We present a nonperturbative theory of the behavior of He in a strong pulsed laser. We address the question of the validity of frozen-core or single-active-electron approximations by providing direct quantitative comparisons with calculations including the complete two-electron behavior with correlation. Our results are illustrated with examples of nonlinear susceptibilities and ionization.

In spite of the fact that cone emission was first observed over a decade ago, there is still no generally accepted theory of the effect. Here we give an outline of a unified approach and point out a mechanism that supports Cherenkov-type radiation due to vacuum fluctuations as a possible source of cone emission. This can be considered as an additional mechanism to the previously discussed four-wave mixing, and initial encoding and follow-up refraction effects. We compare our treatment to previously considered models.

We investigate the implications that a sum rule for the average velocity of a geological fault has on the distribution of earthquake sizes. Under general conditions the exponent B of the Gutenberg-Richter law for the distribution of small seismic moments is shown to obey B<1. This result does not rely on any particular earthquake model and should be equally applicable to the case of friction between two sliding blocks as well as to computer simulations of earthquake dynamics. For the distribution of large earthquakes we specifically consider three different possible models: (a) A single power law exists over the entire range of magnitudes; then B≊1. (b) The large events do not fall in the scaling region; in this case we are able to derive a relationship between the exponents relating their frequency and seismic moment to the total size of the system. (c) The large earthquakes also show scaling behavior but with a different exponent B’; then the sum rule implies that B’>1.

Strong-coupling properties of the Burgers–Kardar-Parisi-Zhang equation describing nonequilibrium interface growth are studied. A physical coupling constant is defined and related to the height-height correlation function for arbitrary substrate dimension. In 1+1 dimensions, the exact universal coupling constant and scaling function are computed using a mode-coupling theory. It is found that a finite surface tension is generated from zero, suggesting that macroscopic properties are not affected by the absence of microscopic surface tension.

We study Artin’s billiard, an extremely chatoic system defined on the fundamental domain of the modular group PSL(2,openZ), and show that its quantum energy levels are given exactly by the nontrivial zeros of a certain Selberg ζ function expressed as an Euler product over the classical periodic orbits. We demonstrate that at least the first 73 energy levels can be determined by using only a finite number of periodic orbits.

An elementary context in which the computing ability of cellular automata is directly related to quantities of physical significance is discussed. In examples, it is found that no single statistic serves to measure the complexity of the behaviors produced. On this basis it is argued that the theory of distributions can be gainfully employed alongside computation theory and information theory in the study of the complexity of dynamical systems.

A simplified method for measuring the director fluctuation intensity below the electrohydrodynamic instability in a thin layer of a nematic liquid crystal is presented.

We present several lattice models for water that belong to the universality class of Angell’s independent bond model. The relevance for real water, the relationship to percolation models for water, and the possible use of these models in simulations of protein-sovlent systems is discussed.

Thermal and structural experiments on thin free-standing liquid-crystal films of n-heptyl-4’-n-pentyloxybiphenyl-4-carboxylate have revealed the existence of a surface-freezing phenomenon, in which surface crystal-E layers coexist with a hexatic-B interior. The crystal-E surface is typically found by electron diffraction to exist in twin domains in which the orientation of the orthorhombic lattice is rotated by 2°–3° with respect to that of the underlying hexatic-B bond-orientational axes. It is shown theoretically that such a rotation minimizes the energy of the strain by the two incommensurate types of structural order, analogous to the Novaco-McTague theory, which applies to the orientational epitaxy of a crystalline overlayer on an incommensurate crystalline substrate.

Binary liquid mixtures are expected to display two interfacial transitions when confined in small pores. One is a wetting transition, similar to such transitions on planar substrates. The other is an interfacial shape transition. We use Monte Carlo simulations to show that an Ising system displays both transitions. We find a wetting phase diagram similar to that of a model with a long-ranged wetting potential, proving that the existence of both transitions is insensitive to the wetting potential shape.

We report a hydrodynamic instability related to the periodic growth of needle-like crystals observed in thin aqueous-solution films of Ba(${\mathrm{NO}}_3$${)}_2$ with the free surface in isothermal conditions. The oscillation of the aqueous-solution film, in which the solution film separates, and re-contacts with the needle-like crystal periodically, has been investigated using in situ observations with interference-contrast microscopy. Our experiments indicate that this phenomenon derives from the interplay of the Marangoni effect and the wetting effect.

The coarsening of a three-dimensional foam is studied with multiple light-scattering techniques. Scaling behavior is observed with the average bubble diameter growing in time as ${\mathit{t}}^{\mathit{z}}$ where z=0.45±0.05. Changes in the packing conditions during coarsening give rise to a dynamical process that also exhibits temporal scaling. Neighboring bubbles undergo sudden structural rearrangement events at a rate per unit volume that decays as ${\mathit{t}}^{\mathrm{-}\mathit{y}}$ where y=2.0±0.2.

We demonstrate that, when the trajectory of a tip is a circle of large radius, the steadily rotating spirals found from the free-boundary formulation are stable, in agreement with numerical simulations of reaction-diffusion systems.

We validate the implementation of a path-integral formalism to compute the thermodynamics of stacking of RNA base pairs, restricting the analysis to intramolecular interactions. We estimate the effective contact volume, an empirical parameter that determines the excluded-volume effects. This is done by comparing the analytical result with the energetics of duplex growth that takes place after a loop closure or nucleation event. As an illustrative example, the derivation is specialized to the polymer (CG${)}_{\mathit{n}}$, n→∞ (C denotes cytosine, G denotes guanosine) in the limit of infinitely long tails.

The coherent overlap of the waves traversing the two legs of a perfect-crystal neutron interferometer is altered by a material with a neutron-nuclear optical potential. The fringe visibility is limited by the longitudinal coherence length. The loss of interference contrast caused by a slab of material of positive optical potential can be restored by placing another slab of material with a negative optical potential in the same leg of the interferometer. This paper describes the observation and detailed characterization of this quantum-mechanical phenomenon, which we call the phase-echo effect.

The concepts and formalism of quasiprobabilities in the space of paths are extended to the relativistic domain. The definition of a relativistic wave function, needed in the construction of the quasiprobability functional, is discussed extensively, together with the formulation and the physical interpretation of an explicitly covariant quantum theory. The Wigner and the Margenau-Hill relativistic phase-space distribution functions are then obtained according to the standard procedures of the formalism.

A description of the adiabatic approximation in terms of the time-evolution operator is presented. Corrections to the approximation are studied, and it is seen that these can be obtained in a simple way in the case of a rapidly oscillating Hamiltonian.

We study, within the framework of nonrelativistic QED, the structure of the electromagnetic field in the neighborhood of a free spinless electron dressed by the interaction with the vacuum field. We introduce a suitable formalism that correlates electron position and field operators. The quantum average value obtained by applying correlated field operator to the dressed state gives the average value of the corresponding field quantity as a function of distance from the electron. The results obtained separately for the electric- and magnetic-field energy density around the particle display contributions that have quantum origin and that cancel in summing of the two, yielding the total energy density. In the total energy density only contributions that have a classical analog remain. These results are compared with those for the electromagnetic structure around a neutral atom, obtained in previous work [R. Passante and E. A. Power, Phys. Rev. A 35, 188 (1987)].

A model for intrinsic decoherence in quantum mechanics is proposed, based on a simple modification of unitary Schrödinger evolution. On sufficiently small time scales the system evolves by a random sequence of unitary phase changes generated by the Hamiltonian. The Schrödinger equation is obtained to zeroth order in the expansion parameter. Higher-order corrections lead to a loss of coherence in the energy basis. The rate of coherence loss becomes very large as the energy scale of the system is increased. The expansion parameter determines an uncertainty in the time step on very short times scales. A number of testable consequences are derived including anomalous dispersion of a free particle, decay of oscillatory systems, destruction of interference-fringe visibility, and a phase shift of interference fringes.

The aim of this paper is to present a careful treatment of time evolution in quantum electrodynamics. I demonstrate the importance of a correct preparation of the initial state to obtain gauge-independent and infrared finite quantities. I also consider the problem of the long-time behavior of spontaneous emission on the basis of a harmonic-oscillator model.