Angle-resolved photoemission spectra have been measured for the polar ($\bar{1}\bar{1}\bar{1}$) and (111) surfaces of GaAs. The results show definitively that although both faces exhibit a (2×2) reconstruction, the local geometries of the unit cells must be distinctly different.

It is shown that, because of the electrostriction and polarization of defects by the modulating electric field, electroreflectance spectra are more affected by defects than are third-derivative spectra from ellipsometry data. The theory of electroreflectance is generalized to include these effects. Plasticity and long-range strains are shown to lead to a first-derivative line shape and polarizable defects to a second-derivative line shape. For defect-rich samples these new terms dominate the usual third-derivative line shape.

Brillouin light scattering has been used to observe the wave-number dependence of the surface-magnon frequency in thin iron films for different in-plane propagation directions relative to the applied in-plane dc magnetic field. The data show that, for propagation directions other than perpendicular, the falloff in magnon frequency with wave number is much faster than predicted by the generally accepted theory. Furthermore, it is found that the surface mode merges into the bulk band at propagation directions much closer to perpendicular than expected from the theory.

A well defined distribution-theory Lagrangian for a classical charged point particle is constructed which reveals the connection between the Lorentz-Dirac equation and a conserved energy tensor. No renormalization is required. Everything is finite.

The depolarization of rotating spins that perform a random walk on a linear chain is investigated. For a large number of steps $n$ the decay of polarization is given by $\exp (-\mathrm{const}\times {\sigma }^{\gamma }n)$, where $\sigma$ is the width of the frequency distribution and $\gamma \approx \frac{4}{3}$. This behavior follows from the distribution of the spans of a one-dimensional random walk; it is derived in an approximation and verified by numerical simulations. A lower bound on the polarization decay is also given which confirms the dependence on $n^1$.

Consider a macroscopic system rendered unstable by both thermal fluctuation and quantum tunneling. Kramer's classical theory of the rate of decay by thermal activation is extended to lower temperatures where quantum tunneling prevails. By means of a functional-integral approach, a general formula for the decay rate is derived which describes the transition between the high- and low-temperature regimes. The influence of dissipation on the decay rate is emphasized.

The Eden model in many dimensions is studied by an exact enumeration method. Our result including the first-order $\frac{1}{d}$ correction has the asymptotic behavior $〈{R_n}^2〉\sim 2 \ln \mathrm{}\left(n\right)\mathrm{}(1+\frac{3}{2d})$, which does not agree with the naive expectation $R\sim n^{\frac{1}{d}}$ [or $d(n^{\frac{1}{d}}-1)$]. This suggests that when $n$ is large, $〈{R_n}^2〉$ might be a singular function of $d$. We also present a result on the Cayley tree.

It is shown that the Zamolodchikov model on the cubic lattice of $N$ sites is equivalent to a spin model on a lattice of $2N$ sites with just two-spin and three-spin interactions. Together with the known transfer-matrix commutativity, this implies that the partition function $Z$ (for $N$ large) factors into a product of single-variable functions. These functions, and hence $Z$, are calculated by using symmetry relations. The model appears to be a critical free-fermion model.

Supersymmetric two-dimensional sigma models which include Wess-Zumino topological terms are constructed, analyzed, and interpreted by means of torsion on the field manifold. One-loop renormalization results are presented geometrically, revealing that an infrared fixed point exists when the torsion parallelizes the manifold. The O(4)/O(3) model is used to illustrate general results.

We show that the quark mixing matrix can be parametrized in exactly unitary forms with the imaginary parts present only at the order of ${10}^{-3\mathrm{}}$. With $s_x=s_1$ and $s_y$ well determined, measurements of ${\epsilon }^{\prime }$ or other $\mathrm{CP}$-nonconservation effects can determine $s_z{s}_{\phi }$. Then after $\left|V_{\mathrm{ub}}\right|\simeq s_z$ is measured, $s_{\phi }$ is known. We also give a simple expression for the $\mathrm{CP}$ asymmetry in the $B^0-B^0$ mixing.

We report mass limits on the ${\tilde{Z}}^0$ and ${\tilde{W}}^{\pm }$, supersymmetric partners of the $Z^0$ and $W^{\pm }$, respectively, using the MARK-J detector at PETRA. The experimental signatures in both cases are acoplanar lepton pairs with missing energy. For the ${\tilde{W}}^{\pm }$, an additional signature is a single energetic lepton. No evidence is found for either the ${\tilde{Z}}^0(M_{{\tilde{Z}}^0}<35\mathrm{}\mathrm{GeV})$ or the ${\tilde{W}}^{\pm }(M_{\tilde{W}}<25\mathrm{}\mathrm{GeV})$.

Atomic continuum-continuum transitions appear to be saturable at realistic light intensities. We discuss some consequences in multiphoton ionization.

Raman heterodyne detection of rf-generated sublevel coherence is investigated in atomic samarium vapor under conditions of velocity-selective optical pumping. In the presence of rare-gas perturbers, velocity-changing collisions determine the characteristics of the rf-laser double-resonance signals by strongly affecting their linewidths and line shapes. The results can be quantitatively explained by a theoretical model that includes velocity-diffusion processes.

We study the stability of the zero-intensity solution of the semiclassical laser equations when the pump parameter increases linearly in time. Equivalently, we study the stability of the trivial fixed point of the Lorenz equations when the Rayleigh number is increased linearly in time. When the time-dependent parameter varies slowly, the stability domain is greatly increased with respect to the stability domain derived under stationary conditions. This corresponds to a dynamical stabilization of an unstable stationary solution.

The cubic nonlinear Schrödinger equation, in the presence of driving and Landau damping, is studied numerically. As the pump intensity is increased, the system exhibits a transition from intermittency to a two-torus to chaos. The laminar phase of the intermittency is also a two-torus motion which corresponds in physical space to two identical solitons of amplitude determined by a power-balance equation.

A new possibility for steady-state current drive based on the use of neoclassical transport processes in tokamak reactors is considered. It is found that the existence of a steady-state self-sustaining plasma current can be ensured with a bootstrap current generated on the magnetic axis by fusion-produced alpha particles. The radial distribution of the driving current turns out to be peaked at a radius close to the magnetic axis.

Our results for superconducting ${\mathrm{U}}_2$Pt${\mathrm{C}}_2$ place it intermediate to heavy-fermion (U${\mathrm{Be}}_{13}$, U${\mathrm{Pt}}_3$) and less anomalous (${\mathrm{U}}_6$Fe) superconductors. Heavy-fermion behavior represented by the electronic specific heat correlates well with spacing between $f$-electron atoms in U, Np, and Ce materials. Two regimes of spacing are evident and arise from direct and indirect interactions between $f$-electron atoms. We suggest that this correlation provides a reasonable criterion for heavy-fermion behavior and a test for theories.

${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ NMR and relaxation have been used to probe heavy-fermion superconductivity in ${\mathrm{U}}_{1-x}{\mathrm{Th}}_x{\mathrm{Be}}_{13}$, $x=0\mathrm{} \mathrm{and} 0.033$. The spin-lattice relaxation rate $\frac{1}{T_1}$ varies approximately as $T^3$ well below the transition temperature. This is consistent with a class of anisotropic pairing models for which the gap vanishes along lines on the Fermi surface. NMR spectra give no indication of magnetic, structural, or charge-density ordering at a second transition for $x=0.033\mathrm{}$.

High-intensity, 75-fs optical pulses have been applied to observe multiphoton and thermally enhanced photoemission from a tungsten metal surface. Experimental results suggest the presence of anomalous heating, a transient nonequilibrium temperature difference between the electrons and lattice. Pump-probe measurements indicate an electron-phonon energy relaxation time of several hundred femtoseconds.