Quantum fluctuations in optical amplification are investigated with a nondegenerate optical parametric amplifier whose internal idler mode is coupled to a squeezed vacuum. Reductions of the inherent quantum noise of the amplifier are observed with a minimum noise level 0.7 dB below the usual noise level of the amplifier with its internal idler mode in a vacuum state. With a small coherent field as the signal input, the amplified output exhibits an improvement in signal-to-noise ratio of 0.5 dB for the case of a squeezed-vacuum as compared to a vacuum state for the amplifier’s internal mode.

We report the observation of light amplification without population inversion. A linear superposition of an m=1 and an m=-1 magnetic substate is populated coherently. Absorption from this superposition to a higher state is canceled by interference of channels, a so-called dark resonance. An inverse transition is used for amplification of light. The temporal behavior of the amplified pulse and the gain are studied as a function of the energy splitting due to a magnetic field. In a vapor of cadmium-112 gain factors of up to 4.3 are observed.

We have measured the crystal-angle dependence and the intensity of the parametric x radiation from a Si single crystal impinged upon by an electron beam with the energy ranging from 200 to 1100 MeV. It has been found that the x-ray intensity is higher and the angular spread is less broad than the predictions of the theoretical model due to Feranchuk and Ivashin. This calls for a more elaborate theoretical treatment of multiple scattering, and encourages research aiming at a new type of x-ray source based on the parametric x-radiation mechanism.

Reynolds number dependence of turbulence energy spectra and higher-order moments of velocity differences is explored by means of numerical integrations of the incompressible Navier-Stokes equation. The simulations have spatial resolutions up to ${512}^3$ and cover the range 15≤${\mathit{R}}_{\lambda }$≤200, where ${\mathit{R}}_{\lambda }$ is the Taylor microscale Reynolds number. Over this range, the energy spectra collapse when scaled by the wave number ${\mathit{k}}_{\mathit{p}}$ of peak dissipation and also by the spectrum level at ${\mathit{k}}_{\mathit{p}}$. It is found that ${\mathit{k}}_{\mathit{p}}$ varies with ${\mathit{R}}_{\lambda }$ in accord with the 1941 Kolmogorov theory. High-order normalized moments of velocity differences over inertial-range distances exhibit an ${\mathit{R}}_{\lambda }$-independent variation with separation distance. Implications of these observations are discussed.

We demonstrate the selection of narrow-band radiation from the output of an infrared free-electron laser operating with electron pulses of a few picoseconds duration. A continuous output with a linewidth of less than ${10}^{\mathrm{-}3}$ ${\mathrm{cm}}^{\mathrm{-}1}$ and a powder in the order of 10 W in macropulses with a duration around 10 μs can be obtained. The number of active cavity modes is reduced by a factor of 40 by inducing coherence between separate optical pulses in the cavity. An external Fabry-Pérot interferometer is used to select one of the remaining modes.

A new class of m=1 magnetic islands is presented. These islands have bounded current-density distributions, so that their resistive evolution is slow. Conservation of the total magnetic flux, however, results in a critical width such that the current density diverges and the separatrix collapses at the X point.

We report ab initio total energy and electronic structure calculations for benzene physisorbed on graphite and ${\mathrm{MoS}}_2$. Our results constitute the first detailed ab initio investigation of the states through which tunneling occurs in the scanning tunneling microscope (STM) through an insulating molecule. At low voltages the density of states near the Fermi energy some way from the surface is dominated by a weak admixture of molecular states into the substrate states; the resulting STM images reflect the details of the molecule-substrate interaction. At higher voltages, images of molecular states are obtained.

The desorption of polymer chains through an overlayer of strongly adsorbed chains was studied to determine the influence of topological constraints on the polymer desorption process. The desorption time of linear chains fits a power law, ${\mathit{M}}^{\mathit{a}}$, where M is a molecular weight and a=2.3±0.2. A comparison of linear and star-branched chains shows that desorption of star-branched chains was greatly suppressed. These findings are reminiscent of entanglement effects in bulk systems. They suggest a unifying perspective from which to analyze polymer mobility at surfaces.

We show, through simulations of a variety of one- and two-dimensional solid-on-solid models, that nonequilibrium surface diffusion processes in the presence of a deposition flux are generically characterized by a nonvanishing inclination-dependent mass current along the surface. The current stabilizes the surface, leading to Edwards-Wilkinson scaling for the surface fluctuations, if it is a decreasing function of inclination, but induces a growth instability otherwise. Both types of behavior generally occur for the same system at different surface orientations.

We develop a dynamical model for phase separation in a system with two disparate energy scales. Monte Carlo computer simulations of this model reveal a ‘‘pinning’’ of the structure factor during spinodal decomposition that obeys new scaling relations. We propose a mechanism for the pinning which allows us to predict exact values for the pinning exponents. Finally, we discuss our model in light of recent experiments.

Universality in the kinetics of systems with O(N) continuous symmetry, under influence of an external time-dependent field, is studied. The exponents that characterize the process are obtained, showing that they are independent of N, for N≥2. We also get analytical expressions for the time dependence of the order parameter (magnetization) in the presence of small fields, in the limit N→∞. Our results show that universality goes beyond the value of the exponents. For small fields, the full dynamical evolution of the order parameter is universal. Similar universality is expected for arbitrary N. Agreement with numerical solutions of the basic nonlinear equations is excellent.

Phase separation dynamics and structural properties of binary liquid clusters with linear dimensions in the nanometer range are investigated. The demixing process proceeds through incipient domain formation and coarsening processes. Strong density correlations are shown to exist within the resulting phase-separated clusters. The results of this study show that cluster binary mixtures display interesting two-phase phenomena which differ from their bulk-phase analogs.

A new method for calculating the boundaries between two solid alloy phases has been developed. It is first principles in that the only input is the atomic numbers of the constituents, and differs from previous efforts in that no interaction potentials are introduced. It is used to calculate the miscibility gap in the palladium-rhodium phase diagram.

We report on torsional oscillator studies of superfluidity in the second and higher layers of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on basal plane graphite. Our measurements reveal several novel features. We observe a reentrant superfluid phase in the second layer of adsorbed helium. Superfluidity disappears before the completion of the second layer and reappears early in the third layer. A well-defined plateau in the transition temperature occurs at the beginning of the fourth layer.

We propose, based on ab initio calculations, that the ground state (001) surface of α-Ga should be covered by two layers of Ga III, a denser phase which is stable in the bulk only at high pressure and temperature. We discuss how this novel prediction can explain recent scanning tunneling microscopy observations, including anomalous thermal stability of this surface.

Scanning tunneling microscopy observations of long hops in the diffusion of Pb atoms on Ge surfaces are explained by the model of a Brownian particle in a periodic potential. The classical turnover theory for barrier crossing predicts a large correlated hopping probability in the underdamped limit, consistent with experiment and in agreement with simulations. The corresponding quantum theory predicts that in the underdamped limit the rate is dominated by tunneling. This causes the quantum correlated hopping probability to vanish in this limit and may be thought of as a new form of quantum localization.

We generalize the exactly solvable Hubbard model with infinitely large on-site Coulomb repulsion recently introduced by Brandt and Giesekus. We point out that the exact ground state can be regarded as a resonating-valence-bond state, and argue that the state may exhibit superconductivity. The existence of a phase with singlet-pair ordering is demonstrated in a simple model on a tree.

Using a combination of interferometric-correlation and time-resolved intensity measurement techniques, we characterize the phase and amplitude of the coherent emission of semiconductor quantum wells resonantly excited by ultrashort optical pulses. We demonstrate that exciton-exciton interaction produces nonlinear phase shifts of the instantaneous frequency relative to the incident laser.

We explain the counterintuitive result that increased short-range order reduces the conductivity in ${\mathrm{Ni}}_{0.8}$${\mathrm{Mo}}_{0.2}$ and suggest an explanation for the ‘‘K-state’’ effect, in which severe deformation can significantly increase conductivity.

We propose a model of an anyon exciton consisting of a hole and several anyons, and apply it to the spectroscopy of an incompressible quantum liquid. Fractionalization of the electron charge makes properties of such entities quite different from those of usual magnetoexcitons. The model describes a number of properties established by few-particle simulations, including an abrupt change in emission vs electron-hole asymmetry of the system. The attractive field of a hole may eliminate the hard core constraint for anyons. The effect of exciton-magnetoroton coupling is discussed.