Recently observed nuclear spin-spin dependence of total cross sections for ${\mathrm{n}}_{\mathrm{pol}{+\mathrm{}}^{27}}$${\mathrm{Al}}_{\mathrm{pol}}$ is accounted for by coupled-channels calculations that include the effects of the static quadrupole moment of ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, and without any spin-spin potentials. Effective spin-spin interactions, of both spherical and tensor kinds, can thus be generated by static nuclear deformation.

Electronic spectra of carbazole⋅${\mathrm{Ar}}_{\mathrm{n}}$ clusters in supersonic beams exhibit specific size-dependent spectral features, showing only sharp intermolecular vibrational bands for n=1 to 3, both broad and narrow spectral bands for n=4 to 6, and only broad bands for n≥7. Monte Carlo simulations indicate that the change from sharp to broad structure is due to surface melting transitions of the solvent clusters relative to the substrate, and predict a finite-width solidlike-liquidlike coexistence range in agreement with the experimental observations.

We have built the first quantum oscillator working on two-photon stimulated emission of radiation. It oscillates continuously on a degenerate two-photon transition between the levels 40S and 39S of rubidium atoms. The oscillation is sustained in a Q≃${10}^8$ niobium superconducting cavity at 68.415 87 GHz. At threshold, this new maser system operates with only a few atoms and a few tens of photons at a time in the cavity.

By measuring the joint probability for the detection of two photons at two points as a function of the separation between the points, we have demonstrated the existence of nonclassical effects in the interference of signal and idler photons in parametric down-conversion. In principle, the detection of one photon at one point rules out certain positions where the other photon can appear.

Large benzene clusters ionize with anomalously high efficiency when excited by ultraviolet radiation. The enhancement of the ionization and multiple-ionization cross sections over molecular values is quantitatively explained by ultrafast fusion of pairs of excitations within a cluster.

We present a complete first-order quantum electrodynamics calculation of the spin polarization in a relativistic electron storage ring. The result differs from previous semiclassical calculations of other authors in several respects. Under ideal conditions a maximum polarization of 99.2% may be obtained.

The Karman vortex street in the wake of a circular cylinder is shown to be due to an absolute instability of the flow in the near wake. A new means of instability analysis is used, involving mappings from the complex k plane to the complex ω plane.

We present an analytical solution to the problem of pulse backscattering from a randomly stratified half-space. It is shown that the power spectrum μ for the backscattered wave is characterized by a function of χ, where χ=(distance traveled by the pulse at time τ)/(the frequency-dependent localization length). For the matched-impedance and the total-reflection boundary conditions, μ is given by χ/(1+χ${)}^2$ and 4χ, respectively. Implications for the time-domain measurement of the localization length are discussed.

The decimated-amplitude scheme (DAS) devised by Kraichnan is applied to a random-coupling model of turbulence originally introduced by Betchov to test the direct-interaction approximation (DIA). Using a system of 32 variables, forced stochastically under appropriate statistical constraints, it is shown that the DAS can accurately represent the autocorrelation function of a full Betchov system of 96 variables. A comparison is also made between the DAS and the DIA.

An initial model of the superfluid λ transition is constructed with use of vortex-ring excitations, as originally suggested by Onsager and Feynman. A real-space renormalization technique generates a screened vortex energy and core size, and gives rise to a transition where rings of infinite diameter are excited as the superfluid density approaches zero at ${\mathrm{T}}_{\mathrm{c}}$. Although the model satisfies the Josephson hyperscaling relation, it is not yet a complete theory: The superfluid density exponent is ν=0.53, and does not match the known value ν=0.67.

The role of thermal spikes in energetic displacement cascades has been investigated by molecular-dynamics computer simulation. For cascade energies of 3 and 5 keV in Cu, which are the highest energies (in reduced units) yet treated by fully dynamical simulations, it is found that local melting occurs and persists for several picoseconds. The implications of this behavior for atomic mixing, Frenkel-pair production, and point-defect clustering are discussed.

We analyze the stationary nonequilibrium state of a spin system evolving under combined flips at temperature ${\mathrm{\beta }}^{\mathrm{-}1}$ and exchanges at β=0. Computer simulations indicate that the phase transition changes, as the exchange rate Γ increases, from second to first order. We interpret this as a changeover from Ising behavior for small Γ to mean-field behavior for Γ→∞ where the magnetization is discontinuous in the ‘‘continuum limit.’’

We report the first observation of the absence of the photodarkening (PD) effect in bulk chalcogenide glasses. In the prototype systems ${\mathrm{Cu}}_{\mathrm{x}}$(${\mathrm{As}}_{0.4}$${\mathrm{S}}_{0.6}$${)}_{1\mathrm{-}\mathrm{x}}$ and ${\mathrm{Cu}}_{\mathrm{x}}$(${\mathrm{As}}_{0.4}$${\mathrm{Se}}_{0.6}$${)}_{1\mathrm{-}\mathrm{x}}$ the PD effect is essentially destroyed for x≥0.01 and 0.05 for the S and Se systems, respectively. A structural model for Cu-As-Se and Cu-As-S systems is proposed, which is based upon the assumptions of covalent bonding and the absence of doping. According to this model, the absence of the PD effect indicates that medium-range interactions among the lone-pair p electrons may be important for the effect to occur. This structural model can be generalized to many other amorphous semiconducting systems.

Using an atomic force microscope, we have observed atomic-scale features on the frictional force acting on a tungsten wire tip sliding on the basal plane of a graphite surface at low loads, ${<10\mathrm{}}^{\mathrm{-}4}$ N. The atomic features have the periodicity of the graphite surface and are discussed in terms of a phenomenological model for the tip motion described by the sum of a periodic tip-surface force and the spring force exerted by the wire.

A discretized version of the reptation model is proposed. The tube is modeled by a one-dimensional lattice and the polymer is modeled by a cluster of walkers, called reptons, on this lattice. Each repton represents a part of the chain. Reptons are allowed to hop between neighboring sites, but the cluster always remains connected. This model is solved analytically and numerically. In the experimentally accessible range of molecular weights M it predicts the diffusion coefficient D∼${\mathrm{M}}^{\mathrm{-}2}$+O(${\mathrm{M}}^{\mathrm{-}3}$) and viscosity ${\mathrm{\eta }}_0$∼${\mathrm{M}}^{3.4\pm 0.1}$. .AE

X-ray absorption spectra of titanium-diffused ${\mathrm{LiNbO}}_3$ indicate that a strong change in the Ti center appears as the concentration is increased above 5×${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$, involving a tendency toward displacement in the x-y plane and disorder. This change in the Ti center occurs within the concentration range used for integrated-optical-device fabrication, and may account for the abrupt decrease in rate of change of the ordinary optical index (${\mathrm{n}}_0$) with increasing Ti concentration reported in the literature.

We discuss a Ginzburg-Landau model of two even-parity superconducting order parameters (e.g., of s-wave and d-wave symmetry with transition temperatures $T_0$ and $T_2$, respectively) as a model for ${\mathrm{U}}_{\mathrm{x}}$${\mathrm{Th}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Be}}_{13}$. The critical temperatures for the pure system are assumed such that $T_2$>$T_0$. We suggest that impurity scattering strongly suppresses $T_2$ but barely affects $T_0$ so that the above inequality is reversed for an impurity concentration x>$x_c$. For special choices of order parameters, the second component is switched on continuously in the pure system (x<$x_c$) but via a second-order phase transition in the impure system (x>$x_c$).

We report the first measurements of the normal-state reflectivity and superconducting energy gap of ${\mathrm{Y}}_1$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ single crystals. With the electric field in the metallic, a-b plane of our platelike crystals we observe a Drude normal-state reflectivity, and an enhancement of the reflectivity at temperatures below ${\mathrm{T}}_{\mathrm{c}}$(90 K) which follows the Mattis-Bardeen form. The superconducting energy gap obtained from these measurements is found to be temperature dependent, increasing to a value of 2Δ/${\mathrm{kT}}_{\mathrm{c}}$≃8 below ≃50 K. Within the Eliashberg-BCS framework this unusually large gap value suggests that ${\mathrm{Y}}_1$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ is a very strongly coupled superconductor.

We have observed ultrafast heat transport in thin gold films under femtosecond laser irradiation. Time-of-flight (from-pump back-probe) measurements indicate that the heat transit time scales linearly with the sample thickness, and that heat transport is very rapid, occurring at a velocity close to the Fermi velocity of electrons in Au.

Analysis of four-terminal magnetoresistance data (published recently) from single-crystal cylinders of potassium indicates that open orbits, created by a charge-density-wave structure, are present. Apparent negative resistance for some orientations of the magnetic field shows that large charge-density-wave Q domains can cause the current to follow a serpentine path having a reversed direction between the voltage contacts.