Multiple Andreev Reflections in Weak Links of Superfluid 3He-B

We calculate the current-pressure characteristics of a ballistic pinhole aperture between two volumes of B-phase superfluid 3He. The most important mechanism contributing to dissipative currents in weak links of this type is the process of multiple Andreev reflections. At low biases this process is significantly affected by relaxation due to inelastic quasiparticle-quasiparticle collisions. In the numerical calculations, suppression of the superfluid order parameter at surfaces is taken into account self-consistently. When this effect is neglected, the theory may be developed analytically like in the case of s-wave superconductors. A comparison with experimental results is presented.Comment: 12 pages, 9 figures, RevTeX

Dynamics of fluctuations in an optical analog of the Laval nozzle

Using the analogy between the description of coherent light propagation in a medium with Kerr nonlinearity by means of nonlinear Schr\"odinger equation and that of a dissipationless liquid we propose an optical analogue of the Laval nozzle. The optical Laval nozzle will allow one to form a transonic flow in which one can observe and study a very unusual dynamics of classical and quantum fluctuations including analogue of the Hawking radiation of real black holes. Theoretical analysis of this dynamics is supported by numerical calculations and estimates for a possible experimental setup are presented.Comment: 7 pages, 4 figure

Black hole lasers in Bose-Einstein condensates

We consider elongated condensates that cross twice the speed of sound. In the absence of periodic boundary conditions, the phonon spectrum possesses a discrete and finite set of complex frequency modes that induce a laser effect. This effect constitutes a dynamical instability and is due to the fact that the supersonic region acts as a resonant cavity. We numerically compute the complex frequencies and density-density correlation function. We obtain patterns with very specific signatures. In terms of the gravitational analogy, the flows we consider correspond to a pair of black hole and white hole horizons, and the laser effect can be conceived as a self-amplified Hawking radiation. This is verified by comparing the outgoing flux at early time with the standard black hole radiation.Comment: iopams, 37 pages, 14 figures, 1 table; for associated gif animations, see http://people.sissa.it/~finazzi/bec_bhlasers/movies/ or http://iopscience.iop.org/1367-2630/12/9/095015/media. Published on New. J. Phys. (http://iopscience.iop.org/1367-2630/12/9/095015/). V2: few new comments, modified figure

The Frequency Dependence of Critical-velocity Behavior in Oscillatory Flow of Superfluid Helium-4 Through a 2-micrometer by 2-micrometer Aperture in a Thin Foil

The critical-velocity behavior of oscillatory superfluid Helium-4 flow through a 2-micrometer by 2-micrometer aperture in a 0.1-micrometer-thick foil has been studied from 0.36 K to 2.10 K at frequencies from less than 50 Hz up to above 1880 Hz. The pressure remained less than 0.5 bar. In early runs during which the frequency remained below 400 Hz, the critical velocity was a nearly-linearly decreasing function of increasing temperature throughout the region of temperature studied. In runs at the lowest frequencies, isolated 2 Pi phase slips could be observed at the onset of dissipation. In runs with frequencies higher than 400 Hz, downward curvature was observed in the decrease of critical velocity with increasing temperature. In addition, above 500 Hz an alteration in supercritical behavior was seen at the lower temperatures, involving the appearance of large energy-loss events. These irregular events typically lasted a few tens of half-cycles of oscillation and could involve hundreds of times more energy loss than would have occurred in a single complete 2 Pi phase slip at maximum flow. The temperatures at which this altered behavior was observed rose with frequency, from ~ 0.6 K and below, at 500 Hz, to ~ 1.0 K and below, at 1880 Hz.Comment: 35 pages, 13 figures, prequel to cond-mat/050203

Preferential CO oxidation in hydrogen (PROX) on ceria-supported catalysts PART II. Oxidation states and surface species on Pd/CeO2 under reaction conditions, suggested reaction mechanism

The aim of the PROX reaction is to reduce the CO content of hydrogen feed to proton-exchange membrane fuel cells (PEMFCs) by selective oxidation of CO in the presence of excess hydrogen. Both Pt and Pd on ceria are active in CO oxidation (without hydrogen), whereas Pd is poorly active in the presence of hydrogen. In this paper we explore the reasons for such behavior, using the same techniques for Pd/CeO2 as used for Pt/CeO2 in Part I: catalytic tests, in situ DRIFTS, high-pressure XPS, HRTEM, and TDS. We also examine the reaction mechanism of CO oxidation (without hydrogen), which does not occur via exactly the same mechanism on Pt and Pd/CeO2 catalysts. In the presence of hydrogen (PROX) at low temperature (T = 350–380 K), the formation of Pd β-hydride was confirmed by high-pressure in situ XPS. Its formation greatly suppressed the possibility of CO oxidation, because oxygen both from gas-phase and support sites reacted rapidly with hydride H to form water, which readily desorbed from Pd. Nevertheless, CO adsorption was not hampered here. These entities transformed mainly to surface formate and formyl (–CHO) species instead of oxidation as observed by DRIFTS. The participation of a low-temperature water–gas shift type reaction proposed for the platinum system (see Part I) was hindered. Increasing temperature led to decomposition of the hydride phase and a parallel increase in the selectivity toward CO oxidation. This still remained lower on Pd/CeO2 than on Pt/CeO2, however

A Variational Method in Out of Equilibrium Physical Systems

A variational principle is further developed for out of equilibrium dynamical systems by using the concept of maximum entropy. With this new formulation it is obtained a set of two first-order differential equations, revealing the same formal symplectic structure shared by classical mechanics, fluid mechanics and thermodynamics. In particular, it is obtained an extended equation of motion for a rotating dynamical system, from where it emerges a kind of topological torsion current of the form $\epsilon_{ijk} A_j \omega_k$, with $A_j$ and $\omega_k$ denoting components of the vector potential (gravitational or/and electromagnetic) and $\omega$ is the angular velocity of the accelerated frame. In addition, it is derived a special form of Umov-Poynting's theorem for rotating gravito-electromagnetic systems, and obtained a general condition of equilibrium for a rotating plasma. The variational method is then applied to clarify the working mechanism of some particular devices, such as the Bennett pinch and vacuum arcs, to calculate the power extraction from an hurricane, and to discuss the effect of transport angular momentum on the radiactive heating of planetary atmospheres. This development is seen to be advantageous and opens options for systematic improvements.Comment: 22 pages, 1 figure, submitted to review, added one referenc

Referral rates in Swiss primary care with a special emphasis on reasons for encounter

Questions under study: Referrals from primary to secondary care reflect a crucial role of primary care physicians (PCPs). Most referral rates are based on the number of consultations, rather than on the number of problems addressed during consultations (reasons for encounter = RFE). The aim of the study was to update data on consultations, RFE and referrals in Swiss primary care and calculate a referral rate based on RFE rather than on the number of consultations. Method: Cross-sectional study in Swiss primary care. PCPs collected data on consultations on fifteen different days in three non-consecutive months in 2012/2013. Demographic data of patients and up to six RFE per consultation was collected. For each RFE the PCP had to indicate whether a referral was initiated. Data was analyzed using descriptive statistics. Results: Ninety PCPs (18.9% females) participated and 24’774 consultations with 42’890 RFE (corresponding to 1.73 (SD 1.07) RFE per consultation) were recorded. 2427 RFE (of 2341 consultations) led to a referral, corresponding to a referral rate of 9.44% (95%-CI 9.08-9.81%) based on consultations and 5.65% (95%-CI 5.43-5.87%) based on the number of RFE. Conclusions: An average of 1.7 RFE per consultations and a broad clinical spectrum of problems were presented in primary care; nevertheless 94.3% of all problems were solved in primary care, reflecting the crucial role of PCPs as a coordinator of healthcare

The Excitation Spectrum of a Bose-Einstein Condensate

We report the first measurement of the excitation spectrum and the static structure factor of a Bose-Einstein condensate. The excitation spectrum displays a linear phonon regime, as well as a parabolic single-particle regime. The linear regime provides an upper limit for the superfluid critical velocity, by the Landau criterion. The excitation spectrum agrees well with the Bogoliubov spectrum, in the local density approximation. This agreement continues even for excitations close to the long-wavelength limit of the region of applicability of the approximation. Feynman's relation between the excitation spectrum and the static structure factor is verified, within an overall constant