We report on optical measurements performed on two 53 and 106 Å wide ZnSe quantum wells separated by a 350-Å-thick ${\mathrm{Zn}}_{0.73}$${\mathrm{Mn}}_{0.27}$Se barrier. The measurements were done by means of cw photoluminescence, cw photoluminescence excitation, and time-resolved photoluminescence spectroscopies at low temperature and under a magnetic field up to 5.5 T. They allow us to determine the strain state of the different layers and the valence-band offset between the ternary and binary alloys. Moreover magnetic tailoring of the nature of the fundamental optical transition from a type-I light-hole exciton to a type-II heavy-hole exciton is evidenced. Calculations taking into account the strain, Zeeman, and excitonic effects are presented and support the experimental findings.

Electron tunneling between quantum Hall systems on the same two-dimensional plane separated by a narrow barrier is studied. We show that in the limit where the inelastic scattering time is much longer than the tunneling time, which can be achieved in practice, electrons can tunnel back and forth through the barrier continuously, leading to an oscillating current in the absence of external drives. The oscillatory behavior is dictated by a tunneling gap in the energy spectrum. We shall discuss ways to generate oscillating currents and the phenomenon of natural dephasing between the tunneling currents of edge states. The noise spectra of these junctions are also studied. They contain singularities reflecting the existence of tunneling gaps as well as the inherent oscillation in the system.

Scanning tunneling microscopy (STM), tunneling spectroscopy, and Auger-electron spectroscopy have been used to study the formation of phosphorus-terminated silicon (001) surfaces by the thermal decomposition of phosphine (${\mathrm{PH}}_3$). The STM images show that surface phosphorus atoms readily displace Si from the substrate, dramatically changing the overall surface morphology through the formation of large numbers of islands and an extreme roughening of step edges. The surfaces are terminated with P-P dimers, but also contain large numbers of line defects which act as a strain relief mechanism. STM images are used to determine the symmetry of these line defects and are compared with model defect structures. Line defects in the phosphorus-terminated surface both nucleate islands and also constrain their growth in one dimension, such that the phosphorus-terminated surfaces contain large numbers of extremely anisotropic islands and very rough step edges. At lower phosphorus coverage, the STM experiments are able to identify Si and P atoms individually, revealing the formation of large numbers of Si-P heterodimers and the simultaneous disappearance of the strain-induced line defects. Counting statistics are used to study the equilibrium between Si=Si, Si-P, and P-P dimers, showing that the surface is a nearly random alloy with a slight nonstatistical preference for formation of the Si-P heterodimer. The geometric and electronic properties of Si=Si, Si-P, and P-P dimers on Si(001) are discussed on the basis of tunneling microscopy and tunneling spectroscopy measurements.

We present atomic-resolution images, obtained with scanning tunneling microscopy (STM), of 3C-SiC(001) surfaces with Si adlayers on the Si-terminated surface, formed by in situ cleaning. We propose a unified additional dimer-row model for the surface with extra Si atoms, which is consistent with structural features obtained by our STM and low-energy-electron-diffraction (LEED) observations. The unit cell of the Si-saturated (3×2) surface seen in LEED observations consists of a pair of additional Si dimers on the Si-terminated surface. The dimer pairs form a straight string along a direction through the centers of two constituent atoms of a dimer. Unit cells of (5×2) and a new reconstruction (7×2) also consists of a pair of additional Si dimers. The (5×2) and (7×2) surfaces have 5/3 and 7/3 wider distances between adjacent rows than that on the (3×2), respectively. This model predicts that even number phases such as (4×2), (6×2), . . . , which are so far unobserved, are unfavorable on the excess Si surfaces in terms of structural stability. Further, it is found that the saturation of Si adsorption on the (3×2) surface is derived from a steric hindrance caused by periodic dimer-vacancy strings between the adjacent dimer pairs.

We report an experimental study of the second-harmonic generation in a type-II InP/${\mathrm{Al}}_{0.48}$${\mathrm{In}}_{0.52}$As heterostructure at frequencies of the fundamental beam resonant with the spatially indirect-band-gap transitions. From the comparison between the second-harmonic spectrum of the heterostructure and that of an InP crystal in the region 1.15–1.45 eV, we show an enhancement of the second-order susceptibility in the former that is related to the transitions between electron- and hole-confined states. From a study of the dependence of the second-harmonic generation on the polarization directions and the sample orientation, the symmetry properties of the second-harmonic tensor have been derived. These correspond to a ${\mathit{T}}_{\mathit{d}}$ (bulklike) symmetry class and the data show the predominance, in the visible region, of the contributions to the second-order nonlinear susceptibility coming from transitions involving high-energy nonconfined states. The terms of lower symmetry involving only low-energy confined states give in this case a negligible contribution.

Results of tunneling spectroscopy measurements of the (110) cleaved surface of GaAs, InP, GaSb, InAs, and InSb are presented. These materials form the family of direct-gap III-V binary semiconductors. Spectroscopic measurements are performed in ultrahigh vacuum, using a scanning tunneling microscope (STM). Techniques based on variable tip-sample separation are used to obtain high dynamic range (six orders of magnitude) in the measured current and conductance. Detailed spectra are obtained for all the materials, revealing the conduction- and valence-band edges, onset of the higher lying conduction band at the L point in the Brillouin zone, and various features associated with surface states. The precision and accuracy in determining energetic locations of spectral features are discussed. In particular, limitations in the accuracy due to tip-induced band bending is considered.

We present the exact solution of an electrostatic problem in terms of the potential distribution and density profile of an incompletely depleted two-dimensional electron gas in a heterostructure covered by a half-infinite gate. Using this solution, we calculate configurations of compressible and incompressible (insulating) strips that are formed in the presence of a quantizing magnetic field. We discuss the application of our results to transport experiments on the inter-Landau-level and spin diodes, as well as edge-magnetoplasmon propagation.

We study a ssV-shaped symmetrically graded quantum well, which has been investigated optically. The well width is 125 Å and the Al concentration x varies linearly from x=0.36 at the barriers to x=0 (pure GaAs) in the middle of the well. In the present paper, numerical calculations of the well electronic states are made within the framework of the surface Green function matching method and the ${\mathit{sp}}_3$${\mathit{s}}^{\mathrm{*}}$ empirical tight-binding model within virtual-crystal approximation. An algorithm developed recently by the authors is applied to treat graded composition heterostructures. The e1-hh1, e2-hh2, and e1-lh1 calculated transition energies are in fair agreement with experiment. The dependence of the transition energies on the well width and on the Al concentration is studied. A comparison with the rectangular quantum well is made. The transition energies decrease when the well width increases and are always higher than the same energies of the rectangular quantum well. All the transitions are better resolved energetically for the triangular quantum well. The energies of the transitions under study increase when the Al concentration x increases, but this dependence is much stronger for the triangular quantum well. Spatial distributions of spectral strengths of the well states are also obtained.

We have studied the problem of coherent and sequential tunneling through a double-barrier structure, assisted by light considered to be present all over the structure, i.e., emitter, well, and collector as in the experimental evidence. By means of a canonical transformation and in the framework of the time-dependent perturbation theory, we have calculated the transmission coefficient and the electronic resonant current. Our calculations have been compared with experimental results and turn out to be in good agreement. Also, the effect on the coherent tunneling of a magnetic field parallel to the current in the presence of light has been considered.

We present a theory for the nonlinear current-voltage characteristics of a ballistic quantum constriction. Nonlinear features first develop because of above-barrier reflection from the potential profile, created by impurities in the vicinity of the constriction. The nonlinearity appears on a small voltage scale and makes it possible to determine distances between impurities as well as the magnitude of the impurity potentials.

We have studied the conductance enhancement near zero voltage bias of double-barrier Nb-${\mathit{p}}^{++}$Si-E junctions, where we chose for the counterelectrode E either Nb, Al, or W. The experiments show a large correction, ΔG≊0.1${\mathit{G}}_{\mathit{N}}$, on the classical superconductor–insulator–normal-metal (SIN) conductance. We present measurements of the temperature, magnetic-field, and voltage dependence, and we interpret the observed results within the available theoretical models for coherent Andreev reflection, as provided by several authors.

We study the sliding state of a two-dimensional Wigner crystal in a strong magnetic field and a random impurity potential. Using a high-velocity perturbation theory, we compute the nonlinear conductivity, various correlation functions, and the interference effects arising in combined ac+dc electric effects, including the Shapiro anomaly and the linear response to an ac field. Disorder is found to induce mainly transverse distortions in the sliding state of the lattice. The Hall resistivity retains its classical value. We find that, within the large-velocity perturbation theory, free carriers which affect the longitudinal phonon modes of the Wigner crystal do not change the form of the nonlinear conductivity. We compare the present sliding Wigner crystal in a strong magnetic field to the conventional sliding charge-density-wave systems. Our result for the nonlinear conductivity agrees well with the I-V characteristics measured in some experiments at low temperatures or large depinning fields, for the insulating phases near filling factor ν=1/5. We summarize the available experimental data, and point out the differences among them.

The dispersion of the nonlinear refractive index of direct-gap semiconductors, over the full sub-band-gap spectral region, is determined within the four-band Kane model. The divergences of individual terms in the nonlinear susceptibility, which have prevented previous full calculations, are avoided by selecting suitable combinations of terms and by a consistent application of the analytic continuation technique. The origins of different forms of divergence are analyzed. Results are presented for InSb, InAs, GaAs, CdTe, ZnTe, and ZnSe; these are compared with previous model calculations based on a Kramers-Krönig transformation of the nonlinear absorptive susceptibilities. The present model gives excellent agreement, with no fitting parameters, to spectral midgap experimental results for the alloy ${\mathrm{Al}}_{0.18}$${\mathrm{Ga}}_{0.82}$As.

The effect of intrasubband triple-electron collisions on the kinetics of a nondegenerate one-dimensional electron gas (1DEG) is investigated since binary collisions give zero contribution to the relaxation. The scattering probabilities are derived from the Lippmann-Schwinger equation for the transition matrix. The corresponding relaxation frequency is of the order of η${\mathit{nv}}_{\mathit{T}}$min[1,(${\mathit{e}}^2$/${\mathrm{\varepsilon }}_{\mathit{L}}$ħ${\mathit{v}}_{\mathit{T}}$${)}^3$], where η=${\mathit{e}}^2$n/${\mathrm{\varepsilon }}_{\mathit{L}}$T is the plasma parameter, n, T, and ${\mathit{v}}_{\mathit{T}}$ are the concentration, temperature, and thermal velocity of the 1DEG, respectively, and ${\mathrm{\varepsilon }}_{\mathit{L}}$ is the dielectric constant. The relaxation frequency can reach a value of ${10}^{12}$/s, comparable to that by other scattering mechanisms, and indicates the importance of the effect for the relaxation of a 1DEG.

The relaxations and vibrational properties of dihydride steps on the Si(111):H surface are studied from first principles. Besides the expected large relaxation of the ${\mathrm{SiH}}_2$ unit on the step edge, long-range relaxations are found along a zigzag chain deep into the bulk. Three vibrational modes are found with frequencies 10.9, 22.5, and 56.0 ${\mathrm{cm}}^{\mathrm{-}1}$ higher than the Si-H stretching frequency on the terrace. A scissors mode is identified with frequency 262 ${\mathrm{cm}}^{\mathrm{-}1}$ higher than the Si-H wagging frequency. These frequency differences and mode assignments are in good agreement with experiment.

The energy bands of a finite-width mesoscopic multichannel ring are calculated using the transfer-matrix method. Two approaches are developed, the diagonal approximation and the multichannel model. We apply these results to the calculation of the persistent current in mesoscopic rings enclosing the magnetic flux Φ in the ballistic regime. It is shown that the interchannel coupling in a finite-width mesoscopic ring produces a halving of the fundamental period h/e of the persistent current ${\mathit{I}}_{\mathit{A}}^{\left(\mathit{n}\mathrm{̃}\right)}$(Φ).

We study attractive and repulsive screened-two body interactions in the integer quantum Hall effect. Our approach is based on a computation of the fermion determinant associated with the finite-temperature partition function of the system. A Hubbard-Stratonovich transformation on the two-body interaction term is performed, which yields an effective free energy depending on a scalar field. As a result of extremizing the free energy, we find, in the case of repulsive interactions, stable translationally invariant solutions related to integer filling factors of the Landau levels and nontrivial radial solutions, corresponding to excited states that represent depletion of charge in the gas. In the case of attractive interaction, we find nontrivial solutions representing accumulation of charge, with energy lower than the uniform solutions. The true ground state in this case is not given by the uniform solution, opening the possibility of having noninteger filling factors related to a new collective state.

Transport in a quasi-one-dimensional wire containing a rectangular quantum dot in a strong magnetic field is studied numerically. As the width of the wire is varied, a rectangular dot on the wire introduces peak or dip structures in the conductance. The charge-density distribution and current-flow pattern clearly show that the Landau-level coupling between the zero-dimensional (0D) state and the extended state is the origin of these structures. At the resonant points, which correspond to-0D states in the dot, Landau level coupling becomes essential and the propagation of one Landau level is enhanced or blocked by the 0D state of a different Landau level.

A hierarchy of quantum models with inverse-square pairwise interaction is studied for the system with a confining harmonic potential. An approach based on renormalized harmonic oscillators is used to construct the energy spectrum, which naturally results in the renormalized Hartree-Fock description of the interaction. In the limit of weak confinement, the model may serve as a canonical model for multiple chiral Tomonaga-Luttinger liquids for the edge states of the fractional quantum Hall effect. For the SU(ν) symmetric case, we find that the excitation spectrum is classified in terms of the simple roots and the weights of SU(ν) Lie algebra.

We calculate the tunnel current between two parallel two-dimensional electron systems in a strong perpendicular magnetic field. We model the strongly correlated electron systems by Wigner crystals and describe their low-energy dynamics in terms of magnetophonons. The effects of the magnetophonons on the tunneling processes can be described by an exactly solvable independent-boson model. A tunneling electron shakes up magnetophonons, which results in a conductance peak that is displaced away from zero voltage and broadened compared with the case of no magnetic field. At low temperatures and low enough voltages the tunneling conductance is strongly suppressed and the I-V characteristics exhibit a power-law behavior. We also analyze the effects of excitation of magnetoplasmons, which have a gap equal to the cyclotron energy and therefore give rise to a second peak in the I-V curves. Most of our results are in good quantitative agreement with the recent experiments by Eisenstein, Pfeiffer, and West [Phys. Rev. Lett. 69, 3804 (1992)].