High-resolution angle-resolved photoemission studies of the Ge(001) surface are reported which clearly indicate the existence of a metallic surface state whose emission is observed over a narrow range of parallel momenta near the center of the surface Brillouin zone. The state slowly disappears as the temperature is lowered from room temperature to 77 K. This metal-insulator transition is coincident with a recently predicted and observed transition from a disordered to an ordered $c(4\mathrm{}\times 2)$ structure.

We investigate the pairing, charge-density, and spin-density correlations of a one-dimensional electron-exciton model using Monte Carlo simulations and analytic results from the strong-coupling limit. The zero-frequency susceptibilities characterizing these correlations are studied as a function of band filling and various interaction parameters, particularly retardation. The conditions which are most favorable for a given type of correlation are determined.

The Si $L_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ absorption edge in amorphous silicon nitride ($a-\mathrm{Si}{\mathrm{Ni}}_x:\mathrm{H}$) exhibits a Fano-type antiresonance just below threshold for $x=1.20\mathrm{} \mathrm{and} 1.40$. At the same energy the valence-band photoemission cross section is resonantly enhanced in these samples for those features that carry an appreciable Si-$3s$-derived partial density of states. We argue that the Fano resonance involves Si-Si antibonding states at the bottom of the conduction bands that become localized as their number decreases with increasing nitrogen content compared to that of Si-N bonds.

Microwave magnetoabsorption peaks in single-crystal superconducting Al have been identified by comparison with a detailed calculation of quasiparticle energy levels. The peaks correspond to resonant transitions between states bound in the magnetic-field penetration layer as predicted by Pincus. By fitting the observed spectra to the calculation, we are able to extract the penetration length $\lambda$ and a second parameter describing the shape of the nonlocalfield penetration.

The magnetic flux quantization effect, with period ${\phi }_0=\frac{\mathrm{hc}}{2e}$, is observed with high accuracy in the resistance of a Mg honeycomb network at temperatures $50 \mathrm{mK}<~T<~6 \mathrm{K}$. As expected, the phase of the oscillations and the sign of the magnetoresistance are dominated by the spin-orbit interaction. Our results confirm, for a new geometry, the reality of the Bohm-Aharonov effect for a nonsuperconducting metal in the weak-localization regime.

The statistics of quasiparticles entering the quantum Hall effect are deduced from the adiabatic theorem. These excitations are found to obey fractional statistics, a result closely related to their fractional charge.

A nonrelativistic random-phase approximation has been used to introduce exchange interaction between excitations from spin-orbit-split core levels. For relatively deep levels this may lead to nonstatistical intensity ratios of "white line" resonances, while for giant dipole excitations it leads to a narrow peak (triplet excitation) on the low-energy side of the giant resonance. Effects of Auger decay have also been included.

A suspension of deformable particles is modeled as an assembly of cyclic interacting lattice chains. Monte Carlo calculations show that particle deformability and dispersion structure are controlled by the size of the repulsive interaction between nonbonded segments on the particle membrane. The model has features common to both a macromolecular solution and a conventional colloidal dispersion.

We present an $N=1\mathrm{}$ locally supersymmetric theory in which all constraints for a successful new-inflationary-universe scenario are satisfied and the gravitino mass is on the order of the electroweak scale. Sufficient baryon asymmetry is generated and the overproduction of entropy due to gravitinos and/or coherent field oscillations (which is typical of supersymmetric models) is avoided.

Some solvable fractal models of the percolating backbone are used to investigate the critical behavior of a random, $d$-dimensional, isotropic, elastic medium. The critical exponent $T$ for the elastic moduli is found to be appreciably greater than the conductivity exponent $t$, and the ratio of bulk to shear modulus is found to have the universal value $\frac{4}{d}$. A comparison with the effective-medium and the Clausius-Mossotti-type approximations leads to the conjecture that the result $\frac{4}{d}$ is in fact exact.

The electric charge of a generic SU(2) magnetic monopole is investigated in field-theory models with fermions. The fermionic contribution to the charge is computed by a comparison with Gauss's law and its dependence on the fermion representation is revealed. A disagreement with the results obtained from the bosonization procedure is pointed out and related to a difficulty in fixing the homogeneous solution of Gauss's law.

We use Monte Carlo simulations to examine the $\theta$ dependence of the O(3) $\sigma$ model. The free energy as a function of $\theta$ is determined and its peculiar scaling properties are clarified.

We find an expression for the fractional fermion number of a soliton at finite fermion chemical potential $\mu$ and finite temperature $T$ in the (1+1)-dimensional chiral field theory. We discuss the physical origin of the depletion of the soliton charge by finite $\mu$ and the relevance to lower-dimensional charge-density-wave systems and to topological solitons in higher-dimensional field theories.

We present the results of a Monte Carlo renormalization-group calculation using the $\sqrt{3}$ block transformation introduced by Cordery, Gupta, and Novotny. The $\beta$ function for the SU(2) lattice gauge theory is calculated both along the Wilson axis and along the line $K_A=-0.24\mathrm{}{K}_F$ in the fundamental-adjoint plane.

We describe a method to calculate the renormalized action generated under Monte Carlo renormalization-group transformations. With use of this method, an approximate renormalized trajectory for the SU(2) lattice gauge theory is mapped over a large range of couplings.

We show that in parity-conserving vectorlike theories such as QCD, parity conservation is not spontaneously broken.

High-resolution proton-scattering experiments yield sets of resonance-decay amplitudes with sometimes strong correlations between different decay channels. The joint probability distribution of the amplitudes is shown to be compatible with the Gaussian one, if all experiments are properly averaged and their finite-range-of-data error evaluated.

Doubly differential cross sections were measured for pion production in the reaction ${\pi }^{-}d\to {\pi }^{+}{\pi }^{-}\mathrm{nn}$ at 256 and 331 MeV and in ${\pi }^{+}d\to {\pi }^{-}{\pi }^{+}\mathrm{pp}$ at 256 MeV. At each incident energy observations were made at 35 to 40 points distributed uniformly over the center-of-momentum phase space of the produced pion. Comparison of these measurements with the cross sections for ${\pi }^{-}p\to {\pi }^{+}{\pi }^{-}n$ suggests that they are consistent with the dominance of quasifree production on a single nucleon. Upper limits for production of bound ${\pi }^{-}\mathrm{nn}$ and ${\pi }^{+}\mathrm{pp}$ systems were measured.

Proton-proton correlations were measured with the "Plastic Ball" detector for two systems, Ca + Ca and Nb + Nb, at 400 MeV/nucleon as a function of proton multiplicity. The source-size radii extracted from these measurements were found to have a cube-root dependence on the proton multiplicity of the event. The deduced freezeout density, the density at which collisions between fragments cease, was found to be about 25% of normal nuclear-matter density.

Backward-angle excitation functions have been measured for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ elastic and inelastic scattering between $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=7.4\mathrm{} \mathrm{and} 24.4$ MeV. Three prominent gross structures (18.0, 20.7, and 22.1 MeV) are observed. The corresponding back-angle elastic angular distributions can each be fitted by a $P_{L,0\mathrm{}}^2\left(cos\theta \right)$ function whose $L$ value tracks about one unit above $l_{\mathrm{grazing}}$.