Experiments are reported in which a controllable torque is exerted via nuclear dipole-dipole interactions on hindered methyl groups in crystals at low temperatures, resulting in coherent rotation resisted by molecular friction. The spectroscopic method reveals the lifting of a degeneracy through the breaking of time-reversal symmetry as a result of the frinction. It is connected with the berry quantum mechanical phase law.

A large enhancement, up to a factor of 30, of the photon intensity is observed in inverse photoemission when the emitted photon energy equals the plasmon energy. The effect is explained as a resonance between a discrete channel, i.e., radiative decay of a plasmon, and the inverse photoemission continuum. Examples are shown for various structures of antimony films.

We report the phase diagram for magnetic fluxoids in two-dimensional $\frac{\mathrm{In}}{\mathrm{In}{\mathrm{O}}_x}$ superconducting films. The films are sputtered directly onto high-$Q$ silicon oscillators. The melting of the vortex lattice is signaled by a peak in the attenuation and a shift of the resonant frequency of the oscillator, caused by the rapid change in mobility of the vortex array. The zero-field limit of the melting temperature can be fitted by the Kosterlitz-Thouless model. However, the magnetic field dependence disagree with the current theoretical picture.

We demonstrate that picosecond photoinduced absorption probes the diffusion-limited transport of electrons in the conduction-band tail for intrinsic hydrogenated amorphous silicon. We find that the median trapping time on this scale is controlled by the number of temperature- and intensity-dependent, metastable light-induced defects. We attribute the intensity dependence of the decay to quasimonomolecular recombination and/or the generation of transient, fast-annealing light-induced defects.

Measurements were made of the temperature dependence, strain dependence, defect dependence, symmetry properties, and small-sample statistical properties of the large $\frac{1}{f}$ noise in antiferromagnetic Cr films. The results are consistent with a model based on rotations of the polarization of spin-density-wave domains. Spontaneous electronic symmetry-breaking effects are proposed as a new source of $\frac{1}{f}$ noise in some metals.

Neutron-diffraction experiments on ceramic powders of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ (nonsuperconducting, with $x=0.0\mathrm{} \mathrm{and} 0.15$ have confirmed the existence of long-range, three-dimensional, antiferromagnetic order of the Cu spins. The structure determination was aided by the use of uniaxially oriented powders. The ordering wave vector within a Cu${\mathrm{O}}_2$ plane is (½ ½) and the planes are coupled antiferromagnetically along the $c$ axis. The Néel temperature is 400±10 K for the $x=0.15\mathrm{}\pm 0.05$ sample and ≳500 K for $x=0.0\mathrm{}$.

We consider a noncanonical ensemble which involves a sample thermally connected to a finite heat bath with specific properties. Treating the size of the heat bath as a parameter, we show that static properties of finite samples are ensemble dependent. Monte Carlo simulations of phase transitions in Potts models confirm the analysis and demonstrate significant reductions in computer time (sometimes by a factor of 100) compared to canonical-ensemble simulations. For finite samples, second-order transitions are sharpened and are clearly distinguished from first-order transitions as the heat bath becomes smaller.

We report on a novel experiment sensitive to the anisotropy of the one-way speed of light. The frequency of a two-photon transition in a fast atomic beam is compared to the frequency of a stationary absorber while the direction of the fast beam is rotated relative to the fixed stars. The experiment yields an improved upper limit for the anisotropy: $\frac{\Delta c}{c}\le 3\times {10}^{-9\mathrm{}}$.

We describe a new method for large-scale computer simulation of the effective action in lattice field theories. As a first application, the scalar ${\phi }^4$ model with spontaneous symmetry breaking is studied, and the renormalization of physical parameters is calculated from the effective action. Mean-field scaling behavior with calculable logarithmic scaling corrections at the trivial Gaussian fixed point (vanishing renormalized coupling) is supported by our results. Applications to the electroweak theory are outlined and a bound on the Higgs-boson mass is estimated.

We report a revised determination of charmed-$D$-meson absolute branching fractions based on complete reconstruction of $D\overline{D}$ events at the $\psi \mathrm{}\left(3770\right)\mathrm{}$. Two backgrounds, Cabibbo-suppressed and multi-${\pi }^0$ $D$ decays, are addressed in detail. The first measurement of the decay $D^0\to K^{-}{\pi }^{+}{\pi }^0{\pi }^0$ establishes the sensitivity to hitherto unobserved multi-${\pi }^0$ modes. Removal of both backgrounds reduces the values of our previously reported branching fractions by (21-24)%, leaving their ratios largely unchanged. The new values are unable to account fully for a reported deficit in charm production in $B$-meson decay.

The ratio $R$ of the cross section for $e^{+}{e}^{-}$ annihilation into hadronic final states to the QED cross section for muon-pair production is measured to be 4.34±0.45±0.30 and 4.23±0.20±0.21 at c.m. energies of 50 and 52 GeV, respectively. From these values of $R$ and an analysis of the event shapes we deduce a 95%-confidence-level upper limit for the production rate of new heavy charge $+\frac{2}{3}e$ or $-\frac{1}{3}e$ quarks to be 0.19 units of $R$.

We searched for possible signatures of top-quark production in 508 $e^{+}{e}^{-}$ hadronic annihilation events collected at $\sqrt{s}=52\mathrm{}$ GeV by the TOPAZ detector at the KEK $e^{+}{e}^{-}$ collider TRISTAN. The observed hadronic cross section and shape of hadronic events are consistent with the standard-model predictions without top quarks. A lower limit (95% confidence level) on the mass of the lightest top meson is set at 25.8 GeV.

The neutron spectrum from the reaction ${}_{}{}^{92}{}_{}{}^{}\mathrm{Zr}({}_{}{}^{64}{}_{}{}^{}\mathrm{Ni},1n){}_{}{}^{155}{}_{}{}^{}\mathrm{Er}$ has been measured at an average spin of $52\hslash$ and for excitation energies between 30 and 36 MeV in ${}_{}{}^{155}{}_{}{}^{}\mathrm{Er}$. Channel and spin selection have been obtained from the Darmstadt-Heidelberg Crystal Ball. The level density in ${}_{}{}^{155}{}_{}{}^{}\mathrm{Er}$ is derived within the framework of a Fermi-gas model. A level-density parameter of $a=\frac{A}{(8.8\mathrm{}\pm 1.3)}$ ${\mathrm{MeV}}^{-1\mathrm{}}$ is deduced.

Evaporation residues following the interaction of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ with targets of ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$, ${}_{}{}^{160}{}_{}{}^{}\mathrm{Gd}$, and ${}_{}{}^{120}{}_{}{}^{}\mathrm{Sn}$ were measured in the energy range of $E_{\mathrm{lab}}=5.5\mathrm{}-10$ MeV/nucleon. The experimental system allowed the determination of the cross sections of the complete fusion (CF) and the incomplete fusion (ICF) processes with a sensitivity to ICF processes below the percent level. The ICF processes were observed at all bombarding energies studied, down to energies slightly above the Coulomb barrier. The two processes have very different angular distributions, with the ICF peaked at a significantly larger angle than the CF.

Wavelength tuning of 0.4 μm has been obtained by the addition of 100 Torr of ${\mathrm{H}}_2$ gas to a vacuum free-electron laser operating in the near infrared. This experiment demonstrates that a relativistic electron beam from an rf linac can propagate in a partially ionized gas with sufficient quality to achieve free-electron laser action. The picosecond pulse structure of the beam is believed responsible for the avoidance of plasma instabilities. Gain and wavelength measurements are consistent with theoretical expectations.

Small static fields (≅1 V/cm) are observed to reduce the microwave fields required to ionize highly excited $25\le n\le 50$ Li atoms by about an order of magnitude, from ≅100 to 10 V/cm. This occurs because the static field splits series of multiply degenerate sideband states to form a quasicontinuum of closely spaced states, in which a sequence of resonant adsorptions occurs, allowing ionization at the lower microwave field.

We outline a possible scenario of successive bifurcations to local and global modes on a Ginzburg-Landau model with varying coefficients. It is shown that self-sustained resonances may appear via a Hopf bifurcation when the system exhibits a region of local absolute instability which is sufficiently large. Our findings are in qualitative agreement with experimental observations of spatially developing flows such as wakes and inhomogeneous jets.

We report measurements of the reshaping of 0.3-psec dark pulses due to their passage through 10 m of single-mode optical fiber. The measurements were performed as a function of intensity and the observed strong reshaping agrees qualitatively with the predictions of the nonlinear Schrödinger equation which suggest that we have observed the formation of dark-pulse solitons.

The statistical properties of the quasienergy spectrum are used to measure the influence of quantum effects on the quantum kicked rotator which displays chaotic behavior in the classical limit. A transition from orthogonal-ensemble statistics in the semiclassical limit ($\hslash \to 0$) to Poisson statistics in the quantum regime is observed. In view of previously obtained results for this system the dependence on the irrationality of $\hslash$ is discussed.

Observations of superthermal ion-cyclotron emission from JET Ohmic- and neutral-beam-heated discharges are presented. Previously unobserved narrow-band spectral features correspond in both cases to multiple harmonics and half-harmonics of the proton gyrofrequency at the outer edge of the plasma. In this region, fusion products with large radial excursions and injected fast ions produce anisotropic velocity distributions with positive perpendicular gradient. It is discussed how these may relax by maser action, giving rise to the observed localized radiation.