The energy required to remove an atom from semiconductor surfaces is calculated using a Green’s-function approach. Contrary to intuition, we find that, in some cases, less energy is needed to remove an atom from the nearly full surface than from a nearly empty surface. The results are explained in terms of the relative energies of anion and cation dangling bonds, and the charge transfers between them. The deducted effective pair-interaction energies and their effects on surface morphology and growth perfection are discussed.

The results of the first ab initio calculations performed on the complexes formed between hydrogen and both donor and acceptor atoms in gallium arsenide are reported. The equilibrium geometries and electronic properties are presented. For the first time a parameter-free determination of the vibrational properties of the entire defect complex has been performed, allowing a more complete comparison with infrared-absorption experiments. Local structural distortions far larger than previously expected are proposed to account for the absence of certain absorption lines.

Ionization potentials of ${\mathrm{Al}}_{\mathit{n}}$ and ${\mathrm{In}}_{\mathit{n}}$ clusters (n<80) in supercooled beams have been measured by photoionization spectroscopy. The abrupt leveling near n=5 of the initial linear increase in ionization potentials and the subsequent gradual approach to the classical metallic sphere model are consistent with a simple picture of s-p band hybridization. For n>6, there is clear evidence of electron shell filling, yet substantial deviations from shell-model predictions remain. Crystal-field effects are considered the most probable source of symmetry breaking and give insight to cluster structure.

Time-resolved spectral and spatial measurements of photoluminescence in ${\mathrm{Cu}}_2$O at low temperatures suggest that the gas of paraexcitons undergoes Bose-Einstein condensation at high densities. As a result of the two-component nature of the gas (orthoexcitons and paraexcitons with different ground-state energies and spin multiplicities) and the nonequilibrium nature of photoproduced excitons, the condensation process is quite different from the simple ideal-gas case envisioned by Einstein.

We have observed for the first time conductivity fluctuations in the persistent photoconductivity mode in II-VI mixed crystals with the magnitude following the percolation approach as ‖T-${\mathit{T}}_{\mathit{C}}$${\mathrm{\Vert }}^{\mathrm{-}\mathrm{\beta }}$, where ${\mathit{T}}_{\mathit{C}}$ is the percolation threshold temperature. Experimental results have demonstrated that the random local-potential fluctuations induced by compositional fluctuations are responsible for the persistent photoconductivity observed in II-VI mixed crystals. Different behaviors resulting from the DX centers, the macroscopic barrier, and the random local-potential fluctuations are distinguished.

Soft-x-ray photoemission spectroscopy of metals deposited on GaAs demonstrates that minor misorientations of the (100) surface produce major deviations from Schottky-like behavior via increased chemical interactions. The degree of chemical activity correlates with the density of dangling bonds at the [110], [111]A, and (111)B steps, producing deep levels with acceptor character which dramatically reduce the range of Fermi-level stabilization. These results demonstrate the central role of local atomic bonding in the Schottky-barrier formation.

We have observed localized surface states (Tamm states) intentionally introduced in AlGaAs/GaAs superlattices by a terminating layer of AlAs. The formation of these states is manifested by excitonic interband transitions in photoluminescence excitation spectra. Critical confirmation is provided by photocurrent experiments under an electric field that show additional transitions as well as anticrossing interactions between the Tamm states and the Stark-ladder states associated with the superlattice.

With far-infrared spectroscopy, coupling between electron quantum dots becomes visible in the electronic excitation spectrum. We employ gated GaAs-AlGaAs quantum wells that enable field-effect tuning of the coupling between adjacent dots. For noninteracting quantum dots in a magnetic field we observe the characteristic edge- and bulk-mode spectrum.. The coupling of dots is reflected by a branching of the bulk mode into a cyclotron-resonance-like and a magnetoplasmonlike mode and a splitting of the edge mode. The latter is caused by formation of new edge orbits embracing two adjacent dots.

We determined the nuclear-spin-lattice relaxation rate in the vicinity of the two-dimensional electrons in GaAs heterostructures at liquid-helium temperatures in the magnetic-field range 4<B<8.5 T. The occurring magnetoquantum oscillations are, by a model calculation, proved to be due to interaction between nuclear and 2D-electronic spins (Korringa relaxation), and give new information on the electronic density of states.

Using the Bethe-Ansatz technique, we diagonalize exactly the one-dimensional t-J Hamiltonian for the supersymmetric case t=J. In this limit it is identical with models considered previously by Sutherland and by Schlottmann. The ground state is a liquid of singlet pairs of varying spatial separation for all band fillings. We find two types of gapless excitations with effective Fermi surfaces at 2${\mathit{k}}_{\mathit{F}}$ and ${\mathit{k}}_{\mathit{F}}$ which we identify with the holon and the spinon excitations near half filling.

It is shown that thermal excitation of spin waves in a highly anisotropic antiferromagnet results in a characteristic temperature dependence of sublattice magnetization with a crossover from a 3D to a quasi-2D behavior. The magnetic dynamics in several copper-oxide-based antiferromagnets is analyzed in this context in terms of subtle details of their structural characteristics, and the temperature dependence of the Cu moment is used to determine the planar and interplanar exchange energies.

Large nonlocal contributions to the electron-phonon interaction in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ are found using the linearized-augmented-plane-wave method with the frozen-phonon technique. These nonlocal effects greatly increase the electron-phonon coupling strength and lower the expected isotope effect for oxygen by increasing the coupling strength of the heavy ions. We find large anharmonicity of chain-oxygen motions, which can also enhance the superconducting transition temperature and decrease the isotope effect. Excellent overall agreement with experiment is found for the fifteen Raman-active-mode frequencies.

Strong evidence has been accumulated from our recent small-angle neutron-scattering and phase-separation studies to support the view that protein–sodium-dodecylsulfate (SDS) complexes in solution are polymerlike objects. This conclusion is also consistent with the viscosimetric measurements of Reynolds and Tanford, if we we further assume that the protein-SDS complex is a free-draining perturbed polymer. We propose here a reptation mechanism for the explanation of the migration of the protein-SDS complex in polyacrylamide gel during electrophoresis. Electrophoretic data taken from independent experiments have been analyzed to confirm our conjecture.

Chaos in area-preserving maps is organized on the basis of the unstable periodic orbits (UPO’s) and the partition of phase space into resonances. Each UPO is of a well defined type, which specifies the sequence of resonances visited and the number of rotations performed in a resonance. Chaotic orbits are grouped into sets of well defined type, topological and metric characterizations. The effect of regular motion on global diffusion may be characterized by an infinity of diffusion coefficients, associated with ensembles of chaotic orbits that are trapped in extended sets of resonances.

A new type of resonance phenomenon in a class of bistable oscillators driven by a periodic dichotomous noise process is described. The phenomenon manifests itself as troughs or peaks in the stationary density and has its origin in the appearance of periodic orbits in the noisy dynamics. The stationary density exhibits many self-similar scaling features as the system parameters are tuned, and these features are discussed. Since the noise process is easy to implement and the phenomenon is quite general, it should be possible to observe such resonances in experiments.

It is shown that there is an exact equivalence between the Aharonov-Bohm effect for spin-1/2 particles and the Aharonov-Casher effect. The demonstration of this precise relationship between the two is seen furthermore to be independent of whether relativistic or nonrelativistic kinematics are used. The only remaining substantive distinction between the two effects may well be the fact that the scattering cross section for polarized beams has a considerably greater sturcture in the Aharonov-Casher case despite the mathematical equivalence of the scattering amplitudes for the two effects.

We show how to characterize a strange attractor by a set of integers. These are extracted from the chaotic time-series data by first reconstructing the low-period orbits and then determining the template, or knot holder, which supports all periodic orbits embedded in the strange attractor, and the strange attractor itself. The template is identified by a set of integers which therefore characterize the strange attractor. This identification is explicitly demonstrated for the Pirogon using a relatively small data set (5000 points).

We show that motion with as few as three degrees of freedom (for instance, a particle moving in a three-dimensional potential) can be equivalent to a Turing machine, and so be capable of universal computation. Such systems possess a type of unpredictability qualitatively stronger than that which has been previously discussed in the study of low-dimensional chaos: Even if the initial conditions are known exactly, virtually any question about their long-term dynamics is undecidable.

A gedanken experiment is described in which the noninvasive determination of joint probability densities for the magnetic flux linking a superconducting quantum-interference device is determined using variable threshold superconducting switches followed by superconducting magnetometers. As a result, it is demonstrated that an important prediction of quantum mechanics can be tested in principle against those of macrorealistic theories using superconducting circuits.

Experimentally observed near-field and far-field spatial profiles, frequency spectra, and cone angles are in good agreement with numerical computations of a theory that includes Doppler-broadened Raman-gain amplification of Doppler-broadened resonance fluorescence, four-wave mixing, propagational coupling, self-trapping of the pump beam, and diffraction and pump-induced refraction of the new frequencies. The use of a cw laser beam detuned from resonance by only a few gigahertz makes the computational problem tractable, leading to a simple physical model of this spatiotemporal instability.