The behavior of the binding energy of an exciton near a fractional filling factor ${\nu }_{\mathit{p}/\mathit{q}}$=p/q with q≫1 is investigated. The model under study is similar to the Thomas-Fermi approach to many-electron atoms. A hole plays a role of a nucleus while an electron is assumed to consist of q quasielectrons interacting with one another and with the hole. We assume that the distance between the hole and the two-dimensional electron gas is larger than the magnetic length. A singular contribution to the exciton binding energy is found. It comes from the abrupt change in the polarizability of an electron system near those fractional filling factors where the system displays the fractional-quantum-Hall effect (FQHE). The total energy of an exciton as a function of the filling factor ν shows a singularity of the upward cusp type when ν takes such a fractional value ${\nu }_{\mathit{p}/\mathit{q}}$, and in the vicinity of ${\nu }_{\mathit{p}/\mathit{q}}$ the total energy has a shape of an asymmetric step that is considerably smeared from the low ν side because of the suppression of the metallic screening in the FQHE regime. For the same reason the electron-hole attraction decreases near such filling factors ${\nu }_{\mathit{p}/\mathit{q}}$. This gives rise to a sharp increase of the radiative lifetime. The experimental data on photoluminescence agree qualitatively with the theory.

We consider the Hall conductivity of two-dimensional noninteracting Bloch electrons when the magnetic flux per unit cell is a rational number p/q, where p and q are mutually coprime. We present a counterexample for the naive expectation that the Hall conductivity carried by a band is given by treating gap minima as Dirac fermions. Instead of the above expectations, we show that the change of the Hall conductivity at a gap-closing phenomenon is given by the Dirac-fermion argument. Comparing with the Diophantine equation, our result implies that a band gap closes at q points simultaneously. Furthermore, we show that the dispersion relation is q-fold degenerate in the magnetic Brillouin zone.

The average number of electrons on a small metallic island biased by an external voltage source via a tunnel junction is investigated. The jumps of the electron number as a function of the applied voltage are smeared by quantum fluctuations arising from the finite tunneling conductance of the junction. A systematic expansion of the partition function in terms of the tunneling conductance is given. The average island charge is determined for the realistic case where the electronic bandwidth is large compared to the single-electron charging energy. It is shown that finite-temperature effects remain important for the lowest temperatures presently attainable experimentally.

Miniband transport of electrons in GaAs-based superlattices under quantized magnetic fields normal to the layers is investigated based on nonparabolic balance equations recently developed. We calculate the drift velocity and electron temperature in both low and high electric fields considering impurity, acoustic, and polar-optic-phonon scatterings. As many as 25 Landau levels are included in the calculation of the frictional accelerations and electron-energy-loss rate. Strong oscillation of linear mobility and drastically different behavior of nonlinear drift velocity versus electric field are exhibited in superlattices with narrow miniband width when polar-optic-phonon scatterings are resonantly enhanced or forbidden due to Landau quantization. The results are in good qualitative agreement with recent experiments.

In GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As coupled four-ring and two-ring samples, large Aharonov-Bohm (AB) oscillations have been observed for magnetic fields 0<B<4.5 T. The oscillation spectrum changes systematically corresponding to the subband (Landau level) population inside the one-dimensional channels. Multiple tunneling between the edge states encircling the lithographically defined islands causes well-resolved high harmonics ne/h of the fundamental Aharonov-Bohm oscillation frequency with n≤8. Such high harmonics indicate participation from several islands and require very long phase coherence lengths ${\mathit{l}}_{\mathrm{\phi }}$. The frequencies ${\mathit{f}}_{\mathit{n}}$=ne/h of all harmonics decrease with increasing B in accord with a simple model. These findings are evidence for ballistic transport in these large complex devices. The B dependence of the quantum interference amplitudes ${\mathit{G}}_{\mathit{n}}$(B) for AB oscillations of frequency ne/h is qualitatively different from the previously published results: They increase by about an order of magnitude with increasing B before decaying at very high field. The increase in ${\mathit{G}}_{\mathit{n}}$(B) is exploited to infer ${\mathit{l}}_{\mathrm{\phi }}$ and indicates that ${\mathit{l}}_{\mathrm{\phi }}$(B) increases with B at a rate ≥1.3 μm/T.

There are numerous instances in semiconductor nanostructure physics where effective-mass approximations are deemed insufficient and ‘‘direct diagonalization’’ approaches that retain the microscopic quasiperiodic potential are needed. In many of these cases there are no free surfaces and charge transfer effects are small, so fixed, non-self-consistent potential approaches suffice. To this end we have developed a continuous-space, fully relativistic empirical pseudopotential for AlAs/GaAs, which is carefully fitted to the measured electronic structure of bulk AlAs and GaAs, and to ab initio local-density calculations on short- and long-period AlAs/GaAs superlattices. Variations in the anion-cation charge transfer in AlAs and GaAs are simulated by using an As pseudopotential that depends on the number of Al and Ga nearest neighbors. Excellent agreement is demonstrated between the results of the present empirical-pseudopotential method and experiment or ab initio calculations for crystal structures exhibiting a variety of local atomic arrangements. The method is suited for large-scale electronic-structure calculations, where a realistic, three-dimensional band structure is important. We illustrate this in the context of plane-wave calculations on (i) 512-atom supercells describing (001) AlAs/GaAs superlattices with rough interfaces, (ii) 2000-atom supercells describing (AlAs${)}_{\mathit{n}}$/(GaAs${)}_{\mathit{m}}$ superlattices with randomly selected periods (n,m), and (iii) 512-atom AlAs supercells containing clusters of isoelectronic Ga impurities.

Our main findings are (i) a transition from an L-like to an X-like conduction-band minimum occurs in the (AlAs${)}_1$(GaAs${)}_1$ (001) superlattice as one introduces local interfacial intermixing, (ii) the band-tail states in random superlattices are strongly localized along the growth direction; this is accompanied by a large band-gap reduction, and (iii) while single substitutions in AlAs:Ga do not produce bound states, clusters of ${\mathrm{Ga}}_{\mathit{n}}$ within AlAs produce quasibound impuritylike states already for n=4.

The perturbation-theory study of universal conductance fluctuations is generalized to systems with electron scattering in a static random magnetic field. Our calculation confirms the intuitive expectation that the amplitude of fluctuations coincides with the universal value typical for the unitary random-scattering ensemble. An application of this result to micrometer-size wires with the two-dimensional electron liquid near filling factor ν=1/2q allows us to predict aperiodic Coulomb-type oscillations in this system beyond the Coulomb-blockade regime, which would manifest a correspondence between the Aharonov-Bohm and Aharonov-Casher effects specific to composite fermions.

A coherent scattering approach to current noise is used to analyze the transition from ballistic mesoscopic to dissipative macroscopic conduction. The intrinsic current noise for an energy-independent scattering matrix is presented and used to derive the noise for a beam splitter and two-terminal device in the finite temperature, finite frequency, and finite dissipation cases within the noninteracting electron approximation. The generalized Nyquist spectral density is recovered not only for the equilibrium (zero-bias) case, but also for the nonequilibrium (finite-bias) case in the heavily dissipative limit.

In various recent model calculations on the transport properties of microstructures, transmission resonances have been found that exhibit the asymmetric Fano line shape. In particular, one often encounters points of vanishing transmission or reflection as a resonance is crossed. The interference effects that cause this phenomenon are identified in this paper using first a coupled-channel theory that starts from the full scattering Hamiltonian and second a more general S-matrix approach. The latter is model independent and thus yields predictions for the possible line shapes in a wide variety of systems. Model-independent results are desirable because knowledge of the microstructure potentials is often incomplete. We show for the most general multiprobe, multisubband structure that the total transmission never varies by more than unity on resonance, generalizing a result previously known only for resonant tunneling structures. The role of symmetry is investigated to clarify which features (e.g., reflection zeros) are a consequence of special invariance properties and which are robust in the unsymmetric case. The effect of a resonance is found to decrease with an increasing number of leads in a rotationally symmetric structure. Only in a two-probe geometry can zeros in transmission and reflection occur together for a single resonance. The known result that resonances in symmetric resonant tunneling devices always display exactly unit variation of the transmission is shown to be violated in structures where the nonresonant transmission exceeds 1. Time reversal invariance is not required in the present treatment. Two model systems displaying asymmetric resonances are discussed. Their advantage is that the resonance lifetime can be tuned externally, making it possible to test a scaling property of the Fano line shape that we derive below.

We study the far-infrared response of a confined two-dimensional electron gas (2DEG) with an imbedded simple Coulomb impurity. The system is subject to a quantizing perpendicular magnetic field at low temperature. The electron-electron interaction is treated in the Hartree or the Hartree-Fock approximation. In large enough systems we observe both bulk and edge effects in addition to single particle excitations due to the impurity. The structure of the excitations depends on the filling of the Landau bands and thus the screening properties of the 2DEG.

Adsorbed states of ${\mathrm{NH}}_3$ and ${\mathrm{C}}_6$${\mathrm{H}}_6$ on a Si(111)(√3 × √3 )R30°-B surface have been studied by the use of thermal desorption and high-resolution electron-energy-loss spectroscopy. At 90 K, a bulklike ${\mathrm{NH}}_3$ multilayer, ${\mathrm{NH}}_3$ hydrogen bonded to a chemisorbed one, and molecularly chemisorbed ${\mathrm{NH}}_3$ exist on the (√3 × √3 )R30°-B surface. These desorb at ∼115, 140, and 170 K, respectively. Chemisorbed ${\mathrm{NH}}_3$ has a very low NH stretching energy of ∼370 meV, and its N-H bonds are significantly weakened. A 2.0-eV loss, which corresponds to the electronic transition from an occupied backbond state to an empty dangling-bond state of the Si adatom on the (√3 × √3 )R30°-B surface, is removed upon ${\mathrm{NH}}_3$ adsorption. It is proposed that chemisorbed ${\mathrm{NH}}_3$ is coordinated to the Si adatom on the surface. ${\mathrm{C}}_6$${\mathrm{H}}_6$ does not chemisorb on the (√3 × √3 )R30°-B surface even at 90 K, but only physisorbs at <160 K. The adsorbed states of ${\mathrm{NH}}_3$ and ${\mathrm{C}}_6$${\mathrm{H}}_6$ on the (√3 × √3 )R30°-B surface are quite different from those on clean Si surfaces, and the reactivity criterion of the surface is discussed. It is also found that the (√3 × √3 )R30°-B surface has few defects when a large number of B atoms (∼7×${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$) exists in the subsurface, which was evaluated by analyzing the surface-plasmon loss.

Interpretation of high-resolution electron-energy-loss (HREEL) spectra is dependent on knowledge of the applicable selection rules and therefore on the scattering mechanism involved. HREEL spectra from hydrogen-terminated C(100) demonstrate that impact scattering is the dominant mechanism and that dipole selection rules are not valid on this surface. We discuss reasons for the dominance of impact scattering and implications for the interpretation of HREEL spectra from diamond.

Room-temperature adsorption of oxygen on potassium- and cesium-precovered Cu(110) surfaces was studied by scanning tunneling microscopy. Depending on the alkali-metal precoverage, two different scenarios exist for the structural evolution of the surfaces. For alkali-metal coverages ${\mathrm{\theta }}_{\mathrm{alk}}$≤0.13 ML [${\mathrm{\theta }}_{\mathrm{alk}}$=0.13 corresponds to the (1×3) missing-row reconstructed Cu(110) surface], oxygen adsorption leads first to a transient contraction of the missing rows into islands of a (1×2) structure. After longer exposures it causes the local removal of the alkali-metal-induced reconstruction, and the (2×1) Cu-O ‘‘added-row’’ structure with ${\mathrm{\theta }}_{\mathrm{O}}$=0.5 is formed. In this structure the alkali-metal atoms are incorporated in the Cu-O chains. For higher alkali-metal precoverages, in the range of the (1×2) reconstruction (${\mathrm{\theta }}_{\mathrm{alk}}$≊0.2), more than one-half a monolayer of oxygen can be incorporated into the (1×2) phase with only a minor structural effect before, at higher oxygen coverages, complex oxygen–alkali-metal–Cu structures with oxygen coverages well above 0.5 ML are formed. The saturation oxygen coverage is drastically enhanced beyond ${\mathrm{\theta }}_{\mathrm{O}}$=0.5, the quasisaturation value of the clean surface. Based on mass-transport arguments the substrate is reconstructed for all ratios of oxygen and alkali metal investigated here. Hence, adsorbate-substrate interactions are essential for these structures; they are not dominated by interactions between alkali metals and oxygen, i.e., by adsorbate-adsorbate interactions.

We point out the limitations of linear computer reconstruction algorithms employed previously in holographic crystallography. We show that many of the undesirable artifacts on prior holographic reconstruction of atom positions from point-source diffraction patterns may be traced directly to the nonlinear object-wave terms neglected in traditional holography. This problem is particularly severe in the so-called forward-scattering geometry, represented by, e.g., a subsurface B-atom emitter in the Si(111)-√3 × √3 B surface. We show that, even in this case, unlike the prior linear schemes, the nonlinear algorithm ‘‘atomic-position recovery by iterative optimization of reconstructed intensities’’ is capable of recovering a very accurate, artifact-free and twin-image-free, three-dimensional reconstruction of the positions of nearby scattering atoms even for a single diffraction pattern.

We have performed local-orbitals-based first-principles molecular-dynamical relaxations for seven different configurations of a ${\mathrm{C}}_{60}$ dimer. Of these seven, we find the dimer with adjacent ${\mathrm{C}}_{60}$’s having parallel double bonds to be the only configuration having an energy lower than the energy of two isolated ${\mathit{I}}_{\mathit{h}}$ ${\mathrm{C}}_{60}$’s. We have computed the infrared frequencies and a Raman spectrum for this lowest-energy configuration, and we find these to be in agreement with the corresponding measured spectra of polymerized ${\mathrm{C}}_{60}$. The computed electronic eigenvalues and a simulation of the energy barrier to dimer separation are also compared with experiment. In addition, we have simulated an infinite linear chain of polymerized ${\mathrm{C}}_{60}$’s, and we report the energy, the computed vibrational frequencies and Raman spectrum, and the electronic band structure.

First-principles calculations for slabs as many as 13 layers thick show that the three surface core-level features observed on Be(0001) correspond to core-electron ionizations in its three outermost atomic layers. The calculations also imply that the experimental peak identified with core ionization in the bulk is a composite; theoretical core-ionization potentials for the fourth and deeper layers differ by as much as 90 meV. The sign and surprisingly large magnitudes of the Be(0001) surface core-level shifts (SCLS’s) are attributed to unusually large surface-state contributions to the three outer layers’ local densities of states. Both initial- and final-state effects are substantial in the SCLS’s, and their contributions are additive.

The electronic states of the pristine and alkali-metal-intercalated monolayer graphite (MG)/Ni(111) systems are investigated by using angle-resolved ultraviolet photoelectron spectroscopy and x-ray-photoelectron spectroscopy (XPS). The electronic structure of the MG is modified largely by hybridization of the π orbitals of the graphite layer with the d orbitals of the substrate. The deposition of alkali-metal atoms onto the graphite overlayer at room temperature results in the penetration of the adsorbates into the interface between the MG and the Ni(111) surface. This intercalation causes the change in the band structure of the system because of the dilation of the MG-Ni interlayer distance. The intercalation of the alkali-metal atoms also shows the broadening of the XPS peak of the C 1s core level of the MG.

By spin- and angle-resolved photoemission we have studied the electronic structure of ultrathin Co overlayers on Cu(111). For coverages below 2 ML, we observe electronic states that are different from those of bulk Co but we find no in-plane ferromagnetic order. Above 2 ML the spectra show exchange-split and spin-polarized states, whose binding energy depends very little on the overlayer thickness. A previously unresolved separation of 0.3 eV between two occupied bands of opposite spin character could be observed. The experimentally determined exchange splitting for the two sets of d bands near Γ is thus found to be 1.4±0.15 and 1.15±0.15 eV, respectively. These values are larger than those derived from previous spin-integrated measurements and in closer agreement with band-structure calculations.

Optical plasma-resonance absorption of fcc Co island films consisting of fcc Co particles smaller than about 300 Å in diameter was measured in the photon energy region from 0.5 to 6.5 eV. Comparison of the broadening and the location of this absorption with those of Ni island films shows that the d character of conduction electrons of fcc Co is stronger than that of Ni, and that the number of conduction electrons per atom of fcc Co is large compared to that of Ni. The difference in the character can qualitatively be attributed to the difference in the s-d hybridization, which is larger in fcc Co than in Ni. The difference in the number may be due to the difference in the s-d hybridization.

Using an empirical many-body Hamiltonian fitted to bulk and surface properties of lead, we have studied the temperature-dependent structural transformations on Pb(110) by molecular-dynamics simulation. The surface is observed to undergo an anomalous thermal expansion prior to disordering at T∼500 K, in good agreement with recent ion-scattering and low-energy electron-diffraction studies. Behavior of the layer-resolved structure factors indicates that the disordering process is anisotropic and occurs in a layer-by-layer fashion. Simulation results are also consistent with experimental observations of surface roughening near 400 K as well as the formation of a quasiliquid surface phase above 500 K. The intrinsic stress anisotropy Δg, present on all fcc (110) surfaces, vanishes at T∼590 K, suggesting the possible existence of a liquid film at the surface. By examination of the structural-anisotropy data taken at various temperatures, we confirmed that complete melting of the surface layer is achieved at ∼590 K.