795 research outputs found

    Comment on some proposed mechanisms for attenuation of third sound

    Get PDF
    Two explanations have been proposed for an apparent discrepancy between theoretical prediction and experimental measurement of third-sound attenuation. One of these proposes a new "macroscopic quantum uncertainly principle," and the other proceeds via nonlinear, anharmonic effects due to zero-point fluctuations. We argue that neither suggestion is acceptable

    Probing the helium-graphite interaction

    Get PDF
    Two separate lines of investigation have recently converged to produce a highly detailed picture of the behavior of helium atoms physisorbed on graphite basal plane surfaces. Atomic beam scattering experiments on single crystals have yielded accurate values for the binding energies of several· states for both (^4)He and (^3)He, as well as matrix elements of the largest Fourier component of the periodic part of the interaction potential. From these data, a complete three-dimensional description of the potential has been constructed, and the energy band structure of a helium atom moving in this potential calculated. At the same time, accurate thermodynamic measurements were made on submonolayer helium films adsorbed on Grafoil. The binding energy and low-coverage specific heat deduced from these measurements are in excellent agreement with those calculated from the band structures

    Experiments on Quantum and Thermal Desorption from ^4He Films

    Get PDF
    Desorption of He atoms from thin films may be resolved experimentally into quantum and thermal components. We show that quantum desorption becomes the dominant part of the signal in submonolayer films. We also show that, when all effects of collisions between desorbed atoms are eliminated, quantum desorption is not focused normal to the surface of optically polished sapphire crystals

    Ion mobility discontinuities in superfluid helium: A test of the Huang-Olinto theory

    Get PDF
    A new method has been developed for making sensitive differential measurements of ion mobilities in liquid helium. Using this method, it has been possible to make a definitive test of the part of the Huang-Olinto theory intended to explain discontinuities in ion mobilities in superfluid helium. The theory has been found to be incorrect

    Thermal effects on nuclear symmetry energy with a momentum-dependent effective interaction

    Full text link
    The knowledge of the nuclear symmetry energy of hot neutron-rich matter is important for understanding the dynamical evolution of massive stars and the supernova explosion mechanisms. In particular, the electron capture rate on nuclei and/or free protons in presupernova explosions is especially sensitive to the symmetry energy at finite temperature. In view of the above, in the present work we calculate the symmetry energy as a function of the temperature for various values of the baryon density, by applying a momentum-dependent effective interaction. In addition to a previous work, the thermal effects are studied separately both in the kinetic part and the interaction part of the symmetry energy. We focus also on the calculations of the mean field potential, employed extensively in heavy ion reaction research, both for nuclear and pure neutron matter. The proton fraction and the electron chemical potential, which are crucial quantities for representing the thermal evolution of supernova and neutron stars, are calculated for various values of the temperature. Finally, we construct a temperature dependent equation of state of β\beta-stable nuclear matter, the basic ingredient for the evaluation of the neutron star properties.Comment: 18 pages, 10 figures, 1 table, accepted for publication in Physical Review

    Adsorption and Specific-Heat Studies of Monolayer and Submonolayer Films of He3 and He4

    Get PDF
    A study has been made of the adsorption of He3 and He4 at 4°K on a substrate consisting of a monolayer of argon adsorbed on a sintered copper sponge. The isotherms display distinct steps indicating the completion of first and second adsorbed layers. Comparisons among the adsorption isotherms of helium and of Ar and N2 at 77.4°K yield a self-consistent set of molecular areas. Measurements have been made of the specific heat of five submonolayer coverages of He3 and He4 on Ar-plated Cu sponge. The heat capacities of nearly complete monolayers vary as T2 from 0.3 to 4°K, yielding two-dimensional Debye temperatures Theta (He4)=28±1°K, and Theta (He3)=31±1°K. At lower coverages the molar heat capacities increase and develop contributions linear in T below 1°K. At an intermediate coverage, the heat capacity of He4 exhibits a broad and pronounced maximum near 3°K. Possible mechanisms for the linear terms and the maximum are discussed briefly. Evidence for considerable mobility of He atoms along the surface is adduced from the temperature and coverage dependence of the heat capacity. The T2 behavior for the complete monolayers yields an upper limit of ~10^-11 sec for the lifetime of a He atom in any individual adsorption site, consistent with a theoretical estimate

    Measurement of the SOC State Specific Heat in ^4He

    Get PDF
    When a heat flux Q is applied downward through a sample of liquid 4He near the lambda transition, the helium self organizes such that the gradient in temperature matches the gravity induced gradient in Tlambda. All the helium in the sample is then at the same reduced temperature tSOC = ((T[sub SOC] - T[sub lambda])/T[sub lambda]) and the helium is said to be in the Self-Organized Critical (SOC) state. We have made preliminary measurements of the 4He SOC state specific heat, C[del]T(T(Q)). Despite having a cell height of 2.54 cm, our results show no difference between C[del]T and the zero-gravity 4He specific heat results of the Lambda Point Experiment (LPE) [J.A. Lipa et al., Phys. Rev. B, 68, 174518 (2003)] over the range 250 to 450 nK below the transition. There is no gravity rounding because the entire sample is at the same reduced temperature tSOC(Q). Closer to Tlambda the SOC specific heat falls slightly below LPE, reaching a maximum at approximately 50 nK below Tlambda, in agreement with theoretical predictions [R. Haussmann, Phys. Rev. B, 60, 12349 (1999)]

    Melting in multilayer adsorbed films

    Get PDF
    We present both an improved model and new experimental data concerning the problem of melting in multilayer adsorbed films. The model treats in a mutually consistent manner all interfaces in a stratified film. This results in the prediction of substrate freezing, a phenomenon thermodynamically analogous to surface melting. We also compare the free energies of stratified films to those of homogeneous films. This leads to an orderly classification of multilayer phase diagrams in the vicinity of the bulk triple point. The results of the model are compared with the experimentally known systems. Of these, only methane/graphite exhibits melting from homogeneous solid to homogeneous liquid in multilayer films. The systems Ne/graphite and Ar/graphite, studied by Zhu and Dash, exhibit surface melting and substrate freezing instead. We observe experimentally, by means of pulsed nuclear magnetic resonance, that melting in methane adsorbed on graphite extends below the film thickness at which the latent heat of melting is known to vanish. The multilayer melting curve in this system is a first-order prewetting transition, extending from triple-point dewetting at bulk coexistence down to a critical point where the latent heat vanishes at about four layers, and apparently extending to thinner films as a higher-order, two-dimensional phase transition. It would therefore seem that methane/graphite is an ideal system in which to study the evolution of melting from two dimensions to three dimensions

    Effect of Inhomogeneous Heat Flow on the Enhancement of Heat Capacity in Helium-II by Counterflow near Tλ

    Get PDF
    In 2000 Harter et al. reported the first measurements of the enhancement of the heat capacity ΔCQ[equivalent]C(Q)-C(Q=0) of helium-II transporting a heat flux density Q near Tλ. Surprisingly, their measured ΔCQ was ~7–12 times larger than predicted, depending on which theory was assumed. In this report we present a candidate explanation for this discrepancy: unintended heat flux inhomogeneity. Because C(Q) should diverge at a critical heat flux density Qc, homogeneous heat flow is required for an accurate measurement. We present results from numerical analysis of the heat flow in the Harter et al. cell indicating that substantial inhomogeneity occurred. We determine the effect of the inhomogeneity on ΔCQ and find rough agreement with the observed disparity between prediction and measurement

    Effective Free Energy for Individual Dynamics

    Full text link
    Physics and economics are two disciplines that share the common challenge of linking microscopic and macroscopic behaviors. However, while physics is based on collective dynamics, economics is based on individual choices. This conceptual difference is one of the main obstacles one has to overcome in order to characterize analytically economic models. In this paper, we build both on statistical mechanics and the game theory notion of Potential Function to introduce a rigorous generalization of the physicist's free energy, which includes individual dynamics. Our approach paves the way to analytical treatments of a wide range of socio-economic models and might bring new insights into them. As first examples, we derive solutions for a congestion model and a residential segregation model.Comment: 8 pages, 2 figures, presented at the ECCS'10 conferenc
    corecore