The Hall resistance of a two-dimensional electron gas is measured with gated probes to determine the extent of equilibration among the N current-carrying states in the quantum Hall regime. After traveling macroscopic distances (∼80 μm), current injected into the first state is equilibrated among the N-1 lowest states but equilibration into the highest state varies strongly across the Hall plateau. This is attributed to a change in the Nth state from being localized within a magnetic length of the edge to substantially extending into the sample.

Zero-bias tunneling of electrons between a quantum well and an $n^{+}$ substrate is studied with excitation voltages smaller than $k_B$T. At low temperatures and only with magnetic field applied perpendicular to the plane of the electron gas in the well, the tunneling rate develops a novel temperature-dependent suppression. The suppression strength is roughly independent of Landau-level filling for densities 0.5×${10}^{11}$ to 6×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$. The data are interpreted in terms of a magnetic-field-induced energy gap, at the Fermi level, in the single-particle spectrum of electrons in the well.

Transport in the fractional-quantum-Hall-effect (FQHE) regime is studied in a two-dimensional electron gas (2DEG) employing adjustable barriers as current and voltage probes. We find a fractionally quantized Hall conductance for integer filling factor in the bulk of 2DEG, as a consequence of the fractional filling factor in the probes. We argue that this effect is the first manifestation of adiabatic transport in the FQHE. The results are in agreement with a proposed Landauer-Büttiker formula in which each fractional edge channel contributes a conductance (1/3$e^2$/h.

A remarkably stable microscopic tunnel junction can be made with two crossed wires separated by a monolayer of adsorbed molecules. Inelastic tunneling peaks of hydrocarbons are clearly resolved. A wide Coulomb blockade of single-electron tunneling is observed, indicating that stray capacitance does not influence the microscopic junction.

We have applied radio-frequency signals to 1000 by 1000 arrays of Josephson junctions and observed giant Shapiro steps at voltages 1000 times that expected for a single junction. When a perpendicular magnetic field is applied, fractional giant steps appear at voltages directly related to the number of flux quanta per unit cell. A phenomenological model is proposed that utilizes the spatial arrangement of the vortices in a superlattice relative to the underlying array to explain these novel steps.

A simple derivation of the ideals in the Temperley Lieb algebras is presented together with a convenient basis. Using this basis it is shown how to derive the eigenvalue equations for operators defined on the algebra and a relation to the Bethe-Ansatz equations is noticed. The irreducible content of the spin-(1/2 XXZ model is also discussed.

The scattering of relativistic spin-one-half particles in an Aharonov-Bohm potential is considered. It is shown that earlier approaches to this problem have neglected a crucial delta function contribution to the potential. By formulating the problem with a source of finite radius which is then allowed to go to zero, it is established that this is the delta function alone that causes solutions that are singular at the origin to become relevant. The changes in the amplitude which arise from the inclusion of spin are seen to modify the cross section for the case of polarized beams. Finally, the calculated Aharonov-Bohm amplitude is shown to describe the scattering of particles with arbitrary spin in the c=∞ limit.

We report observations of an interference between optical transition amplitudes for linear and nonlinear excitation of the mercury 6s ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$→6p ${}_{}{}^1{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ transition. The transition probability is varied sinusoidally by changing the relative phase of the two fields inducing these distinct processes.

We report the first observation of the ionization of Rydberg atoms by a circularly polarized 8.5-GHz microwave field. The observed threshold field for ionization is E=1/16${\mathrm{n}}^4$, a value higher than that required for ionization by a linearly polarized microwave field. The origin of this threshold can be understood by transforming to a frame rotating at the microwave frequency. In the rotating frame no transitions occur, and level mixings alone are responsible for the ionization.

We present nonperturbative, time-independent calculations of the photodissociation rate of ${\mathrm{H}}_2^{+}$ in intense laser fields. The energy distribution of the protons consists in a sequence of peaks evenly spaced by half the photon energy, all of equal width but of varying heights. They result from multiphoton absorption above the dissociation threshold, with equal sharing of the excess photon energy between H and ${\mathrm{H}}^{+}$. Surprisingly, the distribution of higher-energy peaks decreases with increasing intensity, due to stimulated free-free emission of the dissociating fragments. We also predict a sharp angular distribution of the protons along the electric field vector of a linearly polarized laser.

Nonlinear mixing of wave vectors present in the field driving a three-state atom may generate long-wavelength optical forces whose effect on the motion of the atom exceeds by orders of magnitude the influence of the forces familiar from two-state atomic models.

The absolute dissociative-electron-attachment cross section for ${\mathrm{O}}_2$ condensed on a Kr film is found to be about 20 times larger than that for gaseous ${\mathrm{O}}_2$. This large enhancement is attributed to a downward energy shift (≊0.9 eV) of the ${\mathrm{O}}_2^{\mathrm{-}}$ ${(}^2$${\mathrm{\Pi }}_{\mathrm{u}}$) intermediate state due to electronic polarization of the neighboring Kr atoms. Based on the theory developed by O’Malley [Phys. Rev. 150, 14 (1966)] and 155, 59 (1967)], we have shown that this downward shift increases the dissociative attachment by a factor of about 21, through a reduction in the autoionization of the intermediate state.

The impact-parameter–dependent probabilities for single- and double-vacancy production in the 3dσ molecular orbital have been determined for 1.5-MeV/nucleon ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ on ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ collisions from triple coincidences between Ni K and Pb L characteristic x rays and scattered particles. The observed double-ionization probability is enhanced relative to the predictions of the independent-particle model based on the measured single-ionization probabilities by factors varying between 2 and 3. The presence of correlated electron processes during close collisions, with probabilities as large as a few percent, could explain the observed enhancement.

Simultaneous profile measurements of the toroidal rotation speed and ion temperature during unbalanced neutral beam injection in the Tokamak Fusion Test Reactor show that the ion momentum and thermal diffusivities are comparable in magnitude (${\mathrm{\chi }}_{\mathrm{\phi }}$≊1.5${\mathrm{\chi }}_{\mathrm{i}}$) and vary similarly with plasma current and minor radius. The correlation of ${\mathrm{\chi }}_{\mathrm{\phi }}$ and ${\mathrm{\chi }}_{\mathrm{i}}$ is consistent with anomalous transport driven by collisionless electrostatic microinstabilities including ion-temperature-gradient-driven modes (${\mathrm{\eta }}_{\mathrm{i}}$ modes) and collisionless trapped-electron modes.

It is shown that Frank’s ‘‘saddle’’ distortion contribution to the nematic liquid-crystal distortional elastic energy is required, in addition to the splay, twist, and bend energies retained in more recent treatments. ‘‘Normal distortion modes’’ for these four distortions are described. Measurement methods and their interpretations are discussed for the corresponding four elastic constants ${\mathrm{k}}_{11}$ (splay), ${\mathrm{k}}_{22}$ (twist), ${\mathrm{k}}_{33}$ (bend), and ${\mathrm{k}}_{44}$ (saddle). Previously reported values for ${\mathrm{k}}_{11}$ and ${\mathrm{k}}_{22}$ are actually for ${\mathrm{k}}_{11}$+${\mathrm{k}}_{44}$ and ${\mathrm{k}}_{22}$+${\mathrm{k}}_{44}$ respectively. The Nehring-Saupe molecular approach yields ${\mathrm{k}}_{44}$=(1/2(${\mathrm{k}}_{11}$+${\mathrm{k}}_{22}$).

We have realized new systems which consist of a periodic packing of a bidimensional magnetic liquid alternate with nonmagnetic fluid. These ‘‘ferrosmectic’’ phases are obtained by incorporating solid magnetic particles in lyotropic lamellar systems. Their existence has been proved by their optical properties and their anisotropic behavior in applied magnetic field. This new system offers a large domain of investigation, both as a special bidimensional ferrofluid, and as an example—the first to our knowledge—of the compatibility between solid particles and structured liquids.

A generalization of the restricted solid-on-solid growth model of Kim and Kosterlitz is studied and found to exhibit a phase transition, as a function of a temperaturelike parameter κ, in 2+1 dimensions. The surface-width exponents cross over from the Kim-Kosterlitz values for small κ, to the low-temperature values α≊0.25, β≊0.14 for large κ. At the transition point ${\kappa }_c$, we find logarithmic growth and logarithmic dependence of the surface width on system size. The amplitude of the correlation function at ${\kappa }_c$ is found to be equal to that observed for roughening at the Kosterlitz-Thouless transition.

The electric dipole interaction of vibrational modes destroys their localization. Real-space renormalization is constructed for the process of delocalization. The renormalization-group equation for the distribution of dipole parameters is similar to the Boltzmann kinetic equation. Conservation laws are found and an H theorem is proven. Stationary distributions form a six-parameter manifold of fixed points. The two-point dynamical correlation function has the form $t^{\mathrm{-}1}$F($t^{\mathrm{-}1/3}$r), where F(x) is a universal function.

It is proposed that ‘‘magic number’’ Si clusters of intermediate size correspond to structures in which exactly all surface atoms participate in reconstructions similar to bulk-Si surfaces. Models are given for the experimentally observed stable clusters of 33 and 45 atoms, which display features similar to the 7×7 and 2×1 reconstructions of the Si(111) surface. The nature of bonding in the energy-optimized atomic geometries is revealed, through first-principles electronic structure calculations, to be covalent.

First-principles total-energy-minimization calculations show that the interface bonds are significantly strained in nominally lattice-matched ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/InP(001) heterostructures, in agreement with recent x-ray measurements. Anion intermixing at the interface reduces this strain. The calculated valence-band offset is sensitive to interface bond lengths so an energetically (meta)stable interface structure must be used. Then the calculated valence-band offset is independent of intermixing (hence measured strain) and in quantitative agreement with experiment.