Monte Carlo simulation of a hard-sphere gas in the planar Fourier flow with a gravity field

By means of the Direct Simulation Monte Carlo method, the Boltzmann equation is numerically solved for a gas of hard spheres enclosed between two parallel plates kept at different temperatures and subject to the action of a gravity field normal to the plates. The profiles of pressure, density, temperature and heat flux are seen to be quite sensitive to the value of the gravity acceleration $g$. If the gravity field and the heat flux are parallel ($g>0$), the magnitudes of both the temperature gradient and the heat flux are smaller than in the opposite case ($g<0$). When considering the actual heat flux relative to the value predicted by the Fourier law, it is seen that, if $g>0$, the ratio increases as the reduced local field strength increases, while the opposite happens if $g<0$. The simulation results are compared with theoretical predictions for Maxwell moleculesComment: 18 pages (LaTex), 7 figures (eps

Non-equilibrium tube length fluctuations of entangled polymers

We investigate the nonequilibrium tube length fluctuations during the relaxation of an initially stretched, entangled polymer chain. The time-dependent variance $\sigma^2$ of the tube length follows in the early-time regime a simple universal power law $\sigma^2 = A \sqrt{t}$ originating in the diffusive motion of the polymer segments. The amplitude $A$ is calculated analytically both from standard reptation theory and from an exactly solvable lattice gas model for reptation and its dependence on the initial and equilibrium tube length respectively is discussed. The non-universality suggests the measurement of the fluctuations (e.g. using flourescence microscopy) as a test for reptation models.Comment: 12 pages, 2 figures. Minor typos correcte

Noise-Free Measurement of Harmonic Oscillators with Instantaneous Interactions

We present a method of measuring the quantum state of a harmonic oscillator through instantaneous probe-system selective interactions of the Jaynes-Cummings type. We prove that this scheme is robust to general decoherence mechanisms, allowing the possibility of measuring fast-decaying systems in the weak-coupling regime. This method could be applied to different setups: motional states of trapped ions, microwave fields in cavity/circuit QED, and even intra-cavity optical fields.Comment: 4 pages, no figure, published in Physical Review Letter

Thermal van der Waals Interaction between Graphene Layers

The van de Waals interaction between two graphene sheets is studied at finite temperatures. Graphene's thermal length $(\xi_T = \hbar v / k_B T)$ controls the force versus distance $(z)$ as a crossover from the zero temperature results for $z\ll \xi_T$, to a linear-in-temperature, universal regime for $z\gg \xi_T$. The large separation regime is shown to be a consequence of the classical behavior of graphene's plasmons at finite temperature. Retardation effects are largely irrelevant, both in the zero and finite temperature regimes. Thermal effects should be noticeable in the van de Waals interaction already for distances of tens of nanometers at room temperature.Comment: enlarged version, 9 pages, 4 figures, updated reference

A model for the time uncertainty measurements in the Auger surface detector array

The precise determination of the arrival direction of cosmic rays is a fundamental prerequisite for the search for sources or the study of their anisotropies on the sky. One of the most important aspects to achieve an optimal measurement of these directions is to properly take into account the measurement uncertainties in the estimation procedure. In this article we present a model for the uncertainties associated with the time measurements in the Auger surface detector array. We show that this model represents well the measurement uncertainties and therefore provides the basis for an optimal determination of the arrival direction. With this model and a description of the shower front geometry it is possible to estimate, on an event by event basis, the uncertainty associated with the determination of the arrival directions of the cosmic rays

Preserving the validity of the Two-Higgs Doublet Model up to the Planck scale

We examine the constraints on the two Higgs doublet model (2HDM) due to the stability of the scalar potential and absence of Landau poles at energy scales below the Planck scale. We employ the most general 2HDM that incorporates an approximately Standard Model (SM) Higgs boson with a flavor aligned Yukawa sector to eliminate potential tree-level Higgs-mediated flavor changing neutral currents. Using basis independent techniques, we exhibit robust regimes of the 2HDM parameter space with a 125 GeV SM-like Higgs boson that is stable and perturbative up to the Planck scale. Implications for the heavy scalar spectrum are exhibited.Comment: 36 pages, 4 figures, 4 tables (Version 3: typographical error in eq. (A.28) corrected

Holographic Description of Finite Size Effects in Strongly Coupled Superconductors

Despite its fundamental and practical interest, the understanding of mesoscopic effects in strongly coupled superconductors is still limited. Here we address this problem by studying holographic superconductivity in a disk and a strip of typical size $\ell$. For $\ell < \ell_c$, where $\ell_c$ depends on the chemical potential and temperature, we have found that the order parameter vanishes. The superconductor-metal transition at $\ell = \ell_c$ is controlled by mean-field critical exponents which suggests that quantum and thermal fluctuations induced by finite size effects are suppressed in holographic superconductors. Intriguingly, the effective interactions that bind the order parameter increases as $\ell$ decreases. Most of these results are consistent with experimental observations in Pb nanograins at low temperature and qualitatively different from the ones expected in a weakly coupled superconductor.Comment: 4 pages, 3 figure