We present a study of electron focusing in two-dimensional electron gases grown on (311)-orientated GaAs substrates. The electronic properties of electron and hole gases grown on (311)-orientated GaAs substrates have anisotropic behavior in the orthogonal [2¯33] and [011¯] directions, however, the electron focusing results presented demonstrate anisotropic effective mass and Fermi surface even though the mobility is anisotropic. We also determine the ballistic length for the electrons.

Scanning tunneling microscopy was used to study the sputtered and annealed (110) surfaces of ${\mathrm{Mo}}_3$Si and ${\mathrm{Cr}}_3$Si. Both surfaces show extended and atomically flat terraces, but in the case of ${\mathrm{Mo}}_3$Si there is also a uniform distribution of Mo crystallites. This difference in morphology is discussed in terms of different preferential sputtering effects. In both cases, measured step heights show that the ideally bulk-truncated surfaces are either purely Si or metal terminated. Atomically resolved images suggest that the ${\mathrm{Mo}}_3$Si surface is Si terminated, but although no atomic resolution could be obtained for the ${\mathrm{Cr}}_3$Si surface, there are indications that this surface is instead metal terminated.

Femtosecond degenerate four-wave-mixing experiments are performed in the two-beam self-diffracted geometry in two inhomogeneously broadened ZnSe/${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$Se multiple-quantum-well samples at the excitonic resonances at 10 K. A large change in the dephasing time across the linewidth of one sample is observed, whereas it is nearly constant in the other, implying a different degree of localization in these two samples. Exciton-exciton interaction rates are determined across the linewidth, and it is shown that the exciton-exciton interaction rate is much less for excitons that are strongly localized compared to those that are extended.

The polarization dephasing time has been measured across the gain region, at the transparency point, and into the absorption region of an optically excited GaAs multiple-quantum-well sample using spectral hole burning and degenerate four-wave mixing techniques. We observe strongly energy-dependent dephasing rates with a minimum at the crossover from gain to absorption. Numerical results for the microscopically calculated carrier-carrier scattering rates for a two-component electron-hole plasma in quantum wells show fair agreement with the experimental findings.

We present the result of ab initio density-functional calculations of selenium deposited on the GaAs(110) surface. For the coverages FTHETA=1/4,1/2,1, and 3/2 a variety of adsorption and exchange geometries are optimized. For submonolayer coverage a detailed analysis of the total-energy surface is given. For nearly effective monolayer Se coverage, Se-As exchange reactions are compared with adsorption geometries. We observe a tendency toward an exchange of As and Se atoms in the uppermost layers accompanied by a segregation of As at the surface. For the energetically most favored geometries we compare the calculated band structures with recent angle-resolved photoemission spectroscopy results.

The dynamics of biexciton radiative decay in semiconductor quantum wells is treated within the excitonic-molecule mode including super-radiance, exciton recoil, and nonzero photon momentum. For parameters describing GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells, we find that the rate for the decay of the excitonic molecule (in the ground state of the relative motion between excitons) into an exciton and a photon is less than the decay rate characteristic of radiative free one-excitons.

The transition from semimetallic Sb contacts to vacuum tunnel junctions is studied using a mechanically controllable break junction. The conductance of an atom-sized contact is shown to be small compared to the quantum unit due to the large Fermi wavelength of the electrons in Sb. Observation of subquantum conductance steps when varying the contact area of atomic dimensions gives clear experimental evidence for rearrangements of atoms building up the contract.

In a very-low-disorder two-dimensional hole system we observe a ul direct transition from a well developed ul high-order fractional quantum Hall state at ν=2/5 to an insulating state at 1/3<ν<2/5. As T→0, ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$→∞ for the insulating phase, while ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$→0 for the fractional quantum Hall states at ν=2/5 and 1/3, yielding quasiparticle excitation gaps of 0.60 and 3.6 K, respectively. The topology of the theoretical disorder versus magnetic field global phase diagram for this system expressly forbids the direct quantum liquid-to-insulator transition we observe, ruling out the disorder-induced ‘‘Hall insulator’’ as the explanation.

Differential photoconductance measurements are used to observe oblique optical transitions between Stark-localized states spanning up to 14 periods in a 30-Å ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/20-Å ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As superlattice. The conduction-band wave-function coherence length is thus deduced to be 1450 Å, which is consistent with Stark-ladder formation when the well-to-well potential drop is about the size of the alloy-broadened linewidth. The 2-K photoluminescence linewidth agrees with the minimum calculated for a perfectly random distribution of atoms on the group III sublattice.

We develop a theory of the longitudinal excitations of the two-dimensional electron gas in the random-phase approximation. The random-phase approximation describes quantitatively the plasmonic modes in the range of q vectors realized in experiments. We show that the first effects that arise when going to higher q vectors: At large magnetic fields we find discontinuities in the plasmon dispersion at integer filling factors related to the incompressible state at these magnetic fields. Second, in the low-magnetic-field regime, modes arise with 1/B-periodic frequency oscillations, which are governed by the geometrical relation of the wavelength and the radius of the cyclotron orbit at the Fermi energy.

We present an analytical expression for the spectral density of the current flowing through a one-dimensional array of identical small-area tunnel junctions. Negligible stray capacitance, zero temperature, and the voltage close to Coulomb blockade threshold were assumed. Single-electron oscillations appear explicitly (i.e., their linewidth becomes less than the average frequency) when the number of junctions is larger than ∼15.

We report measurements of the coverage-dependent electronic and structural properties of submonolayer Li films, grown on Be(0001). We observe a structural phase transition from a disordered state at low coverages to an ordered hexagonally close-packed structure at high coverages. This structural transition is accompanied by a nonmetal-to-metal transition in the overlayer. The Li overlayer structure was determined using low-energy electron diffraction, while angle-resolved photoemission, inverse photoemission, and core-level spectroscopy were employed to characterize the electronic transformation. For the disordered, low-coverage phase the spectroscopies show only one Li-induced electronic state. This level is centered 3.1 eV above ${\mathit{E}}_{\mathit{F}}$ and is completely unoccupied at the lowest coverages studied. At the completion of the first Li adlayer, the overlayer band is metallic.

Photoelectron spectroscopy is used to study iron particles grown by depositing Fe on a fullerene film. For small amounts of iron deposited on the fullerenes, the Fe 3p core level is shifted ∼0.4 eV to greater binding energy and a clear Fermi edge is missing. Theoretical studies by other workers, principally calculations of the electronic structure for small clusters, suggest that the iron is dispersed in the fullerene matrix in the form of small clusters. When the amount of Fe increases and the clusters become larger, some of the Fe valence states move towards ${\mathit{E}}_{\mathit{f}}$ and the valence band develops a Fermi edge indicative of metallization.

Angle-resolved photoelectron spectra from Ag films supported on a Ni(111) substrate show interesting behavior in response to variation of the photon energy. In contrast to previous reports of discrete and dispersionless quantum-well states in metal films,the Ag/Ni system supports quantum-well resonances that exhibit ‘‘ratcheting’’ energy dispersions and oscillatory emission intensities. These observations are reproduced in a model calculation taking into account both the initial and final continuums of states involved in the photoexcitation process.

We investigate theoretically the effects of the adlayer randomness at low coverages on the charge state of alkali adatoms within the Muscat-Newns model [Solid State Commun. 11, 737 (1972)]. The fluctuating electrostatic potential at an adatom site due to a random arrangement of all the other atoms in the adlayer is modeled in analogy to Holtsmark’s theory of static broadening of spectral lines in plasmas. We find that these fluctuations induce a stronger depolarization of the adlayer with increasing coverage than in the Muscat-Newns model, corresponding to a slower drop of the effective work function.

We present the results of an embedded-atom-method molecular-dynamics simulation of the thermal relaxation of Ag(111). Contrary to recent medium-energy ion-scattering experiments [P. Statiris et al., Phys. Rev. Lett. 72, 3574 (1994)], which find that a change in the sign of the surface relaxation takes place upon heating above 750 K (from contraction to expansion), we find the surface layer to relax inwards at all temperatures. The calculated mean-square amplitudes of vibration, on the other hand, agree quite well with experiment. The model, in addition, shows evidence of premelting at a temperature of about 1000 K.

We present the result of fully self-consistent ab initio density-functional calculations for clean C(100) surfaces. Studying 2×2 and 2×1 reconstructed surfaces, we find the symmetric dimer 2×1 geometry to be the ground state. The corresponding electronic band structure is calculated within the GW approximation, i.e., many-body quasiparticle effects are included.

The electronic structure and relative stability of icosahedral ${\mathrm{Al}}_{12}$M (M=transition metal) clusters have been studied using the density functional theory within the local spin density approximation. Our calculations predict large binding energies for clusters with M atom in the middle of a d series in agreement with the occurrence of ${\mathrm{Al}}_{12}$W phase in these alloys and provide an insight into the understanding of the stability of AlM quasicrystals.

The atomic structure of Co films deposited on Cu(111) at room temperature is studied by imaging backscattered secondary electrons. The patterns show in-registry growth for the first two atomic layers. The next two monolayers show features of the fcc as well as the hcp structure. The hexagonal structure characteristic for bulk Co is found for subsequent layers.

The electronic structure and the optical properties of the cubic hydrate ${\mathrm{H}}_2$-${\mathrm{H}}_2$O at high pressure are elucidated by means of first-principles local-density calculations. The results are compared with the Ic phase of ice. It is shown that the encapsulated ${\mathrm{H}}_2$ molecules interact weakly with the ${\mathrm{H}}_2$O network. The valence-band structure is only slightly modified while the conduction band becomes very different. A number of important physical properties including the bulk modulus, optical-absorption spectrum, refractive index, pressure dependence of the gap, etc., are predicted for ${\mathrm{H}}_2$-${\mathrm{H}}_2$O.