Scanning tunneling microscopy investigations on nucleation and growth of the (2×1)O-Cu(110) structure revealed that the new phase is formed by condensation of mobile chemisorbed O atoms with Cu adatoms evaporating from steps and diffusing across the terraces of the substrate surface. This process can be regarded as two-dimensional precipitation from a dilute mixed fluid into a solid phase rather than a solid-solid transformation. The resulting structure is more appropriately described to be of ‘‘added-row’’ rather than of ‘‘missing-row’’ type.

Temperature-dependent surface extended x-ray-absorption fine-structure (SEXAFS) measurements on half a monolayer of O and N atoms on Ni(100) and Cu(100) surfaces indicate enhanced anharmonic contributions in the adsorbate-substrate pair potential, with respect to bulk values. From the temperature dependence of the SEXAFS amplitude and phase we calculate the thermal expansion coefficients. These coefficients are found to be enhanced independent of the system, establishing the observed anharmonicity as a general surface-related property. The implications of these findings for surface reconstructions are discussed.

We give a definite answer to the question whether or not the quasicrystal i-${\mathrm{Al}}_{73}$${\mathrm{Mn}}_{21}$${\mathrm{Si}}_6$ is a true three-dimensional spin glass by measuring the linear (${\mathrm{\chi }}_{\mathrm{ac}}$ and ${\mathrm{\chi }}_{\mathrm{dc}}$) and dc nonlinear susceptibilities [${\mathit{A}}_3$(T),. . .]. We show that the magnetic correlations expand over large distances with decreasing T and that ${\mathit{A}}_3$(T) diverges following the spin-glass transition scaling law with finite ${\mathit{T}}_{\mathit{c}}$. From ${\mathit{A}}_3$ and ${\mathrm{\chi }}_{\mathrm{dc}}$ we deduce separately and in the H=0 field limit the number (1.3% of the Mn concentration) and the moment (large, 7.5${\mathrm{\mu }}_{\mathit{B}}$) of the magnetic entities, which can be attributed to Mn clusters.

We have developed a full, nonperturbative density-matrix treatment to simulate dynamic nuclear polarization (DNP) spectra for the interaction of a single electronic defect with a single nucleus, that incorporates motion. The theory predicts standard DNP theory in the slow and fast motion limits, and smoothly goes between the two as the motional rate increases. Asymmetric DNP line shapes are simulated at intermediate motional rates. Hence, it is not necessary to adduce more than one defect to explain asymmetric DNP spectra.

The spontaneous bulk polarization of wurtzite BeO is studied using ground-state first-principles theory. This quantity, which is not accessible when imposing periodic boundary conditions, is obtained with a novel approach based on the supercell method. The calculated macroscopic polarization is ${\mathit{P}}_3$=1.5×${10}^{\mathrm{-}2}$ C/${\mathrm{m}}^2$, and mostly results from the internal crystallographic distortion. No experimental value of ${\mathit{P}}_3$ exists, but the calculated differences in polarization, between different geometries, compare well with available data.

We have detected and spectrally analyzed the emission of near-ir continua and identifiable bands by the surface of several solids under uv and visible irradiation. This luminescence is clearly distinguishable from the accompanying thermal radiation. This detection implies that internal vibrational relaxation and conversion to acoustic phonons are not always as radical as usually assumed, at least in the case of simple surface functional groups.

Ultraviolet photoelectron spectra (UPS) have been measured for bare unsupported copper clusters in the 1–410 atom size range as mass selected from a supersonic, negatively charged, cluster ion beam. Using an ${\mathrm{F}}_2$ excimer laser at 7.9 eV, the UPS patterns display the evolution of the 3d band structure as a function of cluster size. Clusters larger than a few atoms were found to display an increasingly sharp feature at the onset of the 3d band similar to that seen for bulk copper UPS at low photon energies, suggesting that these larger clusters may already be crystalline.

The atomic geometries of Ga and B antisites in GaAs are proposed to be strongly bistable. As the Fermi level is lowered towards the valence-band maximum, a structural change from fourfold to threefold coordination is predicted to occur. The Ga antisite is found to be metastable with respect to atomic exchange in the presence of an As interstitial. The antisite-interstitial complex has a large 2.9-eV binding energy but a small 0.3-eV barrier to Ga-As exchange.

The magnetotransport of double-quantum-well structures reveals a high-magnetic-field regime in which quantum Hall states at odd-integer ν are missing, where ν is the number of filled energy levels. In these double wells, the ν=odd quantum Hall states originate from the symmetric-antisymmetric energy gap. With increasing barrier thickness between the two wells, initially the magnetic field destroys only the ν=1 state, then both the ν=1 and ν=3 states disappear. We attribute their destruction to the onset of interwell electron density fluctuations.

We present the first calculation of the energies and absorption spectra of GaAs/${\mathrm{Ga}}_{0.65}$${\mathrm{Al}}_{0.35}$As superlattice excitons in the presence of an applied electric field. The resulting spectra differ substantially from the single-particle Stark-ladder results, particularly at low field strengths. We find very good agreement with the experimental photocurrent spectra of Agulló-Rueda, Mendez, and Hong [Phys. Rev. B 40, 1357 (1989)], and suggest other superlattice structures to test the quantum coherence of the exciton wave functions.

We report the observation of quantum beats in the decay of the coherent polarization of intrinsic excitations in GaAs/GaAlAs quantum wells. The beating arises from interference of excitons with slightly different quantum confinement energy due to well width fluctuations.

Theoretical and experimental results are reported which provide the first evidence for biexciton states in semiconductor quantum dots. The theory predicts an increasing biexciton binding energy with decreasing dot size. Unlike bulk semiconductors, quantum dots have excited biexciton states which are stable. These biexciton states are observed as pronounced induced absorption features on the high-energy side of the bleached exciton resonances in femtosecond and nanosecond pump-probe experiments of quantum dots in glass matrices.

A self-consistent study of structural and electronic properties of semi-infinite Ge(001) terminated by a monolayer of sulfur is reported. Employing local-density theory together with Green’s functions, we find from total-energy minimization that S adsorbs in bridge positions. The electronic spectrum saliently exhibits a dangling-bond and a bridge-bond band, which originate from the four lone-pair electrons at each sulfur adatom. They are both fully occupied. The calculated dangling-bond band is in very good agreement with photoemission data. The bridge-bond band is predicted in this Letter.

Here we give a critical examination of the possibility of realizing a quantum pump of electric charges. The physics is based on the theory of quantum adiabatic particle transport initially due to Thouless. We present theoretical guidelines on the experimental conditions for observing this phenomenon. We argue that potentially very accurate quantization of charge transport or electron current can be achieved, once these conditions are approximately satisfied. An example of experimental setup is outlined to demonstrate the practical possibilities. Some comments are also made on the scientific significance of such a quantum device.

The magnetic-field response of strongly localized electrons is probed by examining the quantum interference of forward scattering paths. With no disorder, there is a rich structure in the tunneling rate, which is sensitively dependent upon commensurability of the flux per plaquette with the flux quantum. With nonmagnetic impurities, there is a positive magnetoconductance, and the localization length appears to increase as ${\mathit{B}}^{1/2}$. With impurities breaking time-reversal symmetry, the distribution of tunneling rates is not changed by the magnetic field.

Ferromagnetic resonance (FMR) and SQUID magnetometry measurements have been made on multilayers of amorphous ${\mathrm{Fe}}_{70}$${\mathrm{B}}_{30}$/Ag. The dependence of the magnetic surface anisotropy constant ${\mathit{K}}_{\mathit{s}}$ on the magnetic layer thickness 2L has been determined in the range 1.6 Å<2L<90 Å using more than twenty samples. It is found that ${\mathit{K}}_{\mathit{s}}$ is constant for 2L>16.5 Å, but decreases monotonically towards zero as 2L decreases from 16.5 Å towards zero. The FMR results can be well described by a theory developed for ultrathin amorphous ferromagnetic layers.

The current-voltage characteristic of an ultrasmall tunnel junction is calculated for arbitrary frequency dependence of the impedance presented to the junction by its electromagnetic environment. It is shown that the Coulomb blockade of tunneling is washed out by quantum fluctuations of the charge on the junction capacitor except for ultrahigh impedance environments. Two simple cases where the environment can be treated as an inductor or resistor are examined in detail. Effects of finite temperatures are discussed.

For a high-${\mathit{T}}_{\mathit{c}}$ layered superconductor, we study a flux creep when the magnetic field is parallel to the layers (the CU-O planes) and vortices move across them. In a homogeneous material, the pinning of vortices results from their interaction with the layered structure of the superconductor provided the vortex-lattice period is commensurate with the interlayer spacing. The flux-creep activation energy is proportional to ${\mathit{j}}^{\mathrm{-}1}$ at small currents due to a large size of the activated vortex bundle.

The large-distance behavior of the spin-spin correlation function of the one-dimensional repulsive Hubbard model is evaluated analytically in the strong-coupling regime at quarter filling. In this case, its power-law decay is characterized by an exponent γ=3/2. We have found that this behavior is generally valid at any nonzero doping, although our argument is not mathematically rigorous away from quarter filling. These results strongly suggest that the renormalization-group scaling to the Tomonaga-Luttinger model is exact in the U→∞ Hubbard model.

We consider a model of a Kondo-like impurity interacting with a band of fermions for which the density of states is zero or small near the Fermi energy. Renormalization-group arguments and a large-degeneracy technique are used to demonstrate that this model has a nontrivial zero-temperature phase transition at a finite coupling constant, in contrast to the zero-coupling-constant transition of the ordinary Kondo model. Possible experimental realizations of this model include anisotropic superconductors, narrow gap semiconductors, and systems with flux phases.