Telecommunication applications of millimeter waves

For abstract see A81-4430

Martin boundary of a reflected random walk on a half-space

The complete representation of the Martin compactification for reflected random walks on a half-space $\Z^d\times\N$ is obtained. It is shown that the full Martin compactification is in general not homeomorphic to the ``radial'' compactification obtained by Ney and Spitzer for the homogeneous random walks in $\Z^d$ : convergence of a sequence of points $z_n\in\Z^{d-1}\times\N$ to a point of on the Martin boundary does not imply convergence of the sequence $z_n/|z_n|$ on the unit sphere $S^d$. Our approach relies on the large deviation properties of the scaled processes and uses Pascal's method combined with the ratio limit theorem. The existence of non-radial limits is related to non-linear optimal large deviation trajectories.Comment: 42 pages, preprint, CNRS UMR 808

Effect of Radiative Levitation on Calculations of Accretion Rates in White Dwarfs

Elements heavier than hydrogen or helium that are present in the atmospheres of white dwarfs with effective temperatures lower than 25,000 K, are believed to be the result of accretion. By measuring the abundances of these elements and by assuming a steady-state accretion, we can derive the composition of the accreted matter and infer its source. The presence of radiative levitation, however, may affect the determination of the accretion rate. We present time-dependent diffusion calculations that take into account radiative levitation and accretion. The calculations are performed on C, N, O, Ne, Na, Mg, Al, Si, S, Ar, and Ca in hydrogen-rich white dwarf models with effective temperatures lower than 25,000 K and a gravity of log g = 8.0. We show that in the presence of accretion, the abundance of an element supported by the radiative levitation is given by the equilibrium between the radiative and gravitational accelerations, unless the abundance predicted by the steady-state accretion is much greater than the abundance supported by the radiative acceleration.Comment: 6 pages, to be published in the proceedings of the 17th European White Dwarf Workshop that was held in Tubingen, Germany, on August 16-20, 201

Thermodynamics in the vicinity of a relativistic quantum critical point in 2+1 dimensions

We study the thermodynamics of the relativistic quantum O($N$) model in two space dimensions. In the vicinity of the zero-temperature quantum critical point (QCP), the pressure can be written in the scaling form P(T)=P(0)+N(T^3/c^2)\calF_N(\Delta/T) where $c$ is the velocity of the excitations at the QCP and $\Delta$ is a characteristic zero-temperature energy scale. Using both a large-$N$ approach to leading order and the nonperturbative renormalization group, we compute the universal scaling function \calF_N. For small values of $N$ ($N\lesssim 10$) we find that \calF_N(x) is nonmonotonous in the quantum critical regime ($|x|\lesssim 1$) with a maximum near $x=0$. The large-$N$ approach -- if properly interpreted -- is a good approximation both in the renormalized classical ($x\lesssim -1$) and quantum disordered ($x\gtrsim 1$) regimes, but fails to describe the nonmonotonous behavior of \calF_N in the quantum critical regime. We discuss the renormalization-group flows in the various regimes near the QCP and make the connection with the quantum nonlinear sigma model in the renormalized classical regime. We compute the Berezinskii-Kosterlitz-Thouless transition temperature in the quantum O(2) model and find that in the vicinity of the QCP the universal ratio \Tkt/\rho_s(0) is very close to $\pi/2$, implying that the stiffness \rho_s(\Tkt^-) at the transition is only slightly reduced with respect to the zero-temperature stiffness $\rho_s(0)$. Finally, we briefly discuss the experimental determination of the universal function \calF_2 from the pressure of a Bose gas in an optical lattice near the superfluid--Mott-insulator transition.Comment: v1) 16 pages, 10 figures. v2) Revised versio

Reexamination of the nonperturbative renormalization-group approach to the Kosterlitz-Thouless transition

We reexamine the two-dimensional linear O(2) model ($\varphi^4$ theory) in the framework of the nonperturbative renormalization-group. From the flow equations obtained in the derivative expansion to second order and with optimization of the infrared regulator, we find a transition between a high-temperature (disordered) phase and a low-temperature phase displaying a line of fixed points and algebraic order. We obtain a picture in agreement with the standard theory of the Kosterlitz-Thouless (KT) transition and reproduce the universal features of the transition. In particular, we find the anomalous dimension \eta(\Tkt)\simeq 0.24 and the stiffness jump \rho_s(\Tkt^-)\simeq 0.64 at the transition temperature \Tkt, in very good agreement with the exact results \eta(\Tkt)=1/4 and \rho_s(\Tkt^-)=2/\pi, as well as an essential singularity of the correlation length in the high-temperature phase as T\to \Tkt.Comment: v2) Final version as published (with revised title): 10 pages, 10 figure

Quantum Hall effect anomaly and collective modes in the magnetic-field-induced spin-density-wave phases of quasi-one-dimensional conductors

We study the collective modes in the magnetic-field-induced spin-density-wave (FISDW) phases experimentally observed in organic conductors of the Bechgaard salts family. In phases that exhibit a sign reversal of the quantum Hall effect (Ribault anomaly), the coexistence of two spin-density waves gives rise to additional collective modes besides the Goldstone modes due to spontaneous translation and rotation symmetry breaking. These modes strongly affect the charge and spin response functions. We discuss some experimental consequences for the Bechgaard salts.Comment: Final version (LaTex, 8 pages, no figure), to be published in Europhys. Let

Spin Anisotropy and Slow Dynamics in Spin Glasses

We report on an extensive study of the influence of spin anisotropy on spin glass aging dynamics. New temperature cycle experiments allow us to compare quantitatively the memory effect in four Heisenberg spin glasses with various degrees of random anisotropy and one Ising spin glass. The sharpness of the memory effect appears to decrease continuously with the spin anisotropy. Besides, the spin glass coherence length is determined by magnetic field change experiments for the first time in the Ising sample. For three representative samples, from Heisenberg to Ising spin glasses, we can consistently account for both sets of experiments (temperature cycle and magnetic field change) using a single expression for the growth of the coherence length with time.Comment: 4 pages and 4 figures - Service de Physique de l'Etat Condense CNRS URA 2464), DSM/DRECAM, CEA Saclay, Franc

Large Deviations Analysis for Distributed Algorithms in an Ergodic Markovian Environment

We provide a large deviations analysis of deadlock phenomena occurring in distributed systems sharing common resources. In our model transition probabilities of resource allocation and deallocation are time and space dependent. The process is driven by an ergodic Markov chain and is reflected on the boundary of the d-dimensional cube. In the large resource limit, we prove Freidlin-Wentzell estimates, we study the asymptotic of the deadlock time and we show that the quasi-potential is a viscosity solution of a Hamilton-Jacobi equation with a Neumann boundary condition. We give a complete analysis of the colliding 2-stacks problem and show an example where the system has a stable attractor which is a limit cycle