We have calculated variationally the binding energy of a hydrogenic donor and a Wannier exciton in a quantum well with a potential shape proportional to ‖z${\mathrm{\Vert }}^{2/3}$ as a function of well width and barrier height. The exciton binding energy compares favorably with the experimental result of Sputz and Gossard. A comparison of the results relating to a rectangular well and those of the ‖z${\mathrm{\Vert }}^{2/3}$ well is also presented.

The radiative lifetime of biexcitons in ${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$Se-${\mathrm{ZnS}}_{\mathit{y}}$${\mathrm{Se}}_{1\mathrm{-}\mathit{y}}$ multiple quantum wells has been studied by means of time-resolved luminescence spectroscopy under high-density excitation. It is shown that the rise of the biexciton luminescence becomes more rapid with increasing excitation energy density and that the biexciton luminescence decays with a double exponential form. It is found that the decay-time constant of the faster-decay component in the double-exponential decay corresponds to the radiative lifetime of the biexciton. Its value is about 6 ps at 2 K and is about one-seventh of that in bulk ZnSe (∼40 ps).

Temperature and electric-field tuning of the vacuum Rabi splitting of the coupled exciton-photon state in a semiconductor microcavity is reported. A very good fit to the coupling of the heavy- and light-hole excitons to the cavity photon mode, as a function of temperature, is obtained from a theoretical model for the reflectivity spectra. In addition, the variation of Rabi splitting with an applied electric field is well accounted for by the calculated variation of exciton oscillator strength with the electric field. Electro-optic control of the vacuum Rabi splitting is demonstrated.

An in-plane magnetic field suppresses the quantum coupling between electrons in a double-quantum-well structure. The microscopic theory of this effect is developed and confirmed experimentally. We have shown that the decrease of the resistance resonance peak is sensitive to the mutual orientation of the current and the in-plane magnetic field. The characteristic field required for the suppression of the resonance depends on the elastic-small-angle and electron-electron scattering rates. The study of the characteristic field allows us to verify the temperature and Fermi-energy dependence of the electron-electron scattering rate, providing another experimental tool for its determination.

The radiative decay rates of excitons localized by interface disorder in type-I indirect-gap quantum wells are explored as a function of the disorder-induced coherence area. In contrast to direct quantum wells, disorder is shown to enhance the radiative decay rate, and to provide the dominant radiative channel at low temperature.

It is shown that the absolute value of the persistent current in a system with toroidal geometry is rigorously less than or equal to eħNα/4π${\mathit{mr}}_0^2$, where N is the number of electrons, ${\mathit{r}}_0^{\mathrm{-}2}$=〈${\mathit{r}}_{\mathit{i}}^{\mathrm{-}2}$〉 is the equilibrium average of the inverse of the square of the distance of an electron from an axis threading the torus, and α≤1 is a positive constant, related to the azimuthal dependence of the density. This result is valid in three and two dimensions for arbitrary interactions, impurity potentials, and magnetic fields.

By using a recently derived upper bound on the allowed equilibrium current in a ring, it is proved that the magnitude of the group velocity of a Bloch electron in a one-dimensional periodic potential is always less than or equal to the group velocity of the same Bloch state in an empty lattice. Our inequality also implies that each energy band in a one-dimensional crystal always lies below the corresponding free-electron band, when the minima of those bands are aligned.

Predictions of the four-position model of the surface reconstruction of W(001) induced by ${\mathrm{H}}_2$ chemisorption are obtained by Monte Carlo simulation. Results are compared with those of the simplified two-position model, finding qualitative differences, and with experiments.

A semiempirical phenomenological equation is proposed to interpret the Monte Carlo simulation experiments of the kinetics of physisorption and chemisorption with attractive and repulsive lateral interaction on a solid having an energetically heterogeneous surface.

We have performed a first-principles electronic-structure study of a planar structural form of a hypothetical ${\mathrm{C}}_3$${\mathrm{N}}_4$ solid and compared its energy to a fully three-dimensional form. We use a mixed basis of pseudoatomic orbitals and low-kinetic-energy plane waves to determine the single-particle Kohn-Sham wave functions. These results are tested against fully converged plane-wave-basis calculations. We find that the proposed planar form of ${\mathrm{C}}_3$${\mathrm{N}}_4$ (p-${\mathrm{C}}_3$${\mathrm{N}}_4$) is lower in energy than the hexagonal form β-${\mathrm{C}}_3$${\mathrm{N}}_4$. However, the energy difference between the two phases is small. The hexagonal β-${\mathrm{C}}_3$${\mathrm{N}}_4$ phase is insulating, with an indirect gap, while the planar form p-${\mathrm{C}}_3$${\mathrm{N}}_4$ has a semimetallic electronic structure.

We study solids of ${\mathrm{Si}}_{20}$ fullerenes, ${\mathrm{Si}}_{46}$ and ${\mathrm{Na}}_2$${\mathrm{Ba}}_6$${\mathrm{Si}}_{46}$, in the framework of density-functional theory. The electronic structure of ${\mathrm{Si}}_{46}$ is remarkably different from that of the diamond Si lattice due to its pentagonal network. The valence-band top becomes deeper, resulting in a narrow valence-band width and a wide fundamental gap. Also, another gap appears within the valence bands. In the Na- and Ba-codoped phase, Ba states show strong hybridization with ${\mathrm{Si}}_{46}$ states giving a very high Fermi-level density of states, which should be of essential importance for the superconductivity observed in ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{Ba}}_{\mathit{y}}$${\mathrm{Si}}_{46}$.

We study contributions to the spin-exchange scattering amplitude at two-loop (1/${\mathit{N}}^2$) order for the metallic phase of the two-dimensional t-J model, for which a well-defined 1/N expansion is possible. We find for small hole dopings and small exchange couplings (t≫J) a positive spin-exchange Landau parameter, which suggests that the spin-density fluctuations show a well-defined collective mode near the metal-insulator filling. This behavior results from the fact that the chirality fluctuations decouple from and are suppressed relative to the charge fluctuations for large mass enhancements. The result is interpreted as paralleling the collective spin and charge modes characteristic of the Luttinger liquid for low wave vectors.

The energetics of hydrogen incorporation into strained Si-Si bonds are examined using first-principles density-functional-pseudopotential calculations. It is found that an increase in the bond length of the initial Si-Si bond leads to a lower formation energy for the Si-H-Si configuration. A simple linear relationship, describing the change in formation energy as a function of the Si-Si bond length, is presented, which applies to bond-stretching as well as bond-bending distortions. The results are discussed in the context of hydrogen interaction with grain boundaries in polycrystalline silicon.

Calculations are carried out for single-particle energy levels and wave functions for electrons in a quasi-one-dimensional charge channel formed on the surface of liquid helium. The expressions for the spectrum are obtained for two different orientations of the magnetic field perpendicular to the channel. The conditions for the applicability of the formulas derived are established and they can be used in experimental studies of spectroscopic transitions between

The interstitial fluoride (${\mathrm{F}}_{\mathit{i}}^{\mathrm{-}}$) of a simple ${\mathit{M}}^{3+}$-${\mathrm{F}}_{\mathit{i}}^{\mathrm{-}}$ dipole in fluorite lattices can occupy any of six nearest-neighbor (nn) sites or eight next-nearest-neighbor (nnn) sites. Previous determinations of the relaxation modes of this center have included only nn-nn, nn-nnn, and nnn-nn jumps, ignoring jumps between adjacent nnn sites. We have solved the more general normal-mode problem allowing jumps between all adjacent sites of either species. This model allows comparison of the time constant of dipolar relaxation and that of approach to equilibrium under all assumptions about the relative magnitude of the rates of the four different possible jumps between adjacent sites. We have measured the time constant and activation enthalpy for dipolar relaxation by thermally stimulated depolarization and for the approach to equilibrium by electron paramagnetic resonance (EPR) below 200 K, and have found similar values for these two processes. We have also determined the relative populations of these sites over a broad range of temperature (100–310 K) from EPR data. Populations of the nn and nnn sites reach equilibrium with each other quickly at temperatures above 200 K; population changes are quenched at lower temperatures. The time constant for approach to equilibrium is nearly the same as that for dipolar relaxation at temperatures between 179 and 200 K, indicating the nnn-nnn jumps are unlikely below 200 K. Other work suggests that above room temperature alternate relaxation modes must be important.

A closed form of the spin-wave Green function is obtained for a system consisting of two different Heisenberg ferromagnets coupled by an interface. The calculations employ a tridiagonal matrix method and the random-phase approximation. The results are used to calculate the localized modes associated with an isolated impurity at or near the interface. Numerical examples of the modes are given, and they are found to exhibit various effects due to the interplay between the bulk and interface modes and the strength of the impurity-related interactions.

The two-magnon absorption of electromagnetic waves in the noncollinear magnetic phase of the four-sublattice antiferromagnet ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$ has been considered. The crystal is shown to exhibit a distinguishing feature that the electric dipole moment associated with the pair of spins has an exchange nature. Consequently, the two-magnon absorption of the electromagnetic waves is electrodipole active in the exchange approximation. We have studied theoretically both the magnetic-dipole and electric-dipole channels of the two-magnon absorption and analyzed the frequency dependences of the coefficient of the absorption. The intensities of the absorption processes involving the exchange magnons are shown to be of the same order of magnitude as that involving the acoustic magnons only.

Ultrathin ferromagnetic films with perpendicular spin anisotropy can possess an alternating up-down stripe-domain structure, with widths L±δ. Considering the two inequivalent types of stripe domains to form a single unit, this structure may be thought of as a two-dimensional smectic crystal. It is subject to a weak stripe orientation energy. With increasing temperature the domain system changes from a smectic crystal phase to an ‘‘Ising nematic’’ phase, and then to a ‘‘tetragonal liquid’’ phase. We discuss its possible phase diagrams, in (${\mathit{H}}_{\mathrm{\perp }}$, ${\mathit{H}}_{\mathrm{\parallel }}$, T) space. This sequence of phases can occur whether or not the system ultimately undergoes a spin reorientation transition to a planar phase.

Electronic structures of Fe on Cr (Fe/Cr) and Cr/Fe/Cr sandwich films, with an Fe coverage of 1–20 Å, have been investigated by using photoemission spectroscopy (PES). The Fe 3d partial spectral weight in Fe/Cr has been extracted as a function of the Fe overlayer thickness. Experimental results are compared with supercell band-structure calculations for a system with monolayer (ML) Fe on each side of five-layer Cr, Fe(1 ML)/Cr(5 ML)/Fe(1 ML), by using the linearized muffin-tin orbital band method. For a very thin Fe overlayer, very interesting features are observed: (i) a sharp emission just at ${\mathit{E}}_{\mathit{F}}$, and (ii) two other structures at higher binding energies which resemble the Cr 3d valence bands. The former feature is expected to originate primarily from hybridization between Fe and Cr 3d electrons at the Fe/Cr interface, and partially from the Fe 3d surface states in the Fe overlayer. The latter features also suggest large hybridization between Fe and Cr 3d states at the Fe/Cr interface, as confirmed by band-structure calculations. The trends observed in PES measurements are qualitatively consistent with those in band-structure calculations for the Fe(1 ML)/Cr(5 ML)/Fe(1 ML) system. This work suggests the importance of electronic structures, such as Fermi surface effects, in determining the character of the exchange coupling as well as the magnitude of magnetoresistance in the Fe/Cr superlattice

The local-spin-selective x-ray-absorption spectrum and the magnetic-circular-dichroism spectrum are measured at the manganese K edge of MnP. Both spectra show the presence of a strong spin polarization at the edge. A comparison of the two techniques makes it possible to determine the energy dependence of the Fano factor. It is found that the Fano factor is -4% at the edge but it decreases quickly over an energy range of 10 eV to values lower than 1%. The local-spin-selective x-ray-absorption spectrum is compared to the empty density of states as determined from a local-spin-density band-structure calculation. The use of the Kβ detection yields a spectrum with considerably better resolution, which offers new possibilities for more detailed comparisons with electronic-structure models.