We demonstrate the existence of transient ordered phases in alloys which, at equilibrium, exhibit a definite tendency toward segregation. This is shown through a Monte Carlo study of the decomposition kinetics of a bcc TiV alloy based on first-principles electronic structure calculations. Specifically, we predict a transient B32 ordering of the quenched-in solid solution before separation into Ti- and V-rich disordered phases. We discuss the requirements for such unusual behavior in relation to experiment, and compare Monte Carlo results for a 2D model system with mean-field predictions.

We study the resonant tunneling of quasiparticles (anyons) through an impurity between the edges of a fractional quantum Hall sample. These quasiparticles are anyons, but their equilibrium one-particle correlation functions are shown to have some quasi Fermi properties. The broken symmetry of many-particle states at the impurity yields a new selection rule for the resonant tunneling: The resonance is suppressed unless an integer number of electrons occupies the impurity. This rule allows an explanation of the scaling behavior observed in the mesoscopic fluctuations of the conductivity in the fractional quantum Hall effect.

Significant gap anisotropy within the a-b plane of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ has been observed with a low temperature scanning tunneling microscope. The energy gap is largest along the a-axis direction, with ${\mathrm{\Delta }}_{\mathit{p}\mathrm{-}\mathit{p}}$=36.6 meV and decreases gradually to a value of 23 meV along the direction 45° from the a axis. The tunneling data have been fit well using a simple model which assumes a d-wave-pairing energy gap, and allows tunneling in all directions close to the normal of the surface.

The cosmic microwave background anisotropy is sensitive to the slope and amplitude of primordial energy density and gravitational wave fluctuations, the baryon density, the Hubble constant, the cosmological constant, the ionization history, etc. In this Letter, we examine the degree to which these factors can be separately resolved from combined small- and large-angular-scale anisotropy observations. We isolate directions of degeneracy in this cosmic parameter space, but note that other cosmic observations can break the degeneracy.

We have performed a theoretical study of the effects of pinning potential and dissipation on vortex tunneling in superconductors. Analytical results are obtained in various limits relevant to experiment. In general we have found that pinning and dissipation tend to suppress the effect of the vortex velocity dependent part of the Magnus force on vortex tunneling.

Neutron scattering measurements show ${\mathrm{UNi}}_2$${\mathrm{Al}}_3$ to be unique among heavy fermion superconductors in that below ${\mathit{T}}_{\mathit{N}}$∼5.2 K it displays long range magnetic order which is incommensurate with its chemical lattice. The ordering wave vector is (1/2±δ, 0, 1/2) where δ=0.110±0.003. The measured superlattice intensities are well described by the presence of a longitudinal spin density wave within the hexagonal basal plane, polarized along ${\mathit{a}}^{\mathrm{*}}$. The magnitude of the maximum ordered moment is found to be very small; ${\mathrm{\mu }}_{\mathrm{ord}}$=(0.24±0.10)${\mathrm{\mu }}_{\mathit{B}}$.

Depositing ${\mathrm{C}}_{60}$ on potassium multilayers creates a K-rich environment for low coverages of ${\mathrm{C}}_{60}$ and photoemission spectra show that not only the ${\mathit{t}}_{1\mathit{u}}$ but also the ${\mathit{t}}_{1\mathit{g}}$ band is occupied. This is direct evidence for the formation of ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ with 6<x<12, which could not be achieved in previous studies where K was evaporated onto solid ${\mathrm{C}}_{60}$ surfaces. However, less than one electron can be transferred to ${\mathrm{C}}_{60}$ from submonolayer coverages of K on Rh(111) and hence even the ${\mathit{t}}_{1\mathit{u}}$ band is mostly unoccupied, despite the large K-induced drop in work function from 5.4 to 1.8 eV.

The peculiar effect of electron correlation in multiband systems is illustrated for the recently proposed ferromagnetism in a Hubbard model having flat bands. Depsite the flatness of the band, an acoustic spin-wave mode with a finite stiffness exists along with optical modes below a Stoner gap. The relevant energy (gap and stiffness) transmutes from the Hubbard U in the weak-coupling limit to t (transfer integral) in the strong-coupling limit. This implies a stable itinerant ferromagnetism with finite band widths.

We report on the first investigation of the effect of ‘‘magnetic dilution’’ on nuclear magnetic properties. We studied this effect in the diluted Van Vleck paramagnets ${\mathrm{Pr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Ni}}_5$. In addition, we investigated the electronic magnetic properties of these alloys. For this purpose, we have measured the susceptibility of ${\mathrm{Pr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Ni}}_5$ with x=0 to 0.2 at 50 μK≤T≤8 mk and at 2.4≤T≤300 K. The observed decrease of the electronic Van Vleck and of the hyperfine-enhanced nuclear susceptibilities as well as of the nuclear Curie and Weiss temperatures with decreasing Pr concentration are compared to the results of mean-field calculations.

A self-consistent description of the ultrafast dynamics of photoexcited carriers in semiconductors based on a generalized Monte Carlo solution of the semiconductor Bloch equations is presented. The problem of photogeneration and its theoretical description are discussed. We show that some of the approaches commonly used fail in describing correctly the effect of carrier-carrier interaction in the low-density limit. By including terms which have the structure of ‘‘in-scattering’’ terms (vertex corrections) for the interband polarization, the experimentally observed features in the carrier dynamics are well described in the whole density range.

We have studied electronic excited states in films of poly(p-phenylenevinylene) using picosecond transient and cw photomodulation, photoluminescence, and their excitation spectra, as well as electroabsorption spectroscopy. We have determined all the important energy levels of singlet excitons with odd and even parity, the onset of the continuum band, the two-electron (biexciton) states, and the two relevant triplet states, and show that good agreement exists with models involving electron correlation.

We report the first photobleaching experiments of single molecules on a surface. Single molecules of rhodamine 6G are detected on a silica surface under ambient conditions using near-field scanning optical microscopy. Photobleaching of an individual molecule appears as an abrupt disappearance of fluorescence after seconds of constant excitation. Other reversible, upward and downward changes in fluorescence are observed, possibly due to reorientation of molecules on the surface.

It is shown that the dynamics of spiral waves in excitable media are organized around a codimension-two point where a Hopf bifurcation from rotating waves interacts with Euclidean symmetry. A simple ordinary-differential-equation model of this bifurcation generates dynamics like the ‘‘meandering’’ of spiral waves.

We report the observation of phospholipid vesicles of high topology, exhibiting strong thermal fluctuations. By deeply affecting the global shape of the vesicle, these differ from the usual local thermal undulations of the membrane. They can be described as positional fluctuations of necks linking two nearby concentric membranes. Using boundary layer methods we determine the shape and elastic energy of the necks and corroborate this analysis by a numerical solution of the minimization problem. This approach leads to a qualitatively correct description of our experiments.

The simplest model that can accommodate a viable nonbaryonic dark matter candidate is the standard electroweak theory with the addition of right-handed (sterile) neutrinos. We consider a single generation of neutrinos with a Dirac mass μ and a Majorana mass M for the right-handed component. If M≫μ (standard hot dark matter corresponds to M=0), then sterile neutrinos are produced via oscillations in the early Universe with energy density independent of M. However, M is crucial in determining the large scale structure of the Universe; for M∼100 eV, sterile neutrinos make an excellent warm dark matter candidate.

Experiments on vertically oscillated granular layers reveal, at a critical acceleration, a well-defined transition from a flat surface to standing wave patterns oscillating at half the excitation frequency. The patterns observed in a cylindrical container are squares or stripes; a continuous transition between these states occurs when the vibration frequency is varied at constant acceleration. The dispersion relation for these parametric granular waves is similar to that for gravity waves in a fluid, but exhibits a well-defined cutoff associated with the particle size.

We calculate the heavy quark potential from the magnetic current due to monopoles in four dimensional U(1) lattice gauge theory. The magnetic current is found from link angle configurations generated in a cosine action simulation on a ${24}^4$ lattice. The magnetic current is resolved into large loops which wrap around the lattice and small loops which do not. It is shown that the long range part of the heavy quark potential, in particular the string tension, can be calculated solely from the large loops of magnetic current.

We calculate the two-loop renormalization group equations for the running gaugino masses in general supersymmetry (SUSY) gauge models, improving our previous result. We also study its consequences on the unification of the gaugino masses in the SUSY SU(5) model. The two-loop correction to the one-loop relation ${\mathit{m}}_{\mathit{i}}$(μ)∝${\mathrm{\alpha }}_{\mathit{i}}$(μ) is found to be of the order of a few percent.

In the hadronic decay mode of a pair of W bosons, ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$→${\mathit{W}}^{+}$${\mathit{W}}^{\mathrm{-}}$→${\mathit{q}}_1$q${\mathrm{¯}}_2$${\mathit{q}}_3$${\mathit{qbar}}_4$, QCD interference effects can mix up the two color singlets ${\mathit{q}}_1$q${\mathrm{¯}}_2$ and ${\mathit{q}}_3$q${\mathrm{¯}}_4$, i.e., produce hadrons that cannot be uniquely assigned to either of ${\mathit{W}}^{+}$ and ${\mathit{W}}^{\mathrm{-}}$. We show that interference is negligible for energetic perturbative gluon emission, and develop models to help us to estimate the nonperturbative effects. The total contribution to the systematic error on the W mass reconstruction may be as large as 40 MeV.

Experimental data on the bin size dependence of charged hadron multiplicity distributions in proton-antiproton collisions and electron-positron annihilation are analyzed in terms of multifractals. Linear relations are found between 〈nlnn〉/〈n〉 and ln〈n〉, and also between ln〈${\mathit{n}}^{\mathit{q}}$〉 and ln〈n〉 for q=2,3,4,..., where n is the multiplicity in a single bin of the (pseudo) rapidity space and 〈 〉 stands for the event average. Generalized dimensions ${\mathit{D}}_{\mathit{q}}$ for q=0,1,2, . . . are determined from the slopes. Our result suggests a power law of the form 〈${\mathit{n}}^{\mathit{q}}$〉=c(〈n〉/${\mathit{n}}_0$${)}_{\mathit{q}}^{\mathit{b}}$.