By means of a high-statistics Monte Carlo simulation, Fisher scaling is studied for pure SU(2) and SU(3) lattice gauge theory. In both cases $2^{++}$ is found to be the lowest state above the vacuum, and the $\frac{m\left(0^{++}\right)}{m\left(2^{++}\right)}$ glueball mass ratio is slightly larger than 1. For SU(2), Monte Carlo data smoothly continue the analytic $\frac{m\left(0^{++}\right)}{m\left(2^{++}\right)}$ small-volume calculation towards the infinite-volume continuum limit. New results for the ratio between the square root of the string tension and the mass gap are also reported.

We report measurements of the two-photon processes $e^{+}{e}^{-}\to e^{+}{e}^{-}{\pi }^{+}{\pi }^{-}$ and $e^{+}{e}^{-}\to e^{+}{e}^{-}{K}^{+}{K}^{-}$, at an $e^{+}{e}^{-}$ center-of-mass energy of 29 GeV. In the ${\pi }^{+}{\pi }^{-}$ data a high-statistics analysis of the $f\left(1270\right)\mathrm{}$ results in a $\gamma \gamma$ width $\Gamma (\gamma \gamma \to f)=3.2\mathrm{}\pm 0.4$ keV. The ${\pi }^{+}{\pi }^{-}$ continuum below the $f$ mass is well described by a QED Born approximation, whereas above the $f$ mass it is consistent with a QCD-model calculation if a large contribution from the $f$ is assumed. For the $K^{+}{K}^{-}$ data we find agreement of the high-mass continuum with the QCD prediction; limits on $f^{\prime }\mathrm{}\left(1520\right)\mathrm{}$ and $\theta \mathrm{}\left(1720\right)\mathrm{}$ formation are presented.

A variational Dirac-Hartree-Fock procedure is introduced, which does not exhibit problems of spurious roots, variational collapse, or continuum dissolution. The optimized eigenvalues converge uniformly from above to the numerical Dirac-Hartree-Fock results as the dimension of the basis set is increased. Results for the $1s^2$, $2s^2$, and $2p_{\frac{1}{2}}^2$ shells are presented as examples.

Energy levels of He adsorbed on a Kr layer on graphite were obtained to high precision by comparison of theoretical calculations and measured resonance oscillations in the intensity of a scattered He beam. An experimental many-body correction was obtained by comparison of these energy levels to the sum of well determined two-body interactions. Theoretical three-body corrections to both the attractive and repulsive interactions were also calculated. A previously reported discrepancy between theory and experiment was resolved when the He-graphite term was remeasured. The agreement is now excellent.

We study low-energy elastic electron scattering from an atomic or molecular target for the case where the initial state of the target is close in energy to target states of opposite parity. We show that under these circumstances there is a range of small scattering angles in which the angular distribution varies rapidly. Experimental tests are suggested.

Extensive studies of a pure electronic transfer $B{}_{}{}^1{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}\to 2{}_{}{}^1{}_{}{}^{}\Sigma _{\mathrm{g}}^{}{}_{}{}^{+}$ in lithium dimers colliding with Li atoms enable the rates to some 600 (${}_{}{}^7{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$) or 400 (${}_{}{}^6{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$) channels from a single level of $B{}_{}{}^1{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}$ to be found. This is the first complete direct analysis of pure electronic transfer in a diatomic molecule. The information-theoretic approach points out the main physical facts and clarifies $\Delta E$ and $\Delta J$ propensity rules.

Time-structure and frequency-spectrum measurements of our free-electron-laser oscillator show that for periods shorter than 5 μs the laser operates in a stable single-cavity mode having a strikingly small fractional frequency bandwidth of ${10}^{-8\mathrm{}}$. This constitutes the first demonstration of high-temporal-coherence operation of a free-electron laser. For periods longer than 5 μs the laser frequency changes in unexpected quantized steps which may be explained in terms of a homogeneously broadened gain profile coupled to a small monotonic drift in electron-beam energy.

The evolution of nonequilibrium initial conditions of an incompressible magnetohydrodynamic $Z$ pinch is described by a three-dimensional, pseudospectral numerical code. Magnetohydrodynamic turbulence develops in the resistive, nonviscous magnetofluid, resulting in the selective decay of the energy relative to the magnetic helicity, at Lundquist numbers of only a few hundred. An interior force-free region grows with time and achieves spontaneous reversal of the toroidal magnetic field at the wall, without the necessity of an external electric field.

We report a high-resolution x-ray study of the fluctuations in the nematic and smectic-$A_1$ phases in heptylphenyl nitrobenzoloxybenzoate. As a function of decreasing temperature, the fluctuations evolve from an incommensurate region, through an intermediate region of coexisting (competing) incommensurate and smectic-$\tilde{C}$ fluctuations, to a region dominated by pretransitional smectic-$\tilde{A}$ fluctuations which exhibit a phase-lockin $\mathrm{smectic}-A_1-\mathrm{smectic}-\tilde{A}$ transition. This behavior directly confirms the current theoretical description of the smectic-$\tilde{A}$ and smectic-$\tilde{C}$ phases as alternatives to the incommensurate phase.

The static dielectric susceptibility of ${\mathrm{K}}_{1-x}{\mathrm{Na}}_x\mathrm{Ta}{\mathrm{O}}_3$ and its field derivatives depend critically on temperature above the transition temperature $T_g$. Below $T_g$, the susceptibility is dispersive and reveals relaxation between marginally stable states. The transition from the paraelectric to the disordered polar phase thus has dual aspects, criticality and metastability, just the same as spin systems with infinite-range interactions.

Unusual corrugations observed in scanning-tunneling-microscope (STM) images of $1T-\mathrm{Ta}{\mathrm{S}}_2$, Si(111)(2×1), and graphite are explained, and are shown to be characteristic of materials where the Fermi surface has collapsed to a point at the corner of the surface Brillouin zone. This is the first clear case where the low-bias STM image is dominated by electronic structure effects rather than surface geometry. Implications for STM spatial resolution are discussed.

We show that scanning tunneling microscopy (STM) images can be dominated by elastic deformations induced by atomic forces between tip and surface. New STM experiments on graphite (0001) showing a strong variation of the giant corrugation amplitudes with varying current at constant voltage are direct evidence of this atomic-force concept. Corrugations up to 8 ÅA on a lateral scale of 2.4 Å are associated with forces up to ${10}^{-8\mathrm{}}$ N for compression and 2 × ${10}^{-10\mathrm{}}$ N for expansion.

It is experimentally demonstrated that an applied dc magnetic field can transform the optical Fréedericksz transition in a nematic film from second order to first order. The results are in good agreement with theoretical predictions.

Atomic steps involved in the reconstruction of the Pt (110) plane, from the (1 × 1) to the (1 × 2) structure, have been directly observed in the field-ion microscope with use of nanosecond-pulsed-laser heating of the surface. The dominant steps are breaking of [110] atomic rows into fragments of two to a few atoms and lateral and cross-channel jumps of these fragments. Within the ∼ 5-ns time resolution of the experiment, atoms are seen to jump in pairs or groups, and jumps of single atoms are rarely seen. The (1 × 2) surface is stabilized by an attractive adatom interaction at ∼ 2.8 and ∼ 8 Å, and a repulsive interaction at intermediate distances.

Photoinduced crystallization is investigated by Raman scattering in Ge${\mathrm{Se}}_2$ and Si${\mathrm{Se}}_2$ glasses. Crystallization into two different structures is observed according as the incident laser power is above or below the threshold. To interpret it a new stochastic random-network model composed of two types of molecules is proposed for the structure of chalcogenide glasses.

The x-ray diffraction profiles of four mixed alkali halides-alkali cyanides have been studied. At the threshold concentration where the systems change over from ferroelastic ordering to the glass state, the profiles are highly unusual. The diffracted intensity of transverse scans along the cubic axes follows $I\sim \exp (-\alpha |\xi \mathrm{}\left|\right)\mathrm{}$, where $\xi$ is the reduced wave vector, measured from the line center.

Time-resolved resonant light scattering in a disordered system is theoretically investigated. It is found that pulse excitation enables one to observe directly the relaxation of elementary excitations which is caused by disorder. In the case of weak disorder, the rapid scatteringlike and slow luminescencelike components distinctively appear. In the case of strong disorder, a continuous change from scattering to luminescence appears, which reflects the dephasing dynamics in the excited states of a disordered system.

We adopt the view that the structure of disordered systems is itself determined by the formation energy of structural deviations from an ideally bonded network. By developing an equilibrium statistical mechanical model for disorder we predict properties of electronic band tails, defects, defect densities of states, and impurity doping.

Large-angle-diffraction oscillations in the intensity of helium atoms elastically scattered from randomly stepped Pt(111) are reported. A formalism based on the concept of reflection-symmetry interference explains the observed features and the data analysis yields a value of 4.4-4.6 Å for the hard-core repulsive cross section of a single step. The observed oscillations are sensitive to the presence of adsorbates at step sites.

Recent photoemission experiments on sodium tungsten bronzes, ${\mathrm{Na}}_x{\mathrm{Ta}}_y{\mathrm{W}}_{1-y}{\mathrm{O}}_3$, have revealed a depletion of the density of states near the chemical potential in insulating samples. We propose that this depletion is a Coulomb gap, and that these experiments represent the first direct measurement of a Coulomb gap in a disordered insulator. Our conclusion is supported by physical estimates and numerical calculations of the density of states for a classical model of ${\mathrm{Na}}_x{\mathrm{Ta}}_y{\mathrm{W}}_{1-y}{\mathrm{O}}_3$. The width and shape of the calculated bands agree well with the experiments.