We performed low-temperature (77 K) photomodulated transmission in two different ${\mathrm{In}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs superlattices with fixed built-in fields. Sample parameters are such that in one of them electrons are in the Stark-Wannier field regime while in the other they are in the Franz-Keldysh regime. In both samples, however, light holes are seen to be in the Franz-Keldysh regime. From this we infer that the light-hole valence bands are essentially lined up on both sides of the ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs heterojunction.

We report diagonal- and Hall-resistance measurements of the localized two-dimensional electron system in silicon in the high-field extreme quantum limit. The Hall resistance has been found to be almost independent of temperature and close to its classical value, B/${\mathit{n}}_{\mathit{s}}$ec, while the diagonal resistivity diverges at T→0. There is no evidence for nonlocal resistance due to special conductivity paths. These results support existence of the recently predicted Hall insulator state in silicon in a high magnetic field.

The photon-absorption edge in a weakly interacting one-dimensional electron gas is studied, treating backscattering of conduction electrons from the core hole exactly. Close to threshold, there is a power-law singularity in the absorption, I(ε)∝${\mathrm{\epsilon }}^{\mathrm{-}\mathrm{\alpha }}$, with α=3/8+${\mathrm{\delta }}_{+}$/π-${\mathrm{\delta }}_{+}^2$/2${\mathrm{\pi }}^2$, where ${\mathrm{\delta }}_{+}$ is the forward-scattering phase shift of the core hole. In contrast to previous theories, α is finite (and universal) in the limit of weak core-hole potential. In the case of weak backscattering U(2${\mathit{k}}_{\mathit{F}}$), the exponent in the power-law dependence of absorption on energy crosses over to a value α=${\mathrm{\delta }}_{+}$/π-${\mathrm{\delta }}_{+}^2$/2${\mathrm{\pi }}^2$ above an energy scale ${\mathrm{\epsilon }}^{\mathrm{*}}$∼[U(2${\mathit{k}}_{\mathit{F}}$)${]}^{1/\gamma }$, where γ is a dimensionless measure of the electron-electron interactions.

We analyze the contributions of electron heating to the generation of time-independent conductance fluctuations as a function of voltage in mesoscopic wires. We discuss both harmonic generation and direct measurements of differential conductance versus voltage. Electron heating may easily dominate the properties of the fluctuations. We propose that observations of conductance fluctuations with a small correlation voltage may be a consequence of heating, rather than being related to a small intrinsic energy scale such as the energy-eigenvalue spacing.

We have investigated the low-temperature I(V) characteristics of a gated submicrometer resonant-tunneling diode in the presence of a magnetic field applied perpendicular to the tunnel barriers. As the negative gate voltage is increased, additional maxima and minima are generated in forward bias with a typical voltage separation of 50 mV. This extra structure is only weakly dependent on magnetic field. In contrast the reverse bias I(V) closely resembles that of a large-area diode in all respects, including the appearance of additional peaks associated with optic-phonon-assisted tunneling between Landau levels. The structure in forward bias is similar to that observed by other groups in the I(V) of similar devices which was attributed to lateral quantum confinement. However, we show that simple models based on lateral confinement cannot account for our data.

We have investigated reflection of edge states from the boundary between a disordered region and a high-mobility region of a two-dimensional electron gas. α-particle bombardment has been employed to reduce selectively the mobility. We observe an exponential increase in reflection of the innermost edge state from the disordered region as the temperature decreases. The results are explained in terms of the Milne reflection of electrons which are elastically scattered back before thermalization can occur.

We have resolved the bound states of a one-dimensional quantum wire by resonant electron tunneling. The atomically precise quantum-confined structure was prepared by cleaved-edge overgrowth in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As molecular-beam epitaxy. Tunneling proceeded, in a planar geometry, from the edge of a two-dimensional electron source through the bound states of the quantum wire into the edge of a two-dimensional electron drain. The experimental resonance positions, as well as their density dependences, are well reproduced by our self-consistent theoretical calculations.

In Brillouin light-scattering (BLS) measurements of an ion-milled Si(001) surface grating with grating wavelength ${\mathrm{\Lambda }}_{\mathit{G}}$=0.35 μm, we have observed numerous high-order zone-folded surface acoustic modes between the second and third zone boundaries associated with the grating. A surprisingly intense signal from a zone-folded longitudinal resonance was observed in the absence of direct hybridization with the Rayleigh modes. The relative intensities of all of the modes have been calculated by allowing for coupling between modes related by up to two grating reciprocal-lattice vectors, and for a grating profile with nonzero first and second Fourier amplitudes. The Fourier amplitudes inferred from the BLS measurements and calculations agree very well with those obtained from a direct measurement of the grating profile using atomic-force microscopy.

The phonon widths of outermost core-electron photoemission spectra from (110)-oriented overlayers of Na, K, and Rb metals show the expected Debye behavior for the bulk atoms, but significant deviations for the surface atoms. The data indicate a softening of the surface vibrational mode above 200 K. This effect, which is weak in Na but strong in K and Rb, demonstrates that the vibrational mode normal to the surface is anharmonic.

Highly ordered thin films of ${\mathrm{C}}_{70}$ have been grown on GaAs(110). Temperature-dependent photoemission studies show changes in the valence-band electronic structure that are related to solid-state phase transitions at ∼280 and ∼340 K. The average on-site correlation energy for ${\mathrm{C}}_{70}$ is determined to be 1.0±0.2 eV, based on comparison of the C KVV Auger spectrum with a self-convolution of the valence band obtained from photoemission.

The electronic dc transport along a spatially disordered chain of scatterers is described within the Landauer-Büttiker approach. The chain is composed of single scatterers which allow for elastic as well as for inelastic processes. The scattering matrix of the disorderd chain is calculated with a recursive method. In the absence of dissipation the transmission coefficient and the dc conductance decrease exponentially with the length of the chain. We show that these disorder effects are gradually suppressed with increasing dissipation. For large dissipation the conductivity of the chain becomes even independent of the disorder. The results allow us to relate the onset of the dc conduction at finite temperature to the loss of phase memory during inelastic collisions of the electrons. Further we analyze the decrease of the chemical potential along the chain in presence of disorder. For small dissipation we find a strikingly nonuniform behavior, which is characterized by large steps and nearly constant plateaus in between.

Nonlinear transport behavior, which shows a sudden breakdown transition to zero- or negative-differential resistance in the high field, is observed in the quenched thallium- or tungsten-doped blue bronzes ${\mathrm{K}}_{0.15}$${\mathrm{Tl}}_{0.15}$${\mathrm{MoO}}_3$ and ${\mathrm{Tl}}_{0.3}$${\mathrm{Mo}}_{1\mathrm{-}\mathit{x}}$${\mathrm{W}}_{\mathit{x}}$${\mathrm{O}}_3$ at 77 K. It is found that the critical breakdown transition threshold is unstable for the induced field and decreases with increasing cycling times of the conducting current and, finally, reaches a definite value. When the current increases from 30 to 100 mA, the voltage between the two ends of the sample does not show a clear increase, i.e., a similar phenomenon with zero-differential resistance. On the downward cycle of the current, the voltage-current curve exhibits a large electronic hysteresis over the nonlinear region. On the other hand, in the critical region, the breakdown transition is usually accompanied by a voltage pulse with a large amplitude and exhibits a negative-differential resistance. These effects of an electric field on a pinned charge-density wave are phenomenologically interpreted in analogy with the mechanical deformations of the crystal solid.

We study an elastic composite described by the position-dependent mass density ρ(r), the longitudinal speed of sound ${\mathit{c}}_{\mathit{l}}$(r), and the transverse speed of sound ${\mathit{c}}_{\mathit{t}}$(r). For a spatially periodic composite—a ‘‘phononic crystal’’—we derive the eigenvalue equation for the frequencies ${\mathrm{\omega }}_{\mathit{n}}$(K), where n is the serial number of the band and K is the Bloch wave vector. This is applied to the special case of a binary composite and, further, to the case of infinite cylinders that form a two-dimensional lattice. For this configuration (and no wave-vector component parallel to the cylinders) there are two independent modes of vibration. The elastic displacement u(r) is parallel to the cylinders for one of them—the transverse polarization mode. The other one is a mixed (longitudinal-transverse) polarization mode with u(r) perpendicular to the cylinders. Specifically we consider circular cylinders that form a square lattice. We compute the band structures for the transverse modes of nickel alloy cylinders in an aluminum alloy host, and vice versa. In both situations we find band gaps which extend throughout the Brillouin zone. Within these gaps the transverse vibrations, sound, and phonons are forbidden. We also investigate the dependence of the band gap on the filling fraction and on the material parameters.

We find the electron eigenenergies in the field of a short-range scatterer and in a magnetic field. We extensively study the case when a short-range potential can be approximately replaced by the point potential.

We present a simple scheme to extend the earlier single-site coherent-potential-approximation (CPA) theories by utilizing connections of the CPA solution with the exact solution of the Falicov-Kimball model in infinite dimensions, and with the single-impurity Anderson-type models. We study the local spectral density of the model at n=1; in the metallic regime, this exhibits a narrow Abrikosov-Suhl resonance and satellite peaks, which correspond, respectively, to the quasiparticle and Hubbard subband structures. This collective resonance disappears in the split-band (insulating) regime, where the CPA is found to be a good approximation. Comparisons are made with numerical works on finite-sized two-dimensional lattices, and good agreement is obtained.