We have measured accurate x-ray crystallographic data from the InSb(111)2×2 reconstructed surface using synchrotron radiation from the DORIS storage ring at DESY. We have analyzed these independently of all models and find a structure with seven atoms in the unit cell, implying that one site is unoccupied. The bonding configuration is reasonable for a III-V semiconductor surface, and is topologically identical (in projection) to the "vacancy-buckling" model proposed for the GaAs (111)2×2 surface. The differences between the InSb and GaAs structures are significant and reflect chemical trends in the periodic table.

It is shown for the first time that the controlled deformation of the electron wave function leads to modulation of the two-dimensional electron mobility by as much as 56% even when the electron concentration is kept constant. The deformation is controlled by use of two different gating modes (front gating and back gating) in a novel $n$-AlGaAs-GaAs heterojunction field-effect-transistor configuration. The observed modulation of electron mobility is in accordance with the theoretical prediction and demonstrates the feasibility of the recently proposed concept of a velocity-modulation transistor.

Far-infrared magnetospectroscopy has been carried out on shallow donor impurities doped in the central region of GaAs quantum wells in $\mathrm{GaAs}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ multiple-quantum-well structures. Quantum-well widths between 80 and 450 Å were investigated. Results are in very good agreement with recent effective-mass calculations for isolated impurities at the center of GaAs quantum wells.

In the upper incommensurate phase of biphenyl, spin relaxation is relatively rapid, because of low-frequency excitations (phasons). The relaxation follows $T_1^{-1\mathrm{}}\sim T^{\frac{4}{3}}{\omega }^{-\frac{1}{3}}$; this is the first report of a Larmor-frequency dependence of $T_1$ in an incommensurately distorted solid. The frequency dependence suggests that the phasons are gapless. In the low-temperature incommensurate phase, peaks in $T_1^{-1\mathrm{}}$ are evident at low frequencies. The motion responsible is thermally activated and may kinetically inhibit commensuration.

In an ESR study on the rare-earth ion ${\mathrm{Er}}^{3+}$ in the intermediate-valence single crystal Sm${\mathrm{B}}_6$ a spectrum is found which is markedly different from those of comparable systems with integral valence. We analyze the unusual experimental results in terms of a dynamic Jahn-Teller effect in the ${\Gamma }_8$ cubic-crystal-field ground state. The characteristic features of our experimental data are well reproduced by the calculations, which suggest that the substitutional rare-earth ion ${\mathrm{Er}}^{3+}$ suffers enhanced electron-phonon coupling mediated by the valence-fluctuating host.

The spin-lattice relaxation time $T_1$ of ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ nuclei is measured near the metal-insulator transition as a function of temperature and magnetic field. The observed relaxation rate in the metal is up to ${10}^3$ times larger than that for nuclei interacting with free, degenerate electrons. The enhanced relaxation rate and its magnetic field dependence suggest the existence of intrinsic, quasistatic spins with an ${\omega }^{-1\mathrm{}}$ spectrum. These spins may cause strong spin-flip scattering and may therefore be responsible for the anomalous critical exponent of the electrical conductivity.

The filled $M$ shell of potassium, set into oscillation by the electric field of a photon, leads to interband transitions that explain a large absorption peak at 8 eV. Interference between this collective perturbation and the ordinary A · p coupling (between a photon and conduction electron) shows that the sign of the $V_{110}$ pseudopotential is negative.

We report the first use of angle-resolved ultraviolet-photoemission spectroscopy to study electron dynamics in the picosecond time domain on a photoexcited semiconductor surface. Transient population in a normally unoccupied surface state on InP(110) was directly observed. The band minimum for this state was found to lie at $\overline{\Gamma }$ and the surface-band dispersion near $\overline{\Gamma }$ (effective mass) was measured along two surface symmetry directions.

We investigate the effect of excess excitonic populations created by resonant ultrashort excitation on the optical-absorption properties of GaAs quantum wells. We find that under these conditions at room temperature excitons produce more absorption bleaching than equal densities of free-carrier pairs. This bleaching partly recovers as the excitons ionize to give free carriers. Hence, we directly measure the thermal ionization time of excitons at room temperature for the first time, and find that $\tau \sim 300$ fs.

These infrared studies of acetanilide together with the existence of two equivalent structures for the hydrogen-bonded chain suggest the possibility of a topological defect state rather than a Davydov soliton as suggested previously. Acetanilide is an example of a class of one-dimensional materials where solitons are a consequence of a twofold degenerate structure and the nonlinear dynamics of the hydrogen-bonded network.

The first direct observation of zero-field NMR spectra of ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$ in $\mathrm{Cu}\mathrm{Mn}$, $\mathrm{Ag}\mathrm{Mn}$, and $\mathrm{Au}\mathrm{Mn}$ spin-glasses allows a detailed study of the concentration and $T$ dependence of the Mn local magnetization for $T\lesssim 0.2T_g$. The quantum reduction of the local spin is found smaller than numerical estimates, while the thermal part is found quadratic in $T$. Such a result agrees with expectations for magnons with a linear dispersion relation and a large stiffness constant, at short times, for the spin system.

It is shown that an effective attractive interaction between nearest-neighbor antiparallel spins a-rises in the Hubbard and Anderson lattice models in the limit of large on-site electron-electron repulsion. Results of Monte Carlo simulations of the Hubbard model show enhancement of anisotropic single-pairing correlations and suppression of triplet-pairing correlations. It is proposed that this interaction leads to an anisotropic singlet superconducting state in the heavy-fermion superconductors.

For $d$-dimensional short-range Ising spin-glasses with local spin-flip dynamics, the correlation function $〈s_i\mathrm{}\left(0\right)\mathrm{}{s}_i\mathrm{}\left(t\right)\mathrm{}〉$ is argued to be bounded below by a function of the form $\exp [-c{\left(\ln t\right)}^{\frac{d}{\mathrm{}(d-1\mathrm{})\mathrm{}}}]$ in a temperature range above the spin-glass transition. The slow relaxation of large isolated clusters of unfrustrated spins is responsible for this bound. We suggest that this is the signature for an intermediate Griffiths phase between the spin-glass and the paramagnetic phases.

We use scanning electron microscopy and optical data to show that the pore spaces of several sandstones are fractal geometries and we use the fractal statistics to predict the correct porosity. Steady-state crystal growth during rock formation is a plausible cause of the self-similar geometry. The fractal-dimension values and a systematic analysis of rock conductivity data both mediate against percolation models as suitable models of rock pore-space geometry.

We consider the two-dimensional Ising model on a rectangular lattice with one row of horizontal coupling strengths changed. We shall present our results for correlation functions with two energy-density operators in arbitrary positions, restricting ourselves here to the scaling limit. These exact results show an unexpected complexity.

Global scaling behavior for period-$n$ windows of chaotic dynamical systems is demonstrated. This behavior should be discernible in experiments.

A numerical simulation of electric breakdown is performed by means of a deterministic model of a two-dimensional network of resistors with nonlinear and irreversible characteristics. A breakdown of a resistor induces successive breakdowns of other resistors and finally the broken resistors form a percolation cluster. It has dendritic structure and is found to be a fractal with its dimension about 1.6.

The quantum continuum action for gauge theories in two dimensions is constructed in the axial gauge. For non-Abelian groups the action differs from the usual expectations by nonperturbative, noncoercive corrections, whose role is to ensure gauge invariance for the quantum field theory outside of perturbation theory. Consequently, ordinary perturbation theory and semiclassical approximations become inapplicable. Similar conclusions are expected to apply in more dimensions.

It appears possible to simulate the results of molecular-dynamics calculations of intramolecular and decomposition reactions by means of relatively simple one-dimensional maps.

Exploiting the solvability of the $d=2\mathrm{}$ nearest-neighbor Ising model we construct the critical surface in a thirteen-dimensional parameter space in the vicinity of the nearest-neighbor transition point. We see if the flows generated from this point by recent Monte Carlo renormalization-group transformations lie on this surface. We clarify and quantify the effects of truncation. We unearth a truncation scheme that should speed up the convergence to the fixed point even in $d=3\mathrm{}$.