We report a bimodal speed distribution for sodium atoms cooled in ‘‘optical molasses.’’ The temperatures of the two velocity groups differ in their dependence on laser detuning, sensitivity to beam misalignment and imbalance, polarization, and the presence of static magnetic fields. We believe that this behavior is evidence for two distinct cooling mechanisms.

In this Letter we obtain an exact solution for a relativistic Langmuir wave with nontrivial space and time dependence. We find explosive behavior in the electron density and hence also in the electric field gradient. Relativistic effects are thus found a possible candidate to explain this interesting type of behavior, recently found in numerical simulations (though it is too early to give a quantitative treatment).

We present measurements of the real squashing mode (RSQ) frequency at temperatures approaching T=0. The data are obtained through repeated frequency sweeps that yield the frequency of the RSQ attenuation maximum over a broad range oaf temperatures, at fixed pressure. We are thus able to record the value to which the RSQ frequency tends as T→0, a number that is largely independent of temperature scale. Essential to these measurements is the use of nonresonant ultrasonic transducers.

Variational Monte Carlo and several many-fermion Green’s-function Monte Carlo (GFMC) algorithms are used to study the ground state of liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. We report the first mirror potential GFMC calculations in a many-fermion problem, comparing them with transient estimation and fixed-node studies to illustrate the strengths and weaknesses of each. GFMC results with the Aziz HFDHE2 interaction are in good agreement with experiment, yielding energies within approximately 0.1 K per particle. In addition, each of these calculations predict a kinetic energy per particle of between 12 and 12.5 K.

We report on synchrotron x-ray scattering studies of multimembrane systems in the fluid ${\mathrm{L}}_{\mathrm{\alpha }}$ phase. By embedding cosurfactant molecules in a model biological membrane (di-myristoyl-phosphoridyl-choline) system, we dramatically reduce the bending rigidity of the fluid bilayer. This effect leads to a crossover from a microscopically driven to a fluctuation-induced universal regime for intermembrane interactions. Studies of a homologous cosurfactant series, with varying hydrocarbon chain length between 5 and 12, allow us to attribute this reduction in ${\mathrm{k}}_{\mathrm{c}}$ to the thinning of the membrane.

The detailed structural changes that occur during ion-beam–induced amorphization of a solid have been experimentally determined for the first time using single-crystal surfaces of lead pyrophosphate that were implanted with varying doses of ${\mathrm{Pb}}^{3+}$ ions. For all Pb-implant doses, the structure of the amorphous layer produced by ion bombardment was found to be significantly different than the structure of lead pyrophosphate glass prepared via thermal quenching.

We present the first molecular-dynamics study of the amorphization of a crystalline alloy (${\mathrm{NiZr}}_2$) induced by chemical disorder. We used a n-body potential in conjunction with isobaric-isothermal molecular dynamics. The behavior of the pair distribution function suggests that the instability leading to the amorphous state is a first-order phase transformation.

The dependence of pair interactions at a metal surface upon interatomic distance as well as orientation has for the first time been explored by mapping out the two-dimensional distribution function for two metal atoms at equilibrium on W(110). Observations with the field ion microscope have been made for the pairs Re-Pd, W-Pd, and Ir-Ir. For all these interactions are highly anisotropic, with repulsion along [11¯0] and [001], but long-range oscillatory attraction along [11¯1]. This anisotropy is consistent with elastic calculations; the observed decay of attractions is not.

The electronic structure and enthalpy are calculated as a function of pressure for several phases of molecular and metallic hydrogen. For the molecular solid we offer an alternative interpretation of the recently observed transition at 1.5 Mbar. For the intermediate-pressure range from 3.8±0.5 to 8.6±1 Mbar, the most stable structures have low coordination numbers and are anisotropic. Among them a metallic filamentary primitive hexagonal phase is particularly stable. For higher pressures, the bcc structure is the most stable.

We present a method to compute exactly the partition function of quantum many-body systems. This approach involves the evaluation of a set of coefficients for the continued-fraction expansion of a resolventlike operator. The main advantage of this method is that this set of coefficients is unique and allows one to calculate the partition function at any temperature. As an illustration, the method is applied to a simple one-dimensional spinless model.

High-resolution spin-polarized electron-energy-loss data on Ni(110) are presented. Exchange scattering is found to be the dominant loss mechanism. The Stoner excitation spectrum shows a sharp threshold around 65-meV energy loss (Stoner gap). The energy-loss probability for spin-down electrons is up to 3 times larger than for spin-up electrons. An interpretation in terms of the Ni 3d band structure is given.

We introduce a new method for the exact and unified treatment of the Schrödinger equation and the calculation of the electronic density of states of materials with reduced symmetry (e.g., surfaces and interfaces) many of which are outside the reach of methods based on Bloch’s theorem. This method is based on a self-consistent equation determining the t matrices of those parts of a system characterized by semi-infinite periodicity, without recourse to reciprocal space. The method is illustrated through a calculation of the density of states for bulk Cu and for a (100)/(111) planar Cu interface.

We present a unified quantitative explanation of the temperature dependence of the drift mobilities of photoinjected electrons in naphthalene for all directions and temperatures. The theory proposed is based on a polaronic description of the charge carriers and the calculational scheme employs site-local states. Excellent agreement of the theory and experiment occurs for known and reasonable parameters.

We investigate the pressure-induced metal-insulator transition of silicon in the diamond structure. Quasiparticle theory (QPT) calculations are performed within the GW approximation, and Kohn-Sham theory (KST) results are obtained by using an exchange-correlation potential derived from the GW self-energy operator, not using the common local-density approximation (LDA). In both KST and the LDA, metallization occurs at a much larger volume than in QPT. These results suggest that the metallization point and Fermi surface of the Kohn-Sham electrons are not necessarily those of the real system.

A novel type of magnetoresistance oscillations is observed in high-mobility AlGaAs/GaAs heterojunctions with a holographically induced lateral periodic modulation in one direction. Theoretically, the effect is shown to result from an oscillatory dependence of the bandwidth of the modulation-broadened Landau levels of the two-dimensional electron system on the band index, which leads in high-mobility samples to a strongly anisotropic oscillating contribution to the conductivity tensor. The oscillations, periodic in ${\mathrm{B}}^{\mathrm{-}1}$, reflect the commensurability of cyclotron diameter at the Fermi energy and modulation period.

We investigate the magnetoresistance of a quasi-two-dimensional electron system subject to a one-dimensional superlattice potential created by field effect in gated AlGaAs/GaAs heterojunctions. At low temperatures this potential gives rise to a new type of magneto-resistance oscillations with a period governed by the ratio of the classical cyclotron diameter 2${\mathrm{R}}_{\mathrm{c}}$ to the superlattice period a. The oscillations are quantitatively explained in period, phase, and magnitude by the formation of Landau bands in the two-dimensional electron system under the influence of the periodic potential.

The Hall conductance of a wide two-dimensional electron gas has been measured in a geometry in which two quantum point contacts form controllable current and voltage probes, separated by less than the transport mean free path. Adjustable barriers in the point contacts allow selective population and detection of Landau levels. The quantization of the Hall conductance is determined by the number of quantum channels in the point contacts and is independent of the number of occupied bulk Landau levels. A theoretical description based on the Landauer-Büttiker formalism is given.

The first observation of the optical Stark effect of electron-hole continuum states in addition to the bound-exciton states is reported under femtosecond excitation conditions chosen to minimize the generation of real carriers. The experimental results agree well with calculations using the generalized semiconductor Bloch equations. The theory shows that the commonly used adiabatic approximation is correct only for pulses which are longer than the coherence decay time. For shorter pulses coherent dynamic effects strongly influence the spectral changes.

We show that in quasi-2D GaAs quantum wells, below-resonance low-intensity excitation produces a pure shift of the excitons, contrary to ac Stark shifts in atomic systems. At high pump intensities two-photon 3D-real-carrier generation competes with the virtual 2D-exciton effects. We also show that dc fields applied perpendicular to the layers reduce the excitonic ac Stark effect.

Spin dynamics at planar sites in oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{x}}$ have been investigated by nuclear spin-lattice relaxation measurements using ${}_{}{}^{63}{}_{}{}^{}$,65Cu nuclear quadrupole resonance and ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR. A sharp decrease of ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear relaxation below 100 K in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.7}$ suggests the possible onset of spin pairing in individual planes well above the transition to three-dimensional bulk superconductivity. Comparison of the relaxation rates for the 60-K superconductor ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.7}$ with those of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7.0}$ in the range 100–300 K shows that the planar Cu(2) spin dynamics are strongly affected by changes in carrier concentration.