Collisionless magnetic reconnection in regimes where the mode structure is characterized by global convection cells is found to exhibit a quasiexplosive time behavior in the early nonlinear stage where the fluid displacement is smaller than the equilibrium scale length. This process is accompanied by the formation of a current density sublayer narrower than the skin depth. This sublayer keeps shrinking with time.

Dense z-pinch plasmas initiated from a single 25–100 μm diam aluminum wire using a 100–350 kA, 100 ns current pulse have been studied with ≤1 ns time resolution. Rapid unstable expansion of a coronal plasma formed around the wire was observed with a subnanosecond pulsed nitrogen laser, while a dense core, which expanded more slowly and stably, was observed with 1–2 ns x-ray backlighting pulses. The core contained most of the initial wire mass, but there appeared to be little or no current flowing in it.

The continuum equations of Burton, Cabrera, and Frank are extended to include thermal fluctuations and used to derive a nonlinear stochastic equation describing the meandering of an isolated step on a crystal face grown from a vapor. Meandering is found to result from a unique competition between thermal noise, which dominates close to equilibrium, and deterministic noise (spatiotemporal chaos), which becomes increasingly dominant beyond the morphological instability point. Numerical and analytical results characterizing the step roughness as a function of the supersaturation and the noise strength, ${\mathit{k}}_{\mathit{B}}$T/γ${\mathit{x}}_{\mathit{s}}$, are presented.

We have studied solid HD to pressures in excess of 100 GPa and to liquid helium temperatures. We have found the broken symmetry quantum phase transition of orientational order by means of Raman scattering. The pressure-temperature phase line has a very unusual reentrant behavior, different from the homonuclear isotopes when measured on the same time scale. The phase line is explained in terms of the rapid thermalization of rotational levels with changing temperature.

The effect of inhomogeneous couplings of the scaling behavior in critical phenomena is considered using renormalization group analysis, Monte Carlo simulations, and other numerical tests. Novel scaling relationships result from nonuniform perturbations of a uniform fixed point at criticality. An effective dimension entering scaling equalities is elucidated. Nonuniversal behavior is found in fully connected systems with exchange couplings arrayed in a hierarchical manner.

In the compressed incommensurate phase at low temperatures, the molecular axes of CO on graphite are ordered with respect to each other, forming a pinwheel structure. A sharp heat capacity peak is found upon warming through the (orientational) order-disorder transition of this pin wheel phase. Critical analysis of the peak indicates that this transition is continuous and belongs to the two-dimensional Ising universality class, in agreement with theoretical expectations.

Debye-Hückel (DH) theory predicts phase separation in the primitive model electrolyte: hard spheres of diameter a with charges ±q. The coexistence curve (CC) is acceptable with ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$≡${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$a/${\mathit{q}}^2$=1/16, which is roughly correct, but the critical density, ${\mathrm{\rho }}_{\mathit{c}}^{\mathrm{*}}$≡${\mathrm{\rho }}_{\mathit{c}}$${\mathit{a}}^3$=1/64π, is far too low. Allowing for association into ideal dipolar pairs, following Bjerrum, improves ${\mathrm{\rho }}_{\mathit{c}}$ and leaves ${\mathit{T}}_{\mathit{c}}$ unchanged but yields an unphysical CC. Extending DH theory to compute the dipole-ionic-fluid coupling yields a mean-field description with sensible CC and (${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$,${\mathrm{\rho }}_{\mathit{c}}^{\mathrm{*}}$) close to Monte Carlo based estimates: (0.057±$1_5$ , 0.030∓8).

We show that Gaussian quantum fluctuations, even if infinitesimal, are sufficient to destroy the superfluidity of a disordered boson system in 1D and 2D. The critical disorder is thus finite no matter how small the repulsion is between particles. Within the Gaussian approximation, we study the nature of the elementary excitations, including their density of states and mobility edge transition. We give the Gaussian exponent η at criticality in 1D and show that its ratio to η of the pure system is universal.

Submonolayer Fe films on a Au(001) surface were found to exhibit atomic place exchanges with Au atoms during room temperature growth by using the angular profile measurement of high-resolution low-energy electron diffraction. The place exchanged atoms result in 2D Au surface islands and Fe subsurface structures. These inverted 2D islands form a structure with a roughly equal island size and spacing in ∼0.5 monolayer FeAu(001) surfaces. The observed mechanism of the atomic place exchange is consistent with recent first-principles calculations for a heteroepitaxial system.

We study a recently proposed mean-field theory relevant for diffusion-limited growth at finite undercooling. This theory exhibits a morphology transition from convex patterns, typical of the dense-branching morphology envelope, to concave ones, typical of the dendritic morphology envelope. This transition occurs as the undercooling is lowered at fixed anisotropy, but does not involve any discontinuous behavior of growth velocity.

An O(N) and rapidly convergent method for calculating the bond order potential and forces for the atomistic simulation of covalent systems is described. A derivation of a novel many-atom expansion, which allows the Green’s functions, not only to handle the symmetry problem correctly but also to retrieve equivalence between the site-diagonal and intersite descriptions for the bond energy, is given. The convergence of the bond orders and bond energies is demonstrated for d-valent transition metals.

We report the first direct observation of the cyclotron motion of new quasiparticles in a 2D electron system near 1/2 filling factor. Just as electrons in a metal geometrically resonate with a sound wave when a magnetic field is applied perpendicular to the sound propagation direction, near 1/2 filling factor we observe resonance of a surface sound wave with cyclotron orbits of charge carriers. The presence of a Fermi surface and geometric resonance of a sound wave with Chern-Simons gauge transformed fermions were explicitly predicted in a recent theory of the half filled Landau level.

According to recent theories, a system of electrons at the half-filled Landau level can be transformed to an equivalent system of composite fermions at zero effective magnetic field. In order to test for these new particles, we have studied transport in antidot superlattices in a two-dimensional electron gas. At low magnetic fields electron transport exhibits well-known resonances at fields where the classical cyclotron orbit becomes commensurate with the antidot lattice. At ν=1/2 we observe the same dimensional resonances. This establishes the semiclassical behavior of composite fermions.

The dynamics of flux vortices in Pb films has been investigated with single fluxon resolution. With increasing magnetic field, there is a crossover from a region where single vortex effects dominate to one where vortex motion becomes correlated. We have measured the flux bundle correlation radius directly and tracked single vortices or correlated bundles as they moved under the action of an applied dc current or in response to a change in temperature. The signatures of flux flow and flux creep have been identified, enabling us to determine the pinning force at individiual pinning sites.

The Hall electric field ${\mathit{E}}_{\mathit{y}}$ is observed to be a strongly nonlinear function of the current density ${\mathit{J}}_{\mathit{x}}$ in the vicinity of the vortex-glass transition of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films in a magnetic field. Much like the longitudinal electric field ${\mathit{E}}_{\mathit{x}}$,${\mathit{E}}_{\mathit{y}}$ apears to scale critically with ${\mathit{J}}_{\mathit{x}}$, with a Hall-related critical exponent λ=3.4±0.3. Furthermore, the Hall resistivity ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$=${\mathit{E}}_{\mathit{y}}$/${\mathit{J}}_{\mathit{x}}$ and the longitudinal resistivity ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$=${\mathit{E}}_{\mathit{x}}$/${\mathit{J}}_{\mathit{x}}$ are found to obey ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$∝${\mathrm{\rho }}_{\mathit{x}\mathit{x}}^{2.0\pm 0.2}$ over a wide range of current densities and temperatures. The results quantitatively verify recent theoretical predictions.

The microscopic magnetic field distribution in the mixed state of single-crystal ${\mathrm{Bi}}_{2.15}$${\mathrm{Sr}}_{1.85}$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ has been measured using the muon spin rotation technique. A study of the line shapes as a function of temperature and magnetic field gives evidence for an abrupt change of flux-lattice structure which is attributed to the presence of a flux-lattice melting line in the B-T phase diagram. In addition, the data suggest the existence of a crossover field above which the flux structure is more 2D in nature.

We have studied the one-dimensional Kondo chain at half filling using a density matrix formulation of the numerical renormalization group. The charge gap is larger than the spin gap for all antiferromagnetic values of the exhange coupling J. A new type of excitation, a neutral spin singlet, consists of a particle and a hole which are repulsive for J≳5t and attractive for J≲5t. As J→0, RKKY interactions become more important and the staggered susceptibility χ(q=2${\mathit{k}}_{\mathit{F}}$) diverges. We have also studied χ(q) and the dispersion of the low-lying spin excitations as a function of wave vector q.

The exchange coupling in multilayers of iron and chromium is calculated in the local spin density approximation, as a function of the Fe magnetic moment. We find a short-period and a long-period oscillation, which we attribute to a harmonic of the short period. For small moments, the interlayer coupling is found to be bilinear in the Fe moments, as assumed in RKKY theory. For moments appropriate to real iron the coupling tends to saturate, as assumed in quantum-well models, demonstrating that the quantum-well model more correctly describes the real iron-chromium system, though both models give correct periods of the oscillation.

The cyclotron resonance on open orbits is suggested to demonstrate the one dimensional character of electron motion along the main conducting axis in the presence of a perpendicular magnetic field for organic materials with warped open Fermi surfaces. Singular behavior at resonant frequencies in the surface impedance and the helicon waves spectrum are obtained.

We demonstrate experimentally that the exciton luminescence rise times in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells oscillate as a function of laser excess energy. We interpret these results as the occurrence of a selective optical-phonon assisted exciton formation. Experiments on doped quantum wells confirm our exciton formation model.