A fundamental limitation of LEED as a surface-structure probe is the complexity of the theory compared with x-ray scattering from solids. By making one full dynamical calculation for a reference structure we express diffraction from modifications of this structure in first-order perturbation theory. This gives a theory analogous to the x-ray case in that it can be separated into a structure factor and a form factor which is a tensor in the dynamical regime. In many cases the linear dependence on the structure factor gives computational savings approaching ${10}^3$ over conventional methods.

The quasiharmonic phonon dispersion relation of Pd${\mathrm{T}}_{0.7}$ has been determined by coherent neutron scattering. Calculation of the phonon terms entering the current models for superconductivity in PdH, PdD, and PdT shows conclusively that the magnitude of the measured anharmonic frequency shifts is not sufficient to explain the reverse isotope effect in these systems. The effect of the large zero-point motion of the hydrogen isotopes on both the electronic structure and the electron-phonon interaction is estimated, and shown to be necessary to explain the experimental data.

Argon multilayers adsorbed on graphite display heat-capacity anomalies attributed to surface roughening and surface melting. Melting begins 3° or 4° below the triple point, producing strong peaks with power-law risers $\sim {(T_t-T)}^{-r}$, $r=1.35\mathrm{}\pm 0.03$. The exponent and the thickness dependence of peak temperature agree with the theory of surface melting under long-range forces. Rounded anomalies at $\sim 0.8T_t$ are identified with roughening transitions. The two types of anomalies are well separated, indicating that melting and roughening are distinct forms of surface disorder.

Using optical second-harmonic generation as a probe, we have observed polar ordering of $4^{\prime }-n-\mathrm{octyl}-4-\mathrm{cyanobiphenyl}$ (8CB) molecules in a monolayer at an 8CB-glass interface, but nonpolar ordering at the free surface of 8CB as well as in the bulk with the molecules forming quadrupole-type pairs. The surface polar ordering was independent of temperature and unaffected by the phase transitions in the bulk 8CB.

We show that a PbTe-type narrow-gap semiconductor with an antiphase boundary (or domain wall) has currents of abnormal parity and induced fractional charges. A model is introduced which reduces the problem to the physics of a Dirac equation with a soliton in background electric and magnetic fields. We show that this system is a physical realization of the parity anomaly.

It is shown that the indirect-exchange interaction, or Ruderman-Kittel-Kosuya-Yosida (RKKY) interaction, between localized moments in metals can be evaluated numerically for realistic band structures. The numerical method gives results in excellent agreement with the analytical RKKY results for free-electron bands and is applied to the calculation of the indirect-exchange interaction between nuclear moments in Cu. We find the nearest-neighbor exchange-interaction constant to be $J=-22.6\mathrm{}$ nK compared to the free-electron RKKY value of $J=-8.2\mathrm{}$ nK with the same normalization procedure.

High-resolution angle-resolved photoemission studies of a potassium-doped Cu (111) surface have allowed extraction of the zone-center $\mathrm{sp}$ surface-state evanescent decay length as a function of energy location in the $L_{2^{\prime }}-L_1$ projected band gap. The results are well matched by recent first-principles calculations, but are in quantitative discord with expectations based on complex band structures extrapolated from bulk band calculations. Possible sources of this disagreement, including interference from final-state effects and surface-induced perturbations on bulk bands, are discussed.

Defects in amorphous Si are universally viewed as due to undercoordinated atoms. The dominant EPR-active center is identified as threefold-coordinated Si (dangling bond), but evidence is inconclusive. I introduce a new viewpoint, motivated by results on defects in crystalline Si, and conclude that overcoordination defects are as likely. I propose that most EPR-active centers are fivefold-coordinated Si, with an electron in a state that I label "floating bond." This new analysis is favored by experiments and theory and leads to new predictions.

We calculate for the first time the distribution of energy widths for Anderson localized states at a surface and show that electrical noise is produced with a $\frac{1}{f}$ spectrum. The distribution has a natural cut-off at widths of $\frac{\hslash }{l_c}$, where $l_c\propto \exp \left(\frac{{\delta }^{\prime 2}L}{2}\right)$ and ${\delta }^{\prime 2}$ measures the disorder; $l_c$ is typically many hours. The scanning tunneling microscope is proposed as the ideal device for observing these fluctuations.

Tunneling reveals a ∼14-meV resonance mode of Re in ${\mathrm{Mo}}_{1-x}{\mathrm{Re}}_x$, $0.2<x<\mathrm{}0.4\mathrm{}$, which contributes strongly to ${\alpha }^{2}F\left(\omega \right)$ and to strong coupling, $\frac{2\Delta }{k{T}_c}-3.53=\delta >0\mathrm{}$. The anomalous softening and width of the resonance mode are consistent with the standard theory of phonon renormalization by the electron gas. These unexpected results explain the low-$N\left(0\right)\mathrm{}$, high-$T_c$ behavior of ${\mathrm{Mo}}_{0.6}$${\mathrm{Re}}_{0.4}$ and possible ${\mathrm{Nb}}_3$Ge.

A theoretical study of the electronic level structure of tunneling semiconductor superlattices in the Hall configuration is presented. Our self-consistent calculations reveal that surface-type states associated with the depletion regions at the ends of the superlattice appear in the gap between Landau levels during Landau-level emptying. The surface-type states are pinned to the Fermi energy throughout the range of magnetic fields where the quantum Hall plateau occurs. Our results explain the Hall quantization index and the anomalously small activation energy observed recently by Störmer et al.

We observed soliton shape modes by photoinduced absorption in partially isomerized ${\left(\mathrm{CH}\right)}_x$ and ${\left(\mathrm{CD}\right)}_x$, coexisting with the soliton translational modes. The shape modes' ir activity, their dependence on the degree of isomerization, and the photoinduced spectra are explained in detail by a symmetry-breaking interaction which allows coupling of shape modes and translational modes. We also observed soliton shape-mode absorption in fully isomerized samples as phonon sidebands associated with optical transitions of neutral solitons above 1.5 eV.

A simple fluid in uniform isothermal shear flow is studied for conditions used in recent nonequilibrium computer simulations. The short-time dynamics for the local conserved densities is found to be unstable at large shear rates for wave vectors near the peak of the structure factor. The critical shear rates obtained are similar to those for an order-disorder transition observed in the computer simulations.

We show that a variational calculation of the ground-state energy of $\mathrm{SU}\mathrm{}\left(N\right)\mathrm{}$ Hamiltonian lattice gauge theories concurrently gives an estimate of the $0^{++}$ glueball mass. Monte Carlo calculations in three spatial dimensions for $N=3, 4, 5, \mathrm{and} 6$ indicate that the resulting glueball mass begins to scale for $N\ge 5$.

We give the semiclassical evaluation of the energy of electric flux in units of the $0^{++}$ glueball mass for SU(3) gauge theory and predict a crossover in this quantity around $z=1.6\mathrm{}$. The parameter $z$ is the length of the sides of a cubic volume (the three-torus) in units of the inverse glueball mass. We discuss lattice gauge theory in the context of a finite volume and compare our analytic prediction with recent SU(3) Monte Carlo results.

Upper limits for the branching ratio of the decay $\mu \to \mathrm{ex}$ where $x$ is a massive particle in the range 0 to 104 MeV have been determined. Over most of the accessible region $\frac{\Gamma (\mu \to ex)}{\Gamma (\mu \to e\nu \overline{\nu })}\lesssim 3\times {10}^{-4\mathrm{}}$ (90% C.L.).

A basically new structure of quasi-Landau resonances is discovered with the magnetized hydrogen atom around the ionization threshold. The resonances are theoretically rationalized by classical-trajectory calculation. In energy spacing and corresponding resonance time they form a highly regularly organized series with, classically, an infinite number of members.

The use of the Coulomb-explosion technique combined with a new multiparticle detector, extremely thin targets, and low-excitation ion source has enabled a direct measurement of the complete stereo-chemistry of the C${\mathrm{H}}_4^{+}$ molecular ion. Preliminary analysis of the data gives a structure consistent with predictions of the Jahn-Teller-distorted ground state. We outline the techniques by which this method can be generalized to determine directly complex vibrational motions in polyatomic molecules.

Photoionization thresholds of small photolytic silver clusters (${\mathrm{Ag}}_2$, ${\mathrm{Ag}}_3$, and ${\mathrm{Ag}}_4$) formed on silver bromide grain surfaces were evaluated from the threshold photon energy required for inducing the photobleach of the clusters. Threshold energies needed were about 1.6, 0.75, and 1.1 eV for ${\mathrm{Ag}}_2$, ${\mathrm{Ag}}_3$, and ${\mathrm{Ag}}_4$, respectively, showing an obvious even-odd oscillatory trend of the ionization threshold. The oscillation, however, seemed to be smaller than that expected for isolated metal-atom clusters, indicating the importance of interaction with the silver bromide grain surface.

A generic structure is proposed for rapidly growing magnetic fields in a class of steady, incompressible flows which are everywhere chaotic with positive Liapunov exponent. In such a flow at high magnetic Reynolds number $R_m$, the magnetic field is approximately aligned everywhere with the dilating direction of the flow, and may have extremely complicated spatial structure. In the limit of infinite $R_m$, the leading-order growth rate is bounded below by the positive Liapunov exponent of the flow; thus the field is a fast dynamo.