Ionization of semiconductor deep impurity centers has been observed in the far infrared, where photon energies are several factors of 10 smaller than the binding energy of the impurities. It is shown that the ionization is caused by phonon assisted tunneling in the electric field of the high power radiation. This optical method allows the investigation of the tunneling process at electric bias fields well below the threshold of avalanche breakdown.

We present an exact dynamical theory, valid on finite time scales, to describe the fully connected Hopfield model near saturation in terms of deterministic flow equations for order parameters. Two transparent assumptions allow us to perform a replica calculation of the distribution of intrinsic noise components of the alignment fields. Numerical simulations indicate that our equations describe the dynamics correctly in the region where replica symmetry is stable. In equilibrium our theory reproduces the saddle-point equations obtained in the thermodynamic analysis by Amit et al.

It is shown that when a quantum mechanical oscillator is parametrically excited there are special values of the parameters for which the system will pass periodically through a lattice of coherent states associated with the modular group Γ. It is shown that these Γ transits can be used to determine unknown parameters. A method is given for detecting the transits experimentally and is made possible by the existence of three families of states associated with modular forms that are orthogonal to the lattice. For isotropic states the three families occur in D-mode systems with D>10, 14, and 26.

A generalization of the Jordan-Wigner transformation to three (or higher) dimensions is constructed. The nonlocal mapping of spin to fermionic variables is expressed as a gauge transformation with topological charge equal to 1. The resulting fermionic theory is minimally coupled to a non-Abelian gauge field in a spontaneously broken phase containing monopoles.

The crossing processes in an overdamped double-well potential driven by an external colored noise and a weak sinusoidal time-dependent modulation are investigated by means of analog simulation. A new resonance is revealed (besides the so-called stochastic resonance) for frequencies close to the inverse time constant of the colored intrawell dynamics (resonant crossing). A quantitative analysis of the results obtained is also proposed.

Radiative corrections to ${\mathrm{\pi }}_{\mathit{l}2}$ (l=e or μ) decays are examined. Higher order electroweak leading logarithms short-distance QCD corrections, and structure dependent effects are incorporated. The results are employed to (1) test e-μ universality in Γ(π→eν${\mathrm{¯}}_{\mathit{e}}$(γ))/Γ(π→μν${\mathrm{¯}}_{\mathrm{\mu }}$(γ)), (2) extract an ${\mathit{f}}_{\mathrm{\pi }}$ which is used to check the Goldberger-Treiman relation and PCAC-anomaly prediction for Γ(${\mathrm{\pi }}^0$→γγ), and (3) determine the tau partial decay rate Γ(τ→π${\nu }_{\mathrm{\tau }}$(γ)).

We present the first complete quantum treatment of the ionization of a circular Rydberg state by a linearily polarized microwave field. Experimentally accessible ionization threshold fields as well as the dynamics of the resonance eigenfunctions are investigated, together with a comparison to the behavior of a quasi-one-dimensional state subject to a microwave, and of a circular state in an intense, high frequency laser field.

Highly efficient and selective population transfer in NO molecules in the electronic ground state (${\mathit{X}}^2$${\mathrm{\Pi }}_{1/2}$) from the vibrational level v’’=0 to the level v’’=6 is demonstrated. It shows, for the first time with pulsed lasers, that a rather generally applicable scheme for complete control over the level population in atoms and molecules is now available. The efficiency relies on a counterintuitive interaction sequence of two lasers with the molecule in a process of stimulated Raman scattering involving adiabatic passage.

We describe a new experimental configuration suitable for observing the topological phase of Aharonov and Casher in atomic systems. Using this we have been able to show experimentally that the Aharonov-Casher phase is both independent of velocity and proportional to electric field and we have verified the predicted size of the effect with an accuracy of 4%.

We present a solution to the direct determination of the statistical parameters (correlation length and root mean square deviation), characterizing the profile of a random rough surface with large slopes, thus producing multiple scattering. This is done from the specular, coherent, component of the mean intensity of waves scattered from the surface, and, today, is the only reliable, realistic procedure which uses an analytical expression of this component. The method is applicable whenever this specular component may be measured, and is valid for a wide variety of physical problems involving surface scattering.

We report on time-delayed wave-mixing experiments which allowed, for the first time, the observation of ultrafast modulation due to the interference between third- and fifth-order optical polarizations. The new effect was observed in semiconductor doped glasses by controlling the relative phase of the optical fields which induce distinct wave-mixing processes. The experiments allowed us to measure the relative phase between the nonlinear susceptibilities ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ and ${\mathrm{\chi }}^{\mathrm{}\left(5\right)\mathrm{}}$ as well as their relative magnitude. In principle the interference effect may be observed in a large variety of materials and the exploitation of this phenomenon to develop a new technique for nonlinear spectroscopy is also discussed.

The whole needle-crystal solution for three-dimensional (3D) dendritic growth is described analytically. Its construction involves the existing 3D selection theory for the tip of the dendrite (M. Ben Amar and E. A. Brener [Phys. Rev. Lett. 71, 589 (1993]) plus matching of the tail to this tip. This is exhibited here. Both intermediate and final asymptotics of the tail shape are given. This shape, which deviates strongly from an Ivantsov paraboloid, is in qualitative agreement with experiental observations.

We propose a mechanism for the origin of non-Gaussian tails in the probability distribution functions of local variables in nonlinear, diffusive, dynamical systems including passive scalars advected by chaotic velocity fields: Intermittent fluctuations on appropriate time scales in the amplitude of the (chaotic) noise can lead to exponential tails in certain regimes. We provide numerical evidence for such behavior in deterministic, discrete-time passive scalar models. Different possibilities for probability distribution functions are also outlined.

We report on experimental studies of an ultraviolet (UV) free-electron laser (FEL) oscillator driven by low-energy electrons from a radio-frequency linear accelerator. Previous UV FELs have been driven by 350–800-MeV electrons from storage rings. We verify the concept of driving an UV FEL with a low-energy, but high-current, low-emittance electron beam. This and other innovations allowed the FEL to lase at wavelengths from 369 to 380 nm using 45.9–45.2-MeV electrons, and to achieve a peak optical output power of 270 kW. The experimental results are in good agreement with simulations.

The scaling behavior of the time-dependent diffusion coefficient D(t) in a suspension is studied by means of a molecular dynamics simulation. We obtain a visual collapse of the curves by suitable scaling of time axis and vertical axis for D(t) at different concentrations. However, the derivatives of D(t) do not show such a scaling behavior. By analyzing the kinetic regime probed in this simulation we conclude that the absence of scaling is due to the fact that the dynamics is not completely dominated by hydrodynamics processes. This provides further evidence on the purely hydrodynamic origin of the scaling behavior.

We have studied the growth of the wetting layer formed at the surface of a critical mixture of poly(ethylene propylene) and perdeuterated poly(ethylene propylene) during spinodal decomposition. The wavelength of the surface spinodal wave is found to grow as ${\mathit{t}}^{1/3}$ over the entire range of times accessible in the experiment. The composition profiles exhibit universal scaling behavior in the near-surface region. These results are in excellent agreement with a recent numerical study on surface-directed spinodal decomposition.

We report precision specific-heat measurements for two-dimensional helium films in Anopore membranes and in Millipore filter paper. Above the transition temperature and for films less than a superfluid layer thick, we find a distinct specific-heat peak which can be understood in terms of the Kosterlitz-Thouless vortex unbinding mechanism. Although our results are well described by the planar theory, transition broadening becomes evident for thicker films.

The mechanism of migration of missing-dimer vacancies on Si(100)-(2×1) has been investigated theoretically. The migration involves a wavelike concerted motion of four atoms: Two rebonded atoms in the second layer more up to the top layer, while two dimerized atoms in the top layer simultaneously descent into the second layer. The migration is extremely anisotropic, with the fast direction along the dimer rows. The predicted direction of fast diffusion and the activation energy are consistent with experiment.

Far-infrared reflection measurements on Si:P both on the metallic and insulating side of the metal-insulator transition (MIT) were performed at low temperatures. In the metallic regime free-carrier absorption and, additionally, absorption peaks due to interband transitions from the impurity band to the conduction band and to transitions between the broadened valley-orbit split 1s states are observed. This gives clear evidence that the impurity band is formed by the overlap of the 1s(${\mathit{A}}_1$) ground states and is well separated from the conduction band when the MIT occurs. Only far beyond the metallic limit the impurity band merges completely with the conduction band.

We have studied the low temperature phases of Pb/Ge(111) at Pb coverages Θ=1/3, 1, and 4/3 using ab initio molecular dynamics. Stable phases are found to occur at Θ=1/3 and 4/3. For the low-coverage (Θ=1/3) α phase, we confirm the model of Pb atoms adsorbed at ${\mathit{T}}_4$ sites of the Ge(111) substrate. For the high coverage (Θ=4/3) β phase and very low temperature, we determine a structure characterized by the presence of covalently bonded Pb chains parallel to 〈11¯0〉 substrate directions. At room temperature, fluctuations are found to disrupt long-range Pb chains and restore the threefold symmetry of the ideal Ge(111) surface. The calculated structure factors and local density of states agree well with x-ray diffraction and STM data.