P. B. Littlewood

Exciton condensate in semiconductor quantum well structures.

We propose that the exciton condensate may form in a well-controlled way in appropriately arranged semiconductor quantum well structures. The mean-field theory of Keldysh and Kopaev, exact in both the high density and the low density limits, is solved numerically to illustrate our proposal. The electron-hole pairing gap and the excitation spectrum of the exciton condensate are obtained. The energy scales of the condensate are substantial at higher densities. We discuss how such densities could be achieved experimentally by generating an effective pressure.

Long-wavelength behavior, impurity scattering and magnetic excitations in a marginal Fermi liquid

The marginal Fermi liquid hypothesis about the excitation spectrum of the high-temperature superconductors is supplemented to specify the long-wavelength behavior; it is shown that there are no anomalous renormalizations of the compressibility and the uniform paramagnetic susceptibility. Consideration of elastic scattering from impurities leads to the conclusion that as temperature is decreased, the scattering approaches the unitarity limit. We discuss the possibility that at T → 0, a marginal Fermi liquid is either a superconductor or an insulator. Predictions for the magnetic structure factor observable in neutron scattering consistent with the nuclear relaxation rate on copper and on oxygen are also given.

Structure and bonding in narrow gap semiconductors

Abstract Many trends in the behavior of elements and their compounds are immediately visible by comparison with their position in the periodic table. The most obvious trend is that the elements to the left and in the lower part of the periodic table are metals while the rest are covalently-bonded semiconductors or insulators; the coordination number of the covalently-bonded materials is generally given by the 8-N rule (N is the column number). The next clear trend comes in the consideration of binary compounds; here we find that the horizontal separation of the two elements in the periodic table is a good indication of their ionicity. For example, this explains the trends in the forbidden energy gap and the crystal structure along an isoelectronic series (e.g. KBr, Case, ZnSe, GaAs, Ge).

Four-wave mixing and terahertz emission from three-level systems in quantum wells: Effects of inhomogeneous broadening

Using a perturbation theory based on the density-matrix formulation, we study the nonlinear optical responses of a noninteracting three-level model system to consecutive coherent pulsed excitations, as realized in several recent experiments involving excitonic transitions in quantum wells. The terahertz emission, which is a second-order response, and the four-wave mixing signal, a third-order response, are calculated within the rotating-wave approximation, in the presence of detuning, dephasing, and inhomogeneous broadening. We study the quantum beats in the photon echo from transient four-wave mixing experiments of a three-level system. We find that the temporal profile of the photon echo signal in the four-wave mixing experiments depends very strongly on the amount of inhomogeneous broadening of the energy levels involved. Both the position and the intensity of the peaks exhibit a smooth evolution from strong quantum beats to a conventional photon echo as the inhomogeneous broadening increases. These features from our noninteracting model system are compared with a recent experiment, and found to account for a number of experimental observations.

Sliding motion of a two-dimensional Wigner crystal in a strong magnetic field.

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 \ensuremath{\nu}=1/5. We summarize the available experimental data, and point out the differences among them.

Microwave conductivity from magnetic impurities in an s-wave superconductor

We calculate the optical conductivity in an s-wave superconductor interacting with a low density of magnetic impurities. Interactions are treated beyond the Born approximation so that an impurity band develops inside the superconducting gap. We find that the contribution of these states to the low frequency conductivity is large, and that they have a clear spectroscopic signature which would would make them observable in conventional superconductors.