We have used interfering multiphoton ionization processes to measure changes in the relative phase between two optical laser beams. In this work, the phase variation is due to the π phase shift of a focused Gaussian beam as it propagates through the focal region. An array of linear collection electrodes is used to measure the multiphoton ionization rate as a function of the distance from the beam waist when two laser fields are resonant with an atomic transition, one through a linear process, the other through a three-photon process.

An atom in a laser-cooled sample driven by more than one traveling-wave laser beam radiates a coherent signal that contains multiwave-mixing frequencies derived from the Doppler shifts. Wave mixing tends to obscure Doppler velocimetry of atoms, although in the actual experiments of Westbrook et al. [Phys. Rev. Lett. 65, 33 (1990)] the results were not qualitatively altered.

Experiments on the ir intensity dependence of the preparation process for second-harmonic generation in germanosilicate fibers and preforms reveal that a sharp threshold of the order of 5 GW/${\mathrm{cm}}^2$ exists. The results are explained in terms of a charge-delocalization transition that occurs due to the renormalization of the disorder energy between localized states dressed by an intense nonresonant light field.

We have observed antiphase dynamical states in an N-mode laser with an intracavity doubling crystal. We have experimentally observed such states for N=3, while numerical simulations indicate their persistance up to (at least) N=5. Analogous dynamical states have been predicted for Josephson-junction arrays; in both cases, the multiplicity of coexisting attractors can be traced to the global nature of the coupling between degrees of freedom.

The characteristcs of relativistic propagation in plasmas of subpicosecond ultraviolet (248-nm) radiation are studied for both spatially homogeneous plasmas and plasma columns. It is established for the first time that the defocusing properties of the interaction can represent a dynamical mechanism stabilizing the mode of propagation against radial oscillations. The dependence of both quasistabilized modes and pulsing waveguide regimes on the initial transverse intensity distribution is examined and, for the latter, the locus of the first focus produced in a homogeneous plasma is calculated.

The quenching of spontaneous-emission coefficients recently inferred at Princeton from measured intensity ratios of visible and extreme-ultraviolet lines from a common upper level in laser blowoff plasmas is further investigated using a ruby laser. The line intensity ratios of the Civ 3s-3p and 2s-3p transitions, for similar laser targets, show a similar dependence on separation from the target as the previous experiments. Both of these results can, however, be interpreted in terms of density gradients and Stark broadening wihtout invoking changes in Einstein A values.

We study the dynamics of a heavy particle in a Fermi liquid and find localization transition in the strong-coupling limit in a homogeneous system. The ground state is a wave packet of size <1/${\mathit{k}}_{\mathit{F}}$. In the localized phase the particle behaves as a quantum-mechanical object at short times (Γt≪1) and distances (${\mathit{k}}_{\mathit{F}}$x≪1). Classical behavior is found at T=0 in the long-time and large-distance limit due to the destruction of phase correlations in the system. The particle has a temperature-independent mobility when a uniform field is applied. Our results explain the observed constant mobilty of negative ions in liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$.

We use quantum Monte Carlo techniques to study the critical properties of an interacting-boson model in one dimension. The phase diagram consists of a series of (Mott-) insulating phases at commensurate fillings and of a superfluid phase. From the critical behavior of the superfluid density and the compressibility, we measure the exponents ν and z, which agree with predictions based on a scaling analysis.

We propose a new temperature-independent Hamiltonian that displays a vibrational-entropy-driven, first-order phase transition. Models of this kind do not presently exist in the statistical-mechanics literature, but are important in providing a simple realization of diffusionless, solid-to-solid martensitic transitions. The model employs anharmonic couplings between neighboring particles that cause the low-temperature phase to have a lower vibrational entropy (i.e., stiffer restoring forces) than the high-temperature phase. This entropy difference, as opposed to an internal energy difference, produces a sharp transition. In one dimension the model may be solved exactly using transfer-integral techniques, and the solutions show heterophase fluctuations connecting the parent and product phases only for temperatures very close to the transition. Consistent with this behavior, S(q,ω) found from molecular-dynamics simulations shows a central peak that is diffusive in origin.

We consider a square-lattice Ising model with a smooth inhomogeneity in the bulk, where the couplings deviate from the bulk critical coupling by ${\mathit{Al}}^{\mathrm{-}1}$, l being the distance from the center of the inhomogeneity. Scaling exponents describing decay of correlations along the inhomogeneity have been determined exactly by conformal invariance and finite-size scaling. It is shown that the homogeneous model (A=0) with bulk Ising exponents separates the first-order transition regime (A>0) from the region of nonuniversal criticality (A<0).

The standard scaling mechanism for matching asymptotic behaviors on the approach to criticality fails for directed percolation, as indicated by transfer-matrix calculations for sizes up to N=15. A formulation is proposed incorporating irrelevant-variable corrections in a new pattern of asymptotic scaling. Numerical data support these theoretical predictions and, among other results, yield a new estimate, λ=0.37±0.04, for the leading irrelevant-variable exponent.

Combining some results from the classical theory of elasticity with the modern functional derivative approach to nonuniform systems, we obtain a well-defined stress tensor for nonuniform equilibrium fluids and solids. This stress tensor is symmetric and satisfies the force-balance equation, so it provides an unambiguous route to quantities such as the surface free energy. The ambiguities associated with earlier stress-tensor definitions are traced back to their failure to take account of Saint-Venant’s strain compatibility conditions.

We study the divergence of the stress field near the tip of a wedge crack of head angle between π and 2π. A new universal family of solutions is found, where the dominant singularity is characteristic of a pure tension on the external boundaries. Further from the tip, the first correction to this behavior is a power law characteristic of a pure shear on those boundaries. When the head angle of the wedge is inside a certain range, higher-order modifications to the field are found to be periodic in the logarithm of the distance from the tip. Inside this range, oscillatory solutions appear as more remote corrections to the power-law behavior. The relevance of these solutions to sidebranching and to self-similar pattern formation of cracks is discussed.

A model calculation based on the effective-medium theory of the oxygen-induced reconstruction of the (110) and (100) surfaces of Cu is presented. Equilibrium structures are calculated from a minimization of the total energy of the system. Missing-row-type reconstructions are found to be most stable in both cases, and an analysis is presented, showing what the driving force is behind these reconstructions.

The relaxation of persistent infrared spectral holes burned in the SeH vibrational absorption band at 1.5 K is studied for ten glass compositions in the Ge-As-Se system. It is found that the dominant spectral-hole relaxation rate increases by over 3 orders of magnitude as the average atomic coordination number varies from 2.0 to 2.8. Over the composition range studied, the quantitative form of the nonexponential hole relaxation depends solely on the average coordination number, independent of chemical composition.

The optical-absorption spectrum of a complex neutral defect in silicon with a lowest bound-exciton line at 615.0 meV has been studied. Only transitions to s-like excited electronic states are observed and their binding energies are in good agreement with a deep pseudodonor model for the electronic structure of the defect. A comparison with optical-excitation spectra of other complex defects shows that this type of electronic structure explains the optical spectra of a large class of defects in silicon.

The carbon-hydrogen complex in GaAs is evidenced by its stretching local mode of vibration. It is shown that even though carbon and hydrogen are present, this complex is not formed in n-type material; it only appears in p-type material. This is explained by the necessity for hydrogen to be in the positive charge state for the carbon-hydrogen complexes to be formed. The corresponding hydrogen electronic level is shown to be located around 0.5 eV above the top of the valence band.

Plasmon excitations for a lateral superlattice of quantum wires in a modulation-doped GaAs/(AlGa)As multiple-quantum-well system have been investigated by resonant inelastic light scattering. The Raman spectra exhibit a set of resonances with a strongly anisotropic dispersion. For momentum transfer parallel to the wires, a significant dispersion is observed; perpendicular to the wires, the dispersion is nearly flat. This is an experimental proof that Coulomb interaction between neighboring wires is weak and that the excitations represent confined plasmons of the individual wires. They provide a direct measure of the width of the electron channels.

We have measured two-photon absorption spectra of quantum-well structures in static electric fields for photon energies close to half the band-gap energy, and found drastic static-field-induced changes in the spectra. Our experimental results agree with a theory based on an infinite-level model, the validity of which was previously established at zero bias, except that a field-induced growth of the two-photon absorption peak at half the lowest light-hole excitation energy is more drastic than the theoretical prediction. At high bias fields the peak height approaches a large value predicted by another simplified theory based on a two-level model.

Extending a random-matrix theory developed earlier, we show that breaking a basic symmetry in an Anderson insulator (e.g., time-reversal symmetry or spin-rotation symmetry) generically yields a multiplication of the localization length ξ by universal factors. Numerical calculations and magnetoconductance measurements in the Mott variable-range-hopping regime confirm that the removal of time-reversal symmetry by a magnetic field yields ξ→2ξ in the absence of spin-orbit scattering, and ξ→ξ/2 in the presence of spin-orbit coupling.