Third-harmonic (TH) transmission spectroscopy was used to study the dispersion in the ${\mathrm{\chi }}_{\mathit{x}\mathit{x}\mathit{x}\mathit{x}}^{\mathrm{}\left(3\right)\mathrm{}}$ element of the third-order susceptibility in wurtzite-phase gallium nitride (GaN) films on sapphire. Analysis of the nonlinear response showed an enhancement in ${\mathrm{\chi }}_{\mathit{x}\mathit{x}\mathit{x}\mathit{x}}^{\mathrm{}\left(3\right)\mathrm{}}$ when the photon energy of the TH field was tuned to the absorption edge, ${\mathit{E}}_{\mathit{g}}$=3.398 eV, with a peak value of 2.7×${10}^{\mathrm{-}11}$ e.s.u. Comparison with previous band-structure calculations of ${\mathrm{\chi }}_{\mathit{x}\mathit{x}\mathit{x}\mathit{x}}^{\mathrm{}\left(3\right)\mathrm{}}$ in wide-band-gap zinc-blende semiconductors shows that the dispersion in GaN is qualitatively similar to the third-order response in ZnSe near the absorption edge.

A number of surface reconstructions formed on InSb(100) have been observed with atomic resolution using scanning tunneling microscopy (STM). Cycles of low-energy ion bombardment and annealing result in the formation of an In-rich surface with a c(8×2) diffraction pattern. Filled-states images of this indicate a (4×1) periodicity arising from imaging occupied lone-pair orbitals on exposed second-layer Sb atoms. The controlled deposition of ${\mathrm{Sb}}_4$ from a Knudsen effusion source onto the c(8×2) surface at elevated temperatures led to the formation of a (1×1), c(4×4), and an asymmetric (1×3) structure, in order of decreasing Sb/In ratio. Filled- and empty-states STM images of all these Sb-rich reconstructions were generated and the coexistence of the different structures was also observed. Both the (1×1) and asymmetric (1×3) surfaces showed a high degree of disorder while the c(4×4) surface was highly ordered. The c(4×4) structure, which involves the chemisorption of Sb onto an already Sb-terminated surface, is characterized by blocks of three pairs of well-resolved Sb dimers. Structural models are proposed for the ordered structures observed.

The 2×2 structure formed on a Ga-adsorbed Si(100) surface is determined using tensor low-energy electron diffraction. I-V curves of the parallel dimer model are in excellent agreement with those of the experiment, indicating that the actual surface has parallel dimer structure. Specific displacements of the topmost two surface layers (the protrusion of the Ga dimer toward the vacuum, the increase of the bond length of the Ga dimer, the stretching of the Si dimer, and the movement of the Si dimer toward the Ga dimer) with the elongation of the Si dimer back bond are observed in the optimized geometry. The Ga-Si bond angle measured from the Si(100) surface plane is recovered with these displacements from that of the ideal geometry where each bond length is assumed to be the sum of Pauling covalent radii. Subsurface layers are also deformed to keep the bond lengths near their bulk values.

We study the effect of Coulomb blockade (CB) on the Aharonov-Bohm (AB) oscillations, observed in the magnetoresistance of small dots in both the quantum Hall and tunneling regimes. In the quantum Hall regime, where one or more edge states pass freely through the dot, the period of oscillation ΔB is well correlated to the dot size, and the temperature-dependent decay of the oscillations is consistent with the expected form of the single-particle magnetospectrum. In the tunneling regime, however, ΔB is enhanced by more than two orders of magnitude over the value expected from the size of the dot, and the resulting oscillations persist to above a degree Kelvin. This behavior can only be explained by considering the effect of CB on the magnetic depopulation of electrons from the isolated dot; these magneto-Coulomb oscillations are the magnetic analogue of the CB oscillations more conventionally observed using electrostatic techniques. These results demonstrate that by varying the coupling of the dot to the source and drain, the resulting AB oscillations can be used to separately probe both the single-particle and CB-related energy scales of small dots.

Experimental data on the thermoelectric coefficients of a very-low-mobility (μ=0.13 ${\mathrm{m}}^2$/V s) two-dimensional electron gas in a GaAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum well are presented. The measurements span a temperature range of 1–200 K, and have been made in magnetic fields up to 8 T (at which point ωτ∼1). As with high-mobility samples, phonon drag is found to be dominant at low temperatures, showing that the sample mobility is not relevant to this quantity. Data on the thermopower ${\mathit{S}}_{\mathit{x}\mathit{x}}$(B) and the Nernst-Ettingshausen coefficient ${\mathit{S}}_{\mathit{y}\mathit{x}}$(B) are compared with recent theoretical predictions. In the absence of magnetic field, B=0, the phonon mean free path (mfp) is initially determined from the measured thermal conductivity. Then the calculated value of ${\mathit{S}}_{\mathit{x}\mathit{x}}$(0) (thermopower in zero field) is in good agreement with measured values when T<10 K but becomes up to an order of magnitude too small when T>20 K. To avoid this problem when B≠0 the phonon mfp is redetermined from the measured values of ${\mathit{S}}_{\mathit{x}\mathit{x}}$(0) with the diffusion part subtracted. Then the calculated magnetothermopower Δ${\mathit{S}}_{\mathit{x}\mathit{x}}$(B) is in good agreement with the data. The calculated drag contribution ${\mathit{S}}_{\mathit{y}\mathit{x}}^{\mathit{g}}$(B) to the Nernst-Ettingshausen coefficient ${\mathit{S}}_{\mathit{y}\mathit{x}}$(B), however, is only 5.4% of that measured. When the theoretical value of the contribution to ${\mathit{S}}_{\mathit{y}\mathit{x}}$(B) due to phonon drag is increased by a factor of (0.054${)}^{\mathrm{-}1}$=18.5 for all B and T, very good agreement with the data is again obtained over the whole regime investigated experimentally.

The adsorption of ${\mathrm{O}}_2$ molecules on 2×1 reconstructed dimers on Si(001) surfaces and the subsequent oxidation have been investigated by ab initio quantum-chemical calculations. Detailed analyses of the potential-energy hypersurfaces in the spin triplet and quintet states have revealed that the triplet state has the lowest-energy reaction path of the oxidation process. On this lowest-energy reaction path, the electronic state as well as the atomic-level configuration of the molecularly adsorbed metastable precursor of ${\mathrm{O}}_2$ on Si(001) surfaces was clarified. The molecular adsorbate is converted into the atomically adsorbed stable state by the dissociation of the ${\mathrm{O}}_2$ adsorbate to oxygen atoms. This is just the insertion process of an oxygen atom into a Si dimer bond to produce silicon oxide. The activation energy required for this conversion has been calculated to be 60.4 kcal/mol, which is in accordance with the value 60 kcal/mol obtained by experiments at high temperatures. By the inspection of the temperature dependence of the reaction-rate constants, it has been concluded that the reconstructed dimer is hardly oxidized at room temperature and that the origin of the natural oxide of Si(001) surfaces might be defects of the surface reacting with ${\mathrm{O}}_2$ molecules, i.e., the defect-free Si(001) surface is stable against ${\mathrm{O}}_2$ molecules and is not oxidized at room temperature. This conclusion is consistent with recent experimental results that reconstructed dimers on the terraces of Si(001) surfaces were inactive for an exposure of ${\mathrm{O}}_2$ molecules and only defect sites on the same surfaces have reacted with ${\mathrm{O}}_2$ molecules.

A systematic investigation on the conductance through a two-dimensionally confined 1D-1D’-1D quantum system is made. The conductance plateaus of different heights are found. Various effects produced by the external blockade barrier, the propagation length of the electron in the 1D’ region, and the lateral size on the conductance plateau and conductance oscillation are studied. Some phenomena such as the sensitivity of the intensity of conductance oscillation to the external blockade barrier, the tail current produced by the quantum tunneling effect, and the different behaviors of conductance oscillation in the protuberance system and the constriction system are presented. The theoretical results might be helpful to experimental studies.

The variation in the emission of ${\mathrm{Si}}^{+}$ ions from ion-beam-induced oxidized silicon surfaces has been studied. The stoichiometry and the electronic structure of the altered layer has been characterized using x-ray photoelectron spectroscopy (XPS). The XPS analysis of the Si 2p core level indicates the strong presence of suboxide chemical states when bombarding at angles of incidence larger than 30 °. Since the surface stoichiometry or degree of oxidation varies with the angle of incidence, the corresponding valence-band structures also differ among each other. A comparison between experimental measurements and theoretically calculated Si and ${\mathrm{SiO}}_2$ valence bands indicates that the valence bands for the altered layers are formed by a combination of those two. Since Si-Si bonds are present in the suboxide molecules, the top of the respective new valence bands are formed by the corresponding 3p-3p Si-like subbands, which extend up to the Si Fermi level. The changes in stoichiometry and electronic structure have been correlated with the emission of ${\mathrm{Si}}^{+}$ ions from these surfaces. From the results a general model for the ${\mathrm{Si}}^{+}$ ion emission is proposed combining the resonant tunneling and local-bond-breaking models.

We have measured and quantified the effect of alloy composition on the atomic bonding in relaxed molecular-beam-epitaxy-deposited crystalline ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ alloys. X-ray-absorption fine structure (XAFS) and x-ray diffraction were used to examine how the atomic bonding in ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ is affected by changes in alloy composition. In this study, the Ge-Ge and Ge-Si bond lengths were measured using XAFS and compared with the conflicting results of existing analytical models and previous XAFS studies. The measured Ge-Ge and Ge-Si bond lengths were found to be in good agreement with the analytical models, which predict that the Ge-Ge, Ge-Si, and Si-Si bonds maintain distinctly different lengths which change linearly with alloy composition. The topological rigidity parameter ${\mathit{a}}^{\mathrm{*}\mathrm{*}}$ was used to quantify the linear dependence of the bond lengths on alloy composition and a value of ${\mathit{a}}^{\mathrm{*}\mathrm{*}}$=0.63 was calculated from the measured bond lengths. An extensive XAFS error analysis was performed and the error in the topological rigidity parameter ${\mathit{a}}^{\mathrm{*}\mathrm{*}}$=0.${63}_{\mathrm{-}0.13}^{+0.08\mathrm{}}$ was determined. This value of ${\mathit{a}}^{\mathrm{*}\mathrm{*}}$, which is notably different from 0 or 1, indicates that both the bond lengths and bond angles are distorted by changes in composition.

The electronic structure of free-standing (ZnSe${)}_{\mathit{m}}$(ZnS${)}_{\mathit{n}}$ (001) strained-layer superlattices is studied through ab initio non-self-consistent factorized linear combination of atomic orbitals calculations, where the nonrelativistic local-density-functional formalism of Slater is used to represent the crystalline potential. The effects of elastic strain on the crystal superlattice structure are determined using the valence force field or Keating model. The heavy- and light-hole states are found to be localized in the ZnSe sublattice, while the lowest conduction state is delocalized all along the structure. The valence-band offset is determined to be large and increases as the strain on the ZnSe sublattice is increased. The conduction-band offset, though, is determined to be near vanishing, independent of the strain. These results suggest that for all degrees of strain there is a strong confinement of the holes in the ZnSe sublattice and a weak confinement of the electrons. This lack of a strong type-I formation accounts for the weak blue luminescence observed in photoluminescence and absorption measurements at room temperature.

We study the influence of inhomogeneous broadening on results obtained from spectrally resolved transient four-wave mixing. In particular, we study the case where more resonances are coherently excited, leading to polarization interference or quantum beats, depending on the microscopic nature of the resonances. To demonstrate the ideas, we present experimental results from bulk CdSe and from GaAs multiple quantum wells.

Raman scattering spectra of PbTe films, grown by laser-assisted deposition on a KCl substrate, containing the metastable GeS-type phase of PbTe, are investigated at different temperatures in the range 64–300 K. The Raman-active modes of the orthorhombic GeS-type phase of PbTe are detected in the low-temperature spectra of the films containing the predominantly GeS-type phase. The investigation of films with thick upper layers of the stable Raman-inactive PbTe fcc phase indicates that the Raman spectra of the films at RT are governed by the scattering on uncompensated Te atoms in the undoped samples and by second-order scattering in the doped films.

The renormalization of the Landau-level transition energies in the dense quasi-two-dimensional magnetoplasma has been determined from time-resolved luminescence spectra in a strained undoped ${\mathrm{In}}_{0.17}$${\mathrm{Ga}}_{0.83}$As/GaAs quantum well. The neutral electron-hole magnetoplasma was created by 1-ps laser pulses in magnetic fields H≤7.5 T. The renormalization has been found to depend strongly not only on the carrier density but also on their distribution over Landau levels. The dependence is explained by the interplay of the attractive exchange and repulsive Pauli contributions in the transition energies and Coulomb interaction screening effect.

The scattering rates of Wannier states in superlattices in an electric field by acoustic phonons, optical phonons, ionized impurities, and interface roughness are calculated. The explicit analytical expressions are given, and the results calculated for a typical superlattice structure are discussed. As the electric field increases, the scattering rates by acoustic and optical phonons increase first, reach a maximum, and then decrease rapidly. They form the main mechanism of negative differential conduction in the hopping-conduction framework. The critical field, at which the scattering rate reaches a maximum, corresponds to that of the potential drop across the superlattice period equal to the width of the miniband. The variations of scattering rates by impurity and interface roughness with electric field differ apparently from that of phonons.

A theoretical and experimental study of intersubband transitions in modulation-doped p-type Si/SiGe quantum wells is presented for SiGe wells with widths between 26 and 65 Å and Ge contents in the range from 19% to 50%. The SiGe multiple quantum wells are pseudomorphically strained with an in-plane lattice constant equal to the lattice constant of the Si substrate. Calculations of the in-plane dispersion of the quantum-well states are performed within the envelope-function approach, with full inclusion of the degeneracy and warping of the three topmost bulk valence bands described by the strain-dependent Luttinger-Kohn Hamiltonian. Many-body effects such as the Hartree potential and the exchange-correlation interaction are taken into account in a self-consistent manner. The transmission spectra are finally calculated with a dielectric simulation for the multilayer stack. Using the structural parameters determined by high-resolution triple-axis x-ray diffraction, the results of the calculation are in excellent agreement with the observed intersubband absorption that occurs between 480 and 1830 ${\mathrm{cm}}^{\mathrm{-}1}$ for the different samples. As long as the excited states are confined to the SiGe wells, the full widths of the absorption lines are only 20 meV. This value represents the narrowest absorption line so far observed in p-type SiGe quantum wells.

We show the existence of an instantaneous contribution to the time-resolved four-wave mixing (FWM) from GaAs quantum wells near zero time delay (T=0), which is very sensitive to the excitation density and detuning. In spectrally resolved (SR) FWM, this instantaneous contribution corresponds to a broad background, which is stronger in samples with larger inhomogeneous broadening. This spectral background extends well below the heavy-hole exciton, implying that diffraction of laser spectral components with no corresponding density of states occurs near zero delay. Theoretical calculations show that the origin of this instantaneous signal is the free induction decay of off-resonance laser fields, which is dramatically enhanced by the strong pulse overlap and the resulting population grating near T=0. Away from T=0, the delayed, interaction-induced signal dominates. Three-beam SR-FWM also shows the existence of a similar contribution when the first two beams overlap and the third beam is diffracted off the population grating.

Periodic growth and decay of linearly unstable plasmon modes are predicted by numerical solution of the one-dimensional collisionless quantum Boltzmann equation. The nonlinear mode stabilization is accompanied by the generation of higher harmonics. The results are analyzed using quantum generalized quasilinear theory.

Photoluminescence from GaAs quantum wells displays sharp spectral features associated with the free exciton. At high excitation density and low temperature, an additional component appears just below the lowest (heavy-hole) excitonic emission. Previous studies have attributed this feature to the biexciton, consisting of two electrons and two holes, because it appears below the exciton line and grows superlinearly with respect to the exciton intensity. In this paper, we considerably strengthen this identification by analyzing time- and space-resolved photoluminescence data that imply a dynamic chemical equilibrium between the excitons and biexcitons, i.e., a law of mass action. This thermodynamic (complementary to spectroscopic) evidence for biexcitons is based on the interdependent temporal decay and spatial transport behavior of the two components on a picosecond time scale. As the excitons recombine, the law of mass action describes the equilibrium over more than two orders of magnitude in exciton density as determined from the measured photoluminescence intensity and volume of the exciton gas.

We report pressure- and laser-tuned Raman-scattering studies on ${\mathrm{CdS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{-}\mathit{x}}$ nanocrystals. The electron-phonon coupling strength S, known as the Huang-Rhys parameter, was determined as a function of pressure beyond the bulk-phase-transition pressure point. The coupling strength is not drastically smaller, as might be expected theoretically, and also shows no abrupt changes at the bulk-phase-transition pressure point. These results indicate that deep traps play a critical part in the mechanism of the electron-phonon coupling.

Coulomb-blockade oscillations in transport through a lateral quantum dot at small Landau-level filling factors are investigated. We observe both a periodic modulation of the peak conductance as well as a striking modulation of the peak separation. We show that below a Landau-level filling factor of 3 a single-particle description of these effects fails and the depopulation of the Landau levels with decreasing electron number can occur noncyclically. We interpret our measurements in terms of a recently developed phase diagram that characterizes the states of the quantum dot.