We report time-resolved spectroscopy on exciton luminescence in low-dimensional systems. The reduction of dimensionality from two in quantum wells to one in corrugated superlattices can completely change the initial relaxation process. In the one-dimensional system exciton-phonon scattering is, even at high exciton densities, faster than exciton-exciton scattering since the latter is reduced whereas the former is enhanced if the final state falls into the singularity in the density of one-dimensional states.

Chemical composition, core-level line shape, and electron affinity measurements are performed on the Se-terminated (2×1) and Zn-terminated c(2×2) ZnSe(100) surfaces. The (2×1) reconstruction corresponds to a complete monolayer of Se dimers with one filled dangling bond per surface atom. The c(2×2) reconstruction corresponds to a half-monolayer vacancy structure which leaves twofold-coordinated Zn atoms in the top layer. The 150-meV decrease in surface electron affinity at the (2×1)→c(2×2) transition is consistent with this model and results from a dipole induced by the charge transfer required to empty and fill all cation and anion dangling bonds on the c(2×2) surface, respectively.

We present a combined experimental and theoretical investigation of shallow two-dimensional acceptor states in the presence of an external magnetic field for center-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells (QW’s). The calculated 1${\mathit{S}}_{3/2}$(${\mathrm{\Gamma }}_6$)-2${\mathit{S}}_{3/2}$(${\mathrm{\Gamma }}_6$) transition energies of the acceptor state are in excellent agreement with our resonant Raman-scattering data. The g factors obtained from our calculations for varying well width of QW’s are also in good agreement with available experimental data. These results confirm the value of the Luttinger parameter κ=1.2±0.05 for bulk GaAs.

Shubnikov–de Haas oscillations have been studied, at low temperatures, in a high-mobility quantum-Hall-effect sample. A linear increase with increasing magnetic field of the peak values of resistance is observed and is explained quantitatively by a quantum diffusion model with predominantly small-angle scattering. The results are inconsistent with recent suggestions that the peak values of conductivity, between integer-quantum-Hall-effect plateaus, are ${\mathit{e}}^2$/2h.

We show the existence of a stress-induced coupling between upper-conduction-band states and valence-band states in bulk zinc-blende semiconductors and in their strained-layer low-dimensional systems. This coupling is calculated by an empirical tight-binding method which exactly includes the full crystal symmetry of the strained semiconductor compounds. As an application, we report a study of CdTe-ZnTe superlattices having thin ZnTe layers which exhibit an anomalously high energy for the electron to light-hole transition. We show that the large shift of this transition is well accounted for via mixing between valence and upper-conduction bands.

Electron-spin-resonance measurements were performed on the newly developed conducting Langmuir-Blodgett films consisting of a charge-transfer complex of bis(ethylenedioxy)tetrathiafulvalene (BO) and decyltetracyanoquinodimethane. The temperature-dependent spin susceptibility and linewidth clearly indicate that the major region of the films has a metallic nature. The density of states has been estimated as 3.5 states/eV per BO molecule, assuming no electron correlation. No metal-insulator transition has been detected at least down to 50 K, while an anomaly seen in the linewidth around 140 K suggests a certain kind of structural transition.

We present experimental results on magnetization reversal for a class of nanoscale magnetic films with perpendicular magnetic anisotropy and develop a model that describes a variety of related experiments. In this model the sample is divided into identical single domain cells that interact through dipolar fields and a nearest-neighbor domain-wall interaction. Monte Carlo simulations give insights into the relationship between the macroscopic magnetic parameters and the reversal behavior and demonstrate the important role that thermal activation plays in the reversal process.

Guided propagation of neutrons in thin films was directly observed in a ${\mathrm{TiO}}_2$/Ti/Si waveguide by exciting one of its discrete guided modes from an external neutron beam. The condition found experimentally for exciting this particular mode agrees well with theory. The measured intensity of the guided neutrons was also in good agreement with a calculation based on a previous coupling theory.

Optical measurements were performed on thin films of ${\mathrm{Rb}}_{\mathit{x}}$${\mathrm{C}}_{60}$, identified by x-ray diffraction as mostly x=1 material. The samples were subjected to various heat treatments, including quenching and slow cooling from 400 K. The dramatic increase in the transmission of the quenched samples, and the relaxation towards the transmission observed in slow-cooled samples, provides direct evidence for the existence of a metastable insulating phase. Slow cooling results in a phase transition between two electrically conducting phases.

In this paper, the insensitivity of the projected density of states (PDOS) is investigated. Rounding errors are treated as perturbations in the recursion process when calculating the PDOS. A generalized error theory for the PDOS is developed, which includes Paige’s theorem and the effects of truncation of the continued fraction. An analytic expression for the PDOS which isolates the error term is derived. This error term is shown to be exponentially insensitive to perturbations which are distant from the starting state; in contrast to eigenvalues and/or eigenfunctions which are very sensitive to small perturbations. This is what makes calculating PDOS and its integrated quantities a more stable approach compared to computing eigenvalues. This insensitivity is equivalent to a black-body theorem for the PDOS. This result is useful in practical computations because it enables an infinite system to be approximated by a particular finite one, and gives a bound on the error in the computed PDOS.

Heavily doped polypyrrole-hexafluorophosphate, PPy(${\mathrm{PF}}_6$), undergoes a metal-insulator (M-I) transition at resistivity ratio ${\mathrm{\rho }}_{\mathit{r}}$=ρ(1.4 K)/${\mathrm{\rho }}_{\mathit{r}}$(300 K)≊10: for ${\mathrm{\rho }}_{\mathit{r}}$<10, the system is metallic with ρ(T) remaining finite as T→0, whereas for ${\mathrm{\rho }}_{\mathit{r}}$>10, the system is an insulator with ρ→∞ as T→0. In the critical regime, ρ(T) shows a power-law temperature dependence, ρ(T)=${\mathit{T}}^{\mathrm{-}\mathrm{\beta }}$, with 0.3<β<1. The effect of the partially screened Coulomb interaction is substantial at low temperatures for samples on both sides of the M-I transition. In the insulating regime, the crossover from Mott variable-range hopping (VRH) to Efros-Shklovskii hopping is observed. In the metallic regime, the sign of the temperature coefficient of the resistivity changes at ${\mathrm{\rho }}_{\mathit{r}}$≊2. At T=1.4 K, the interaction length ${\mathit{L}}_{\mathit{T}}$=(ħD/${\mathit{k}}_{\mathit{B}}$T${)}^{1/2}$≊30 Å.

Since this is smaller than the inelastic-scattering length, ${\mathit{L}}_{\mathrm{in}\mathrm{\approxeq }}$300 Å, the contribution to ρ(T) from the electron-electron interaction is dominant. Application of high pressure decreases ${\mathrm{\rho }}_{\mathit{r}}$, induces the transition into the metallic regime, and enables fine tuning of the M-I transition. For samples close to the M-I transition, the thermoelectric power is proportional to the temperature in both the metallic and insulating regimes. The correlation length (${\mathit{L}}_{\mathit{c}}$) increases as the disorder, characterized by ${\mathrm{\rho }}_{\mathit{r}}$, approaches the M-I transition from either side. The expected divergence in ${\mathit{L}}_{\mathit{c}}$ at the M-I transition is qualitatively consistent with the values for ${\mathit{L}}_{\mathit{c}}$ inferred from the extrapolated σ(0) in the metallic regime and from analysis of the VRH magnetoresistance in the insulating regime. Thus, by using ${\mathrm{\rho }}_{\mathit{r}}$ to characterize the magnitude of the disorder, a complete and fully consistent picture of the M-I transition in PPy(${\mathrm{PF}}_6$) is developed.

The electron-electron correlation has been considered by using the local-spin-density approximation (LSDA) with on-site Coulomb interaction (LSDA+U) in the first-principles discrete variational cluster method. Based on the correction to LSDA, we have carried out the self-consistent electronic-structure calculations for the strongly correlated electronic systems of NiO, CoO, and FeO. In contrast to LSDA, the LSDA+U calculations reveal the experimentally observed antiferromagnetic and insulating ground state. The values of the energy gap and the magnetic moment for these compounds are in good agreement with experimental results. The calculations suggest that the energy gap changes from the Mott-Hubbard characters for the early-3d-transition-metal monoxide to charge-transfer character for the late-3d-transition-metal monoxide.

A method is described for the calculation of generalized symmetry-adapted Wannier functions by Fourier transformation of Bloch functions obtained by standard first-principles techniques. An additional unitary transformation derived from a Slater-Koster-model Hamiltonian is introduced in order to deal with the nonanalytical behavior of the Bloch functions. The parameters of the Hamiltonian, which are the energy matrix elements of the Wannier functions, reproduce the first-principles band structure with high accuracy. It is considered in detail how problems caused by degeneracies have to be treated by an analysis of transformation properties at symmetry points. An application to fcc transition metals yields highly localized functions of d symmetry, an s function whose localization depends on the strength of relativistic effects, and less localized p functions.

We bosonize a Fermi liquid in any number of dimensions in the limit of long wavelengths. From the bosons we construct a set of coherent states which are related to the displacement of the Fermi surface due to particle-hole excitations. We show that an interacting Hamiltonian in terms of the original fermions is quadratic in the bosons. We obtain a path-integral representation for the generating function, which, in real time, in the semiclassical limit, gives the Landau equation for sound waves and in the imaginary time gives us the correct form of the specific heat for a Fermi liquid even with the corrections due to the interactions between the fermions. We also discuss the similarities between our results and the physics of quantum crystals.

Accurate measurements of the polarized reflectance spectra of the series of isostructural organic conductors (TMTSF${)}_2$${\mathrm{ClO}}_4$, (TMTSF${)}_2$${\mathrm{PF}}_6$, (TMTTF${)}_2$Br, and (TMTSF${)}_2$${\mathrm{PF}}_6$ (where TMTSF denotes tetra-methy tetraselenafulvalene and TMTIF denotes tetra methyletrathiafulvalene) at room temperature are reported. A dimerized molecular chain model is used to analyze these spectra as well as those previously reported [C. C. Homes and J. E. Eldridge, Phys. Rev. B 42, 9522 (1990); J. E. Eldridge and C. C. Homes, ibid. 43, 13 971 (1991)] for (TMTSF${)}_2$${\mathrm{BF}}_4$ and (TMTSF${)}_2$${\mathrm{ReO}}_4$ and the corresponding conductivity spectra obtained by Kramers-Kronig transformation. The complex structures observed in the spectra are successfully accounted for by assuming that the double occupancy of the band states is effectively excluded so that the Fermi level lies inside a narrow gap induced in these materials by a small dimerization of the molecular stacks. The spectral changes observed among different members of the series are reproduced by varying a limited number of model parameters in a way that follows closely the known changes of the crystal structural properties. Fitting of the experimental data enables us to estimate the parameters of the band structure, namely the transfer integral and the dimerization gap amplitude. Use of self-consistent relations inherent in the adopted model allows us to conclude that a static potential rather than a Peierls-type phonon induced mechanism plays the dominant role in driving the formation of charge-density waves and the opening of the gap. The same model analysis accounts for the presence of vibronic structures induced by the coupling of the conduction electrons with intramolecular vibrational modes of TMTSF or TMTTF. It has been thereby possible to evaluate the coupling constants for the individual vibrational modes with a greater degree of reliability than in previous attempts. The general physical picture of the studied materials at room temperature as narrow gap correlated semiconductors is briefly discussed.

Fermionization of the spin operators is allowed only in the restricted Hilbert space of singly occupied sites. A method to implement this single-site occupancy constraint is applied to the frustrated Heisenberg model. This method allows us to study either magnetic or nonmagnetic phases at finite temperatures. A quantitative estimate of the relative weight of quantum fluctuations around the disordered dimer phases and the antiferromagnetic phase is given.

Photonic energy bands in body-centered-cubic bcc materials are analyzed by considering structures having ${\mathit{O}}^8$ (I$4_1$32) space-group symmetry. Such structures can be realized physically by interlacing cylindrical elements oriented along 〈111〉 crystallographic axes. In addition to heterogeneous systems composed entirely of dielectric materials, the possibility of using conducting materials (particularly at microwave frequencies) is studied. We find that (a) band gaps occur in heterogeneous dielectric systems when materials having a dielectric constant of 100 or more are properly placed in the ${\mathit{O}}^8$ unit cell, and (b) utilizing conducting materials can significantly widen the excluded frequency band, the result being that band gaps of more than 20% should be attainable with ${\mathit{O}}^8$ structures at microwave frequencies. Experimental verification of these results should be possible in this spectral region.