A method for direct determination of the threshold energy (${\mathit{E}}_{\mathit{d}}$) for displacing atoms by low-energy ion irradiation to form residual point defects is described. The method is demonstrated for ${\mathrm{Ne}}^{+}$ irradiation of graphite. The damage induced by low ${\mathrm{Ne}}^{+}$ doses (<2×${10}^{15}$ ions/${\mathrm{cm}}^2$) was quantified by means of a defect-sensitive feature in the Auger electron spectrum. The defect production rates at different ${\mathrm{Ne}}^{+}$ energies yield a displacement energy of ${\mathit{E}}_{\mathit{d}}$=35.3±1 eV. This result provides new insight into the elementary collision processes leading to point defect creation.

Steady-state domain walls and kinks have been observed in one-dimensional nonlinear lattices that are damped and parametrically driven. These states are localized robust transition regions between two extended standing-wave domains of definite wave number. The observations are made in an experimental lattice of coupled pendulums and in simplified numerical models. A nonlinear Schrödinger theory is developed for kinks in the upper cutoff mode. There is currently no theory for the domain walls and noncutoff kinks, which are fundamentally new localized structures.

We present a rigorous description of finite-size effects for a large class of models with an asymmetric first-order transition, assuming that all phases contributing to the transition have a finite correlation length. If the model describes the coexistence of two phases, it is shown that, at sufficiently low temperatures, the shift of the transition point due to finite-size effects in a volume ${\mathit{L}}^{\mathit{d}}$ with periodic boundary conditions is O(${\mathit{L}}^{\mathrm{-}2\mathit{d}}$), in contrast to certain claims in the literature. We also discuss different ways to determine the transition point from finite-size data, which involve only exponentially small systematic errors in L.

We consider a temperature-driven first-order phase transition describing the coexistence of q ordered low-temperature phases and one disordered high-temperature phase at the infinite-volume transition temperature ${\mathit{T}}_0$. Analyzing the exponential corrections to the periodic partition function in a box of volume V, ${\mathit{Z}}_{\mathrm{per}}$=${\mathit{tsum}}_{\mathit{m}}$ exp(-β${\mathit{f}}_{\mathit{m}}$V), where ${\mathit{f}}_{\mathit{m}}$ is the (metastable) free energy of the phase m, we propose several definitions of a finite-volume transition temperature ${\mathit{T}}_0$(V) which involve only exponential corrections with respect to ${\mathit{T}}_0$. We test our propositions in the d=2 Potts model for q=5, 8, and 10 by means of Monte Carlo simulations, using the single-cluster update procedure.

The early-stage kinetics of a second-order phase transition with a nonconserved order parameter (the B2-${\mathit{DO}}_3$ transition in ${\mathrm{Fe}}_3$Al) has been investigated using in situ synchrotron x-ray scattering. A line-shape analysis allows the short- and long-range-order components to be determined separately as a function of time during the ordering process. At all temperatures investigated, the correlation length of short-range-order fluctuations ξ(t) grows to a maximum and then decreases, while the long-range-order domain size L(t) increases monotonically from an initial value 1 order of magnitude larger than ξ.

Using the functional integral for many-boson systems, we propose a new formulation of quantum vortices in two-dimensional superfluids. The motion of vortices is described by a generalized Hamiltonian dynamics in which the equation of motion is governed by a deformed canonical structure. The quantum correspondence of this unusual feature is realized by the anomalous commutators for vortex coordinates. In terms of a semiclassical language, this anomaly yields a consequence that vortex quanta behave like ‘‘anyons.’’

We study the many-body exchange-correlation properties of electrons confined to the lowest subband of a quantum wire, including effects of impurity scattering. Without impurity scattering, virtual excitations of arbitrarily low-energy plasmons destroy the Fermi surface of the electrons, whereas the presence of impurity scattering damps out these plasmons and restores he Fermi surface. The electron inelastic scattering rate Γ in the absence of scattering is zero below ${\mathit{k}}_{\mathit{c}}$ corresponding to the plasmon emission threshold, above which Γ diverges as (k-${\mathit{k}}_{\mathit{c}}$${)}^{\mathrm{-}1/2}$ as k→${\mathit{k}}_{\mathit{c}}$. For typical wire widths and electron densities currently available, the band-gap renormalization is found to be 10–20 meV.

We have observed a series of sharp peaks in the low-temperature I(V) characteristics of a gated 1 μm×1 μm GaAs/(AlGa)As resonant tunneling diode, in which the gate is used to reduce the effective cross-sectional area from 0.7 to <0.1 μ${\mathrm{m}}^2$. These peaks, which occur at voltages well below the calculated resonant threshold, show a weak dependence on temperature, magnetic field, and cross-sectional area. We argue that this subthreshold structure is due to an inhomogeneity in the device, which gives rise to a localized preferential current path, and we deduce that the spatial extent of the inhomogeneity is approximately 25 nm. The likely origin of the inhomogeneity is a background donor impurity in the quantum well.

The nonlocal elasticity and the normal modes of an unpinned flux-line lattice (FLL) in a uniaxially anisotropic supeconductor are considered in the low-induction regime b=B/${\mathit{B}}_{\mathit{c}2}$≪1 at oblique angles θ between B and the c^ axis. A novel feature of anisotropic superconductors is strongly dispersive shear moduli when θ>0. Thus, the normal modes of the FLL may become soft away from the zone center, signaling a k≠0 structural instability of the distorted hexagonal FLL. Vortex structures of a novel type are thus possible in anisotropic superconductors.

Though much of the recent work on 2D quantum canted antiferromagnets has been devoted to the study of disordered states, there is evidence that some of these systems have Néel ground states, even for spin 1/2. Following Chakravarty, Halperin, and Nelson, we study the quantum nonlinear sigma model suited to these systems and obtain, for the correlation length, ξ=${\mathit{C}}_{\xi }$[A/(${\mathit{k}}_{\mathit{B}}$T${)}^{1/2}$]${\mathit{e}}_{\mathit{B}}^{2\mathrm{\pi }\mathit{B}/\mathit{k}}$T, where A and B depend on the spin stiffness and spin-wave velocities renormalized by quantum fluctuations. We discuss experiments on ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ adsorbed on Grafoil.

We study the possibility of √3 × √3 antiferromagnetic order in the S=1/2 triangular- and Kagomé-lattice Heisenberg models. An Ising-like anisotropy is introduced into the Hamiltonian, which picks a pair of ground states out of the manifold of the classically ordered states. To study properties of the Heisenberg model, we develop series expansions around one ordered state. We find that the Kagomé-lattice model is disordered, whereas the triangular-lattice model is very close to the critical point for antiferromagnetism; if ordered, the latter has an order parameter much smaller than that predicted by spin-wave theory.

We used a combination of the quantum Monte Carlo technique and the maximum entropy method to calculate the dynamic structure factor of the quantum Heisenberg antiferromagnet. The dynamics is governed by renormalized damped spin waves and we calculate their frequencies and linewidths. The computed spin-wave dispersion is in quantitative agreement with inelastic-neutron-scattering experiments on ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. The low-energy sector exhibits scaling behavior with the characteristic frequency scale, which is inversely proportional to the correlation length.

Gadolinium clusters (N=11–92) in a molecular beam exhibit dramatic size dependences in their magnetic properties. Two principal behaviors are observed: rigid rotor precession and nutation when the magnetic moment is tightly locked to the lattice by crystal anisotropy, and superparamagnetism when the moment is approximately free. Partially locked behavior is also seen. Magnetic moments of about 3${\mathrm{\mu }}_{\mathit{B}}$ per atom are observed, with Curie temperatures substantially above the bulk value. Moments that remain locked above 300 K may permit ultrahigh-density magnetic memory.

Unidirectional charge flow was observed in potassium tantalum niobate, ${\mathrm{KTa}}_{0.6}$${\mathrm{Nb}}_{0.4}$${\mathrm{O}}_3$, films. These films of relaxor-type diffuse phase transition material were formed by metalorganic deposition and had a polarization gradient normal to the surface of the films. Novel charge pumping was observed in the active operation of simple two-terminal structures by excitation with periodic, strong electric fields. Such operation also showed a large pseudopyroelectric response. Effective pyroelectric coefficients as large as 2×${10}^{\mathrm{-}5}$ C/${\mathrm{cm}}^2$ °C have been obtained.

We have observed in real time the nature and motion of the silicon(111)/oxide interface during oxidation in situ in a transmission electron microscope. Oxidation occurs by the reaction of discrete monolayers with no flow of surface steps. This is in dramatic contrast to oxygen etching of silicon at high temperatures, which is initially also a terrace reaction, but is followed by an evaporative process from steps.

We study numerically by Padé approximants the analytic properties of the classical perturbation series parametrizing Kolmogorov-Arnold-Moser curves for the standard map. By complexifying the rotation number we show how the appearance of natural boundaries at diophantine real rotation numbers is a consequence of the accumulation of complex lines of singularities related to resonances. This provides a new intuitive understanding of the breakdown mechanism of invariant curves.

It is shown that the naked singular solutions that form in the gravitational spherical collapse of radiation are of the same nature as the ones which form the collapse of dust matter. The stability, the strength, and other features of the singularities and Cauchy horizons are analyzed. This has implications in a possible formulation of the cosmic censorship hypothesis.

The spontaneous breaking of topological symmetry induced by a vanishingly small symmetry breaking term is investigated. It is shown that, in the presence of Gribov zero modes, topological theories without smearing terms, which are inequivalent to theories with smearing terms, permit spontaneous symmetry breaking. The relation with reducible configurations is briefly discussed.

The most popular model of spontaneous R-parity breaking is analyzed. It is shown that the extremum of the potential found in the model is, in fact, an unstable saddle point, while in the true minimum R parity is not broken. Other existing models involving R-parity breaking are briefly discussed and it is pointed out that none of them can describe successfully the existing experimental data.

The charge asymmetry of leptons from W-boson decay has been measured using p¯p data from the Collider Detector at Fermilab at √s =1.8 TeV. The observed asymmetry is well described by most of the available parton distributions.