Using the method of optimal fluctuation, we investigate the asymptotic behavior of the phonon density of states in the low-frequency limit ω→0. If the pinning potential is not too strong, both the density of states g(ω) and the ac conductivity at zero temperature σ(ω) are found to be proportional to ${\mathrm{\omega }}^4$. This power law is valid in both one- and two-dimensional crystal and does not change in the presence of magnetic field.

It is well known that in a classically strong magnetic field, ${\mathrm{\omega }}_{\mathit{c}}$${\mathrm{\tau }}_{\mathit{t}}$≫1 (${\mathrm{\omega }}_{\mathit{c}}$ is the cyclotron frequency and ${\mathrm{\tau }}_{\mathit{t}}$ is the transport relaxation time), the transverse conductivity is inversely proportional to magnetic field squared, ${\mathrm{\sigma }}_{\mathit{x}\mathit{x}}$=${\mathrm{\sigma }}_0$/(${\mathrm{\omega }}_{\mathit{c}}$${\mathrm{\tau }}_{\mathit{t}}$${)}^2$, wher ${\mathrm{\sigma }}_0$ is the conductivity without magnetic field. Such a dependence results from the lack of correlation of scattering events of an electron. This condition can be violated in a high mobility 2D gas, where the correlation length of the scattering potential is large. A strong correlation between scattering events results in an anomalous dependence of ${\mathrm{\sigma }}_{\mathit{x}\mathit{x}}$ on magnetic field.

The temperature (T) and voltage (V) dependence of conductance signals in metal point contacts, previously asserted to be due to 2-channel Kondo scattering from atomic 2 level tunneling systems, collapse onto a universal scaling curve dependent on eV/${\mathit{k}}_{\mathit{B}}$T, for all T and eV/${\mathit{k}}_{\mathit{B}}$ below a characteristic Kondo scale. Measurements determine a conductance exponent of 1/2, as expected from conformal field theory solution of the 2-channel Kondo model. The magnetic field (H) dependence at low T is nonanalytic at H=0 (∝‖H‖) in contrast to Fermi-liquid theory but in agreement with conformal field theory.

We measured dynamical dI/dV and ${\mathit{d}}^2$I/${\mathit{dV}}^2$ of carbon-wax mixtures near the percolation threshold as a function of dc voltage V. Scaled appropriately, dI/dV at small V fall on the same universal curve as one which also contains the real part of the frequency dependent conductance data. We find that at small q (scaled voltage or frequency), Σ(q)/Σ(0)-1 varies as ${\mathit{q}}^{\mathit{v}}$ where Σ is conductance and v is 1.36±0.06. An empirical form of the full scaling function that agrees well with known facts is proposed. We discuss how our results could be used to probe dangling clusters in the periphery of the conducting backbone, hitherto largely unexplored. Relevance to other systems like amorphous semiconductors is discussed.

We report on the first observation of highly spin-polarized nearly free-electron image-potential states. At the surface Brillouin zone boundary Y¯ of Co(101¯0) spin-polarized inverse photoemission reveals large exchange splittings of 125 meV and 96 meV for two n=1 image states, separated by 0.6 eV due to the surface corrugation potential. The exchange splittings are nearly as large as the intrinsic linewidths so that electrons trapped in these two-dimensional states are highly spin polarized. By their large exchange splitting these image states serve as a test case for a possible spin dependence of the surface-barrier potential.

The influence of charging effects on time-dependent transport in small semicondutor quantum dots with arbitrary level spectra is studied. Starting from an explicit time-dependent tunneling Hamiltonian, a non-Markovian master equation is derived which is also valid in the nonlinear response regime. The many-body nonequilibrium distribution functions of the dot are calculated and the I-V characteristic of the structure including the displacement currents is obtained. New resonant features show up in the Coulomb oscillations and in the Coulomb staircase, and a new possibility to realize electronic pumps is described.

Finite-size corrections to scaling laws in the centers of Landau levels are studied systematically by numerical calculations. The corrections can account for the apparent nonuniversality of the localization length exponent ν. In the second lowest Landau level the irrelevant scaling index is ${\mathit{y}}_{\mathrm{irr}}$=-0.38±0.04. At the center of the lowest Landau level an additional periodic potential is found to be irrelevant with the same scaling index. These results suggest that the localization length exponent ν is universal with respect to the Landau level index and an additional periodic potential.

We propose a way to measure the interference associated with two superconductors with a ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ symmetry coupled by a Josephson weak link. An experimental realization shows no interference effects in a superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ film. The results are consistent with s-wave symmetry.

The first order vortex melting transition in single crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is investigated for magnetic field direction close to H∥ab. For exact field alignment H∥ab, the ‘‘kink’’ in ρ(T) associated with the first order melting transition is replaced with a continuous resistive transition in the presence of intrinsic pinning, indicative of a second order transition with dynamic scaling exponent s=1.4. In addition, we demonstrate the abatement of the ‘‘kink’’ in the temperature dependence of the magnetoresistivity, associated with the first order melting transition for H∥c in the presence of only six twin boundaries.

We present transport measurements in high quality untwinned crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ in fields up to 8 T. We find an extremely sharp resistive transition at the first order vortex freezing transition ${\mathit{T}}_{\mathit{m}}$, with width much narrower than the zero field transition width. The resistive transition as a function of magnetic field is equally sharp with freezing occurring within 10 Oe in a field of about 6 T. There is dramatic asymmetric hysteresis in the field dependence of the resistivity related to the unpinned nature of the vortex liquid state.

We report a neutron scattering study of the spin correlations in the model 2D, S=1/2, square-lattice Heisenberg antiferromagnetic ${\mathrm{Sr}}_2$${\mathrm{CuO}}_2$${\mathrm{Cl}}_2$. The spin correlation lengths obtained agree quantitatively with values deduced from Monte Carlo simulations over a wide range of temperature. The combined data, which cover the length scale from 1 to 200 lattice constants, are predicted accurately with no adjustable parameters by renormalized classical theory for the quantum nonlinear sigma model.

The low frequency dielectric response of relaxor ferroelectrics is analyzed. Following the conventional approach this response is considered as a sum of those of relaxators having an exponentially wide and smooth spectrum of the relaxation times. It is shown that the Vogel-Fulcher relationship for the positions of the temperautre maximum for the real and imaginary parts of the dielectric permittivity can be obtained as a direct consequence of gradual broadening of the spectrum with decreasing temperature. It is shown that such a Vogel-Fulcher–type relationship does not necessarily imply ‘‘freezing’’ in the system.

We observe competition between picosecond photoinduced absorption and stimulated emission in poly(paraphenylenevinylene) which demonstrates that they derive from two different pathways. We conclude that the photoinduced absorption originates from nonemissive ‘‘spatially indirect’’ singlet excitons which are formed with quantum yield as high as 0.9. Previous arguments about maximum possible electroluminescence device efficiencies and about lasers in conjugated polymer films are reexamined in light of the prevalence of this species.

Positron trapping at charged vacancies ${\mathit{V}}_{\mathrm{Hg}}^0$, ${\mathit{V}}_{\mathrm{Hg}}^{1\mathrm{-}}$, and ${\mathit{V}}_{\mathrm{Hg}}^{2\mathrm{-}}$, in ${\mathrm{Cd}}_{0.2}$${\mathrm{Hg}}_{0.8}$Te has been investigated. Doppler measurements reveal the temperature dependence of the trapping rates and provide evidence for the predicted existence of Rydberg precursor states. This first direct observation of the vacancy charge states yielded energies of 18 meV and 41 meV for the singly and doubly ionized states, respectively.

X-ray magnetic circular dichroism measurements reveal that Cu atoms in Co/Cu multilayers exhibit an induced magnetic spin moment in the d shell. The magnitude and sign of the moments and their variation with Cu thickness are in excellent agreement with moments calculated from a local spin density functional theory. The results indicate that the Cu d electrons play a large role in mediating the exchange coupling between successive Co layers.

We report the suppression of electron-stimulated desorption of positive ions (${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$) from a ${\mathrm{TiO}}_2$ (110) surface caused by adsorption of a fractional monolayer of molecular ${\mathrm{NH}}_3$. A linear decrease of ${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$ emission with ${\mathrm{NH}}_3$ coverage is observed. This system allows us to distinguish between neutralization of desorbing ions via interaction with the substrate or with adsorbed molecules. We propose a novel charge exchange mechanism where electron transfer from the occupied orbitals of the adsorbed molecules to the desorbing ions causes the decrease in detected ion yield. Charge transfer cross sections of 2.8(±0.5) and 2.7(±1) (×${10}^{\mathrm{-}15}$) ${\mathrm{cm}}^2$ have been determined for ${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$ ions, respectively.

Pattern formation exhibited by a two-dimensional reaction-diffusion system in the fast inhibitor limit is considered for the point of view of interface motion. A dissipative nonlocal equation of motion for the boundary between high and low concentrations of the slow species is derived heuristically. Under these dynamics, a compact domain of high concentration may develop into a space-filling labyrinthine structure in which nearby fronts repel. Similar patterns have been observed recently by Lee, McCormick, Ouyang, and Swinney in a reacting chemical system.

We have evaluated the sticking probability and activation energy for dissociation of ${\mathrm{H}}_2$ molecules on a Cu(110) surface by a reversible work formulation of quantum transition state theory. Feynman path integrals are used to describe the two hydrogen atoms and a few of the surface atoms, thereby including quantum effects such as tunneling and zero point energy, as well as thermal averaging. At a temperature below 600 K an onset of a quantum regime is observed as the activation energy drops by 30%.

We show that, in a narrow gap in the spectrum of two linearly coupled Korteweg–de Vries equations with opposite signs of the dispersion coefficient, a two-parameter family of solitons of a novel type may exist. These are envelope solitons with decaying oscillating tails, which are radically different from the gap solitons previously known in nonlinear optics. In particular, they may become singular at some value of the velocity, and degenerate into algebraic solitons in another special case. It is demonstrated that gap solitons of the same type may also exist in a nonlinear optical system consisting of focusing and defocusing tunnel-coupled planar lightguides.

We consider the geometric phase factor in quantum mechanics for the case of the continuous spectrum, and we find that it is nothing but the S matrix; for certain specific states the geometric phase is simply twice the usual phase shift.