We present a new laser-cooling scheme based on velocity-selective optical pumping of atoms into a nonabsorbing coherent superposition of states. This method has allowed us to achieve transverse cooling of metastable ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms to a temperature of 2 μK, lower than both the usual Doppler cooling limit (23 μK) and the one-photon recoil energy (4 μK). The corresponding de Broglie wavelength (1.4 μm) is larger than the atomic-transition optical wavelength.

We show how the squeezed vacuum fields generated in collective fluorescence within a high-finesse cavity can be transformed into squeezed coherent states with high brightness but with the same degree of noise suppression. The transformation is achieved by an injected signal which creates a mean polarization and by squeezing in the high, rather than the low branch of the optical bistable collective radiation.

The correlation functions of the transmission coefficients for scalar wave propagation through disordered media are calculated by use of both diagrammatic techniques and numerical simulations. The calculation is valid in the diffusive regime: multiple elastic scattering with negligible absorption or inelastic scattering and a scattering length much longer than the wavelength. In addition to the familiar large local intensity fluctuations we find a novel memory effect and long-range correlations in the transmission coefficients which decay to a positive background value. Implications for light-scattering experiments are discussed.

A new state composed of quasiperiodic traveling waves has been observed in very close vicinity of the convection onset along a stable branch. This state precedes the recently discovered confined traveling-wave state, and consists of left- and right-going traveling waves which periodically alternate between either side of the cell. A time-dependent wave-number spectrum is a characteristic feature of this pattern. Effects of pattern translation and modulational instability can qualitatively explain the observed dynamical behavior.

Experimental results are presented for traveling-wave states near the onset of convection in a fluid mixture in a rectangular container. When the Soret-effect-induced stabilization is sufficiently weak, the nonlinear oscillatory states at onset evolve in time from the linear states of the system and remain in a pattern of parallel rolls with a frequency which is always near the Hopf frequency (i.e., $\frac{\Delta f}{f_0}\lesssim 0.15$). However, these states exhibit erratic spatial and temporal behavior which is apparently due to the non-linear coupling of the counterpropagating traveling waves.

We report new high-resolution measurements of the thermal conductivity very near the superfluid-transition temperature $T_{\lambda }$ on a thicker sample (0.57 cm) than had been used previously (0.22 cm) by Lipa and Chui (LC). Our results deviate less from the conductivity expected for a bulk sample than do the LC data. Our data, the results of LC, and previous measurements on a 0.0025-cm-thick sample are consistent with a simple phenomenological model for the effect of the solid boundaries upon $\lambda$.

A quantum model is proposed to describe the motion of ions in the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ superfluid. The interaction between the ions and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms is expressed in terms of the structure factor and quasiparticle operators of the quantum liquid. This model Hamiltonian is solved by a variational scheme. On the basis of the nonparabolic energy-momentum spectrum of the dressed ions, an explanation is given for the linear temperature dependence of the ion effective mass observed in the experiments. The study reveals some interesting aspects of the interaction between classical particles and quantum fields.

It is suggested that a fast kinetic mode should be observable in a whole class of disparate-mass fluid mixtures, such as gaseous He-Xe, by neutron scattering experiments, at not too high densities and Xe concentrations.

Solid normal hydrogen was compressed in a diamond-anvil cell to pressures above 200 GPa. Spontaneous Raman spectra demonstrate that the solid undergoes a structural phase transformation beginning at 145 GPa at 77 K, as evidenced by an abrupt discontinuity in the intramolecular vibron frequency as a function of pressure. The magnitude of the vibron-frequency shift and the pressure-temperature conditions of the phase transformation are consistent with its identification as the theoretically predicted pressure-induced orientational ordering of the molecular solid.

We introduce a new theoretical approach that clarifies the origin of fractal structures in irreversible growth models based on the Laplace equation and that provides a systematic method for the calculation of the fractal dimension. A specific application to the dielectric breakdown model (including therefore diffusion-limited aggregation) in two dimensions is presented. For fractal growth this new method appears to be more appropriate than the renormalization-group method.

The quenching of an electronically excited adsorbed molecule by gas-phase quenchers within the pore network of several porous silicas has been studied. It is proposed that the early-time quenching rate should scale with the mean pore size, $R_p$, for gas pressures in the Knudsen regime. We have experimentally observed such scaling in what we believe is the first study of these quenching reactions in a homologous series of morphologically similar porous silicas.

A new method to provide interatomic pair potentials from ab initio Hartree-Fock self-consistent-field calculations is proposed; potentials are calculated for model clusters of silica (Si${\mathrm{O}}_2$). They are tested in the molecular-dynamics simulation of crystalline states, in which four known polymorphs of silica are shown to be dynamically stable.

We propose that $\mathrm{DX}$ is a negatively charged defect center resulting from the "reaction" $2d^0\to d^{+}+D{X}^{-}$ where $d$ represents a substitutional donor. The results of our pseudopotential calculations for Si- and S-induced $\mathrm{DX}$ centers in GaAs indicate large dopant-dependent relaxations leading to threefold-coordinated interstitial sites for either the donor or one of its nearest neighbors. A simple expression for the alloy composition and pressure dependence of the $\mathrm{DX}$ binding energy is suggested and used in an analysis of experimental data.

$k$-resolved alloy bowing data for ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ alloys have been taken and used to examine the validity of different alloy formalisms. Although the molecular coherent-potential approximation is considered to be the most complete alloy formalism for zinc-blende pseudobinary alloys, it is not in agreement with the experimental $k$-resolved data. In contrast, these $k$-resolved data are in approximate agreement with the coherent-potential-approximation prediction. This implies that the implementation of the bond-length disorder effect in the molecular coherent-potential approximation needs to be modified.

We report the magnetotransport measurements of a low-density, very low-disorder two-dimensional electron system realized in a GaAs/AlGaAs heterojunction. The diagonal magnetoresistance displays a structure near the filling factor $\nu =\frac{1}{7}$ which is interpreted as evidence for a developing fractional quantum Hall state. We therefore conclude, contrary to recent theoretical and experimental work, that the Wigner crystallization does not occur in this system for $\nu \ge \frac{1}{7}$.

Optical conductivity computed from first principles suggests that the feature near 0.37 eV is due to interband transitions from the bridging O that connects the Cu-O planes to the Cu-O chain. Screened Coulomb interaction between an electron and a hole broadens the interband peaks but does not lead to any new features that could be interpreted as excitonic. Variations in O content cause the low-energy optical feature to disappear as the bridging O states drop well below $E_{\mathrm{F}}$. We suggest that differences between single-crystal and powder data are likely due to compositional differences.

The resistance changes generated by individual microscopic displacements due to dissolved hydrogen ions hopping between neighboring sites within palladium films create intense $\frac{1}{f}$ noise at low temperature. Crossover to one-dimensional diffusion-mediated number-fluctuation noise occurs for $T\gtrsim 150$ K. The measured resistance change per proton displacement is comparable to the resistance per proton as predicted by applicable quantum-interference theories.

An implicit particle-in-cell code has been used to study collisionless magnetic reconnection of a current sheet. The reconnection process is found to occur in three distinct stages: (1) early spontaneous reconnection fed by the free energy of an initial anisotropy in the electron component; (2) coalescence of the resulting small-scale filaments of electron current, accompanied by electron jetting; and (3) oscillatory flow of electrons through the magnetic $X$ point, superposed on continuing nonlinear growth of ionmediated reconneciton. The time evolution of stage 3 is strongly dependent on $\frac{M_i}{m_e}$.

A study of time reversibility reveals that a quantum chaotic system manifests the same potential for mixing as its classical counterpart under the influence of externally applied noise. The quantum-chaotic system, however, reveals its own nature below a quite small "classicalization" threshold of noise intensity: In the case of the kicked rotor there exists a quite wide regime for which the mixing property is understood neither from classical dynamics nor from quantum perturbation theory. The relationship between such a curious behavior and the pure quantum evolution without the noise is elucidated.

Quantum-classical correspondence in many-dimensional quantum chaos is investigated by use of a coupled quantum kicked-rotors model. Even when the number of rotors is only two, results obtained are drastically different from those for a single-rotor system; that is, in the semiclassical limit the coupled system restores the essential features of classical chaos under appropriate conditions. In particular, the time-reversal experiment reveals that the classical chaotic mixing is recovered almost entirely; however, the recovered mixing is "conditional" in the sense that there exists a threshold for the recovery.