowing and cooling in isotropic light, and show that it can be superior to cooling with directed laser beams.

The effect of pendular orientation of the figure axis of ICl on elastic and inelastic scattering by Ar was explored in a crossed beam experiment. The pendular states of ICl are created by hybridizing its rotational states up to J≤3 in a uniform electric field of strength E=100 kV/cm. The resulting change in the distribution of individual J states of ICl is measured state-selectively by laser-induced fluorescence near the zero scattering angle outside the field. We find that the electric field inhibits rotationally inelastic scattering of low-J state molecules, due to a larger spacing between levels.

We report on the first Q-value measurements in charge-transfer collisions using recoil longitudinal momentum spectroscopy. This method is not limited to relatively low beam energies and is easily adaptable to captures involving any number of transferred electrons. A very monoenergetic beam is not necessary. For a 50-keV ${\mathrm{Ar}}^{15+}$ on Ar collision system, Q values coresponding to single through quintuple electron capture were measured and found to be in good agreement with the predictions of the molecular classical overbarrier model.