Dynamics of Nucleation in the Ising Model

Reactive pathways to nucleation in a three-dimensional Ising model at 60% of the critical temperature are studied using transition path sampling of single spin flip Monte Carlo dynamics. Analysis of the transition state ensemble (TSE) indicates that the critical nuclei are rough and anisotropic. The TSE, projected onto the free energy surface characterized by cluster size, N, and surface area, S, indicates the significance of other variables in addition to these two traditional reaction coordinates for nucleation. The transmission coefficient along N is ~ 0.35, and this reduction of the transmission coefficient from unity is explained in terms of the stochastic nature of the dynamic model.Comment: In press at the Journal of Physical Chemistry B, 7 pages, 8 figure

Aluminum arsenide cleaved-edge overgrown quantum wires

We report conductance measurements in quantum wires made of aluminum arsenide, a heavy-mass, multi-valley one-dimensional (1D) system. Zero-bias conductance steps are observed as the electron density in the wire is lowered, with additional steps observable upon applying a finite dc bias. We attribute these steps to depopulation of successive 1D subbands. The quantum conductance is substantially reduced with respect to the anticipated value for a spin- and valley-degenerate 1D system. This reduction is consistent with disorder-induced, intra-wire backscattering which suppresses the transmission of 1D modes. Calculations are presented to demonstrate the role of strain in the 1D states of this cleaved-edge structure.Comment: Submitted to Applied Physics Letter

Classical-to-stochastic Coulomb blockade cross-over in aluminum arsenide wires

We report low-temperature differential conductance measurements in aluminum arsenide cleaved-edge overgrown quantum wires in the pinch-off regime. At zero source-drain bias we observe Coulomb blockade conductance resonances that become vanishingly small as the temperature is lowered below $250 {\rm mK}$. We show that this behavior can be interpreted as a classical-to-stochastic Coulomb blockade cross-over in a series of asymmetric quantum dots, and offer a quantitative analysis of the temperature-dependence of the resonances lineshape. The conductance behavior at large source-drain bias is suggestive of the charge density wave conduction expected for a chain of quantum dots.Comment: version 2: new figure 4, refined discussio

Infrared Hall effect in high Tc superconductors: Evidence for non-Fermi liquid Hall scattering

Infrared (20-120 cm-1 and 900-1100 cm-1) Faraday rotation and circular dichroism are measured in high Tc superconductors using sensitive polarization modulation techniques. Optimally doped YBCO thin films are studied at temperatures down to 15 K and magnetic fields up to 8 T. At 1000 cm-1 the Hall conductivity varies strongly with temperature in contrast to the longitudinal conductivity which is nearly independent of temperature. The Hall scattering rate has a T^2 temperature dependence but, unlike a Fermi liquid, depends only weakly on frequency. The experiment puts severe constraints on theories of transport in the normal state of high Tc superconductors.Comment: 8 pages, 3 figure

Improving the Sensitivity of LISA

It has been shown in the past, that the six Doppler data streams obtained LISA configuration can be combined by appropriately delaying the data streams for cancelling the laser frequency noise. Raw laser noise is several orders of magnitude above the other noises and thus it is essential to bring it down to the level of shot, acceleration noises. A rigorous and systematic formalism using the techniques of computational commutative algebra was developed which generates all the data combinations cancelling the laser frequency noise. The relevant data combinations form a first module of syzygies. In this paper we use this formalism for optimisation of the LISA sensitivity by analysing the noise and signal covariance matrices. The signal covariance matrix, averaged over polarisations and directions, is calculated for binaries whose frequency changes at most adiabatically. We then present the extremal SNR curves for all the data combinations in the module. They correspond to the eigenvectors of the noise and signal covariance matrices. We construct LISA `network' SNR by combining the outputs of the eigenvectors which improves the LISA sensitivity substantially. The maximum SNR curve can yield an improvement upto 70 % over the Michelson, mainly at high frequencies, while the improvement using the network SNR ranges from 40 % to over 100 %. Finally, we describe a simple toy model, in which LISA rotates in a plane. In this analysis, we estimate the improvement in the LISA sensitivity, if one switches from one data combination to another as it rotates. Here the improvement in sensitivity, if one switches optimally over three cyclic data combinations of the eigenvector is about 55 % on an average over the LISA band-width. The corresponding SNR improvement is 60 %, if one maximises over the module.Comment: 16 pages, 10 figures, Submitted to Class. Quant. Gravit

Time-dependent distinguishability: Choosing to be a wave or a particle

Interference experiments with connected parametric down-converters have demonstrated that the possibility, in principle, of identifying the photon path through the interferometer is sufficient to wipe out all interference, irrespective of whether the identification is actually made. The distinguishability of the photon path can be controlled by a time-dependent shutter, which leaves the choice whether the photon behaves as a wave or as a particle in the experimenter's hands. By contrast, in some more recent experiments involving the addition of a low-Q cavity, each idler photon makes the choice whether the associated signal photon behaves like a wave and exhibits interference, or like a particle

Generic Tracking of Multiple Apparent Horizons with Level Flow

We report the development of the first apparent horizon locator capable of finding multiple apparent horizons in a ``generic'' numerical black hole spacetime. We use a level-flow method which, starting from a single arbitrary initial trial surface, can undergo topology changes as it flows towards disjoint apparent horizons if they are present. The level flow method has two advantages: 1) The solution is independent of changes in the initial guess and 2) The solution can have multiple components. We illustrate our method of locating apparent horizons by tracking horizon components in a short Kerr-Schild binary black hole grazing collision.Comment: 13 pages including figures, submitted to Phys Rev

Momentum-Resolved Tunneling into Fractional Quantum Hall Edges

Tunneling from a two-dimensional contact into quantum-Hall edges is considered theoretically for a case where the barrier is extended, uniform, and parallel to the edge. In contrast to previously realized tunneling geometries, details of the microscopic edge structure are exhibited directly in the voltage and magnetic-field dependence of the differential tunneling conductance. In particular, it is possible to measure the dispersion of the edge-magnetoplasmon mode, and the existence of additional, sometimes counterpropagating, edge-excitation branches could be detected.Comment: 4 pages, 3 figures, RevTex

Two-dimensional hole precession in an all-semiconductor spin field effect transistor

We present a theoretical study of a spin field-effect transistor realized in a quantum well formed in a p--doped ferromagnetic-semiconductor- nonmagnetic-semiconductor-ferromagnetic-semiconductor hybrid structure. Based on an envelope-function approach for the hole bands in the various regions of the transistor, we derive the complete theory of coherent transport through the device, which includes both heavy- and light-hole subbands, proper modeling of the mode matching at interfaces, integration over injection angles, Rashba spin precession, interference effects due to multiple reflections, and gate-voltage dependences. Numerical results for the device current as a function of externally tunable parameters are in excellent agreement with approximate analytical formulae.Comment: 9 pages, 11 figure

Optical Conductivity and Hall Coefficient in High-Tc Superconductors: Significant Role of Current Vertex Corrections

We study AC conductivities in high-Tc cuprates, which offer us significant information to reveal the true electronic ground states. Based on the fluctuation-exchange (FLEX) approximation, current vertex corrections (CVC's) are correctly taken into account to satisfy the conservation laws. We find the significant role of the CVC's on the optical Hall conductivity in the presence of strong antiferromagnetic (AF) fluctuations. This fact leads to the failure of the relaxation time approximation (RTA). As a result, experimental highly unusual behaviors, (i) prominent frequency and temperature dependences of the optical Hall coefficient, and (ii) simple Drude form of the optical Hall andge for wide range of frequencies, are satisfactorily reproduced. In conclusion, both DC and AC transport phenomena in (slightly under-doped) high-Tc cuprates can be explained comprehensively in terms of nearly AF Fermi liquid, if one take the CVC's into account.Comment: 5 page