Showing papers in "Journal of Low Temperature Physics in 1992"

Superfluid helium in porous media

Abstract: The superfluid properties of 4He absorbed in porous media are discussed. Emphasis is given to the nature of the superfluid transition for helium contained in a variety of different porous structures. Recent heat capacity and superfluid measurements for helium in Vycor glass and other porous media are presented in some detail. The onset of superfluidity as a function of adsorbed helium coverage at zero temperature, or “boson localization,” is touched on briefly. The problem of dissipation of superflow in porous media, especially in the vicinity of the superfluid transition, is discussed.

Helium crystals under stress: The Grinfeld instability

Abstract: We present the first quantitative experiment on the equilibrium shape of crystals under nonhydrostatic stress By studying very pure helium-4 crystals, evidence was obtained that small nonhydrostatic stresses produce a macroscopic melting, as predicted by calculations based on elastic theory [1] Furthermore, above a certain threshold in strain, large grooves appear on the crystal surface, in agreement with the instability first predicted by Grinfeld [2] Our results also agree with a more qualitative study done by M Thiel et al[3] We finally discuss the relaxation of stresses as a possible consequence of the motion of crystal defects

Electrons at the surface of quantum systems

Abstract: The two lightest elements hydrogen and helium provide unique systems to study surfaces and interfaces of condensed matter in the presence of large quantum fluctuations. Surface phenomena to be mentioned here are, e.g., surface melting, roughening and layering.1 Since in the quantum systems the gas density above the surface is relatively high in the regions of interest, investigations using conventional surface techniques like LEED and EELS are of limited applicability, because they usually require high vacuum conditions. A technique wich has been applied successfully is neutron scattering, which — although not being particularly sensitive to surfaces — can be used if the quantum systems are adsorbed on substrates with a large intrinsic surface area, like exfoliated graphite.2 Information about quantum surfaces has been obtained also from measurements of specific heat, adsorption isotherms, third sound propagation19−22 and, as a somewhat peculiar method, from the investigation of surface state electrons (SSE). It is the latter which is the topic of this paper.

Lectures on the properties of liquid and Solid 3He-4He mixtures at low temperatures

Abstract: We give a concise review of the empirical properties of liquid and solid 3He-4He mixtures and their phenomenological interpretation. The bulk of the paper is about dilute solutions of 3He in liquid 4He at temperatures well below the tricritical point, where the roton and phonon excitations are comparatively unimportant. We describe the thermodynamic properties in terms of the Landau-Pomeranchuk 3He quasiparticles and the effective interaction between them, introduced by Emery and Bardeen, Baym and Pines. The scattering amplitude, needed to fit the low temperature transport properties, and the effective interaction are related, provided the multiple virtual scattering calculated by Fu and Pethick is included. The multiple scattering should always be included, even for very small concentrations. We present the evidence for the velocity dependence of the effective interaction, and urge that this also be taken into account in the interpretation of experiments. We give a short description of spin-polarized liquid mixtures and of the possibility of pairing superfluidity in solutions of 3He in liquid 4He. The existence of supersaturated solutions may be a way to attain p-wave pairing at accessible temperatures. Beacause of phase separation, the concentration of 4He in dilute mixtures of 4He in liquid 3He becomes very small as the temperature is lowered, making it unlikely that a degenerate Bose gas of 4He quasiparticles can be produced. In addition, the superfluid 4He film on the walls of the vessel makes the achievement of a supersaturated solution very difficult. We briefly review the measurements of the phase separation, the density and specific heat, and show that the spectrum of the 4He quasiparticles and the role of their effective interactions are still in doubt. In solid 3He-4He mixtures the thermodynamic properties and the phase diagram, including the crystallographic transformation, fit the regular solution model very well. On the other hand, dilute solid solutions of either 3He or 4He in the other isotope have NMR relaxation times and spin diffusion which agree with the theory of impuritons (mass fluctuation waves) which tunnel more or less freely through the crystal. The reconciliation of these two theories and an explanation of the accuracy of the regular solution model remain mysteries. A deviation from the regular solution model, the fluctuation specific heat above the phase separation temperature, is discussed in detail. The review includes two tables which list most of the experiments on liquid and solid 3He-4He mixtures published between 1975 and 1990.

Circular Dichroism and Birefringence in Unconventional Superconductors

Abstract: We present a theoretical analysis of circular dichroism and birefringe in unconventional BCS superconductors with appropriate broken symmetries. We show that for the effect to exist, that in addition to broken time-reversal and two-dimensional parity symmetries, it is necessary to take into account the weak particle-hole asymmetry of the low-energy excitations of the metallic state. Circular dichroism and birefringence are shown to arise from the order parameter collective mode response of the superconductor; in the clean limit the contribution to the current response from the single-particle excitations does not give rise to circular dichroism or birefringence, even with particle-hole asymmetry. The magnitude of the circular dichroism is found to be small for the classes of superconductors which are thought to be likely candidates to exhibit the requisite broken symmetries, namely the heavy fermions and oxide superconductors. The order of magnitude of the elliptical polarization of a linearly polarized incident wave is Vf/c(ζ/λL) (Δ/Ef) ln(Ef/Δ), which is roughly 10−7−10−8 rad at frequencies of order the gap, and decreases at least as fast as (2Δ/ω)2 at higher frequencies.

4He films on graphite studied by neutron scattering

Abstract: The properties of 4He films adsorbed on graphite have been studied by neutron scattering. In particular excitations of the commensurate phase of the monolayer are discussed. The first two adsorbed layers are solid and the next ones stay liquid. At the boundaries of the superfluid film excitations could be studied. Also the phonons, maxon and rotons of the film are investigated. An explanation of the lower density of the very thin films compared to bulk 4He is given.

Experimental techniques: Methods for cooling below 300 mK

Abstract: There are at present three methods for cooling samples to temperatures below 300 mK: dilution, Pomeranchuk, and nuclear refrigeration. We give the basic principles of these methods with more details concerning dilutions refrigerators. This should allow the construction of a simple all plastic refrigerator for temperatures lower than 15 mK, or an even simpler Pomeranchuk cell. The source of heat leaks and other important points for reaching temperatures in the microkelvin range with nuclear refrigerators are given in the lecture by F. Pobell

Quantum diffusion in solids

Abstract: The lectures are devoted to the problems connected with the tunneling motion of particles in a solid in the presence of strong interaction with phonon and electron excitations. We examine the quantum diffusion in insulators, metals, and superconductors. The interaction with the barrier fluctuations as well as the intrawell interaction are taken into account. Special attention is paid to the role of adiabaticity, playing an important part in tunneling phenomena, revealing the decisive influence of nonadiabatic excitations. We consider the entire temperature interval: coherent diffusion at low T, suppression of the coherent phase correlations and the transition to the incoherent quantum diffusion, and then with the further increase in T the crossover to the tunneling diffusion induced by excitations. The detailed analysis of the quantum diffusion in imperfect crystals and the problems of localization, in particular, caused by the interparticle interaction is given. We discuss the wide spectrum of experimental results displaying the quantum diffusion in different systems.

A possible observation of quantum nucleation in superfluid He4 near crystallization

Abstract: The overpressurization of superfluid He4 under crystallization is studied experimentally The effect of a substrate and He3 impurities on the nucleation rate W is observed The dependence of the metastable state lifetime versus the overpressurization and temperature is measured in the range 10−7–1 s and 052–175 K The results are compared with the theoretical predictions for the classic and quantum nucleation The surface relief effect is considered It is shown that relative to a flat surface, a cone-shaped pit is a site corresponding to a smaller nucleation energy In this case a change of the nucleation rate versus the metastability degree and temperature in the classic region is analyzed As a result, the most reliable qualitative manifestation of quantum nucleation is the independence of the metastable degree under the constant rate W upon temperature, which is observed below 1 K in the experiment

Extraordinary behavior of 4 He on hydrogen and deuterium surfaces

Abstract: Using third sound resonance below 1 K, for4He on solid H2 and D2 surfaces, we have found new phenomena. On H2 we find surface superfluidity for4He coverages less than one layer, with the zero temperature Kosterlitz-Thouless (KT) transition extrapolated to a coverage of 0.35±0.05 layers. On both H2 and D2 we find a new line of transitions below the KT transitions. On H2, at zero temperature, the new line of transitions starts at zero temperature at a coverage of 0.6±0.1 layers of4He and has a slope of 0.58 (K/layer) of the universal KT slope. We also find, on H2, two modes of surface wave propagation between the new transition and the KT transition. Below the new transition we also find anomalous third sound absorption which increases exponentially with temperature by five orders of magnitude.

Spin diffusion in dilute, polarized 3He-4He solutions

Abstract: Spin dynamics for arbitrarily polarized and very dilute solutions of 3He in liquid 4He are described. We began at a very fundamental level by deriving a kinetic equation for arbitrarily polarized dilute quantum systems based on a method due to Boercker and Dufty. This approach allows more controlled approximations than our previous derivation based on the Kadanoff-Baym technique. Our previous work is here generalized to include T-matrix interactions rather than the Born approximation. Spin hydrodynamic equations are derived. The general equations are valid for both Fermi and Bose systems. By use of a well-known phenomenological potential to describe the 3He-3He T-matrix we calculate longitudinal and transverse spin diffusion coefficients D ⊥ and D ¦ and the identical-particle spin-rotation parameter Μ. We confirm that these two diffusion constants differ at low T with D ⊥ approaching a constant as T → 0, and D¦~1/T2. Estimates of errors made by our approximations are considered in detail. Good agreement is found in comparison with data from both Cornell University and the University of Massachusetts. We find that the s-wave approximation is inadequate and that mean-field corrections are important. Comparison is also made between theory and the recent UMass viscosity measurements.

Electric dipole moment and spin supercurrent in superfluid 3 He

Abstract: The SU(2) gauge invariant theory of the relativistic interaction of the electrically neutral superfluid3He with electric and magnetic fields is formulated. The spin supercurrent response on the electric field is calculated for this interaction. The comparison with the nonrelativistic flexoelectric effect, arising due to the distortion of the atomic shell by the gradients of the superfluid order parameter, is made.

Helium wetting and prewetting phenomena at finite temperatures

Abstract: Helium wetting and prewetting phenomena at temperatures T>0 are addressed Emphasis is given to the weak-binding substrates Cs and H2, which have been predicted and observed to be nonwet and wet, respectively, by4He at low temperatures Calculations of the wetting temperature TW and the prewetting line near TW for Cs, as well as the prewetting line near T=0 for H2, are given Predictions concerning the interplay between superfluid onset and prewetting are made, and prewetting critical temperatures are estimated

Eliashberg theory of the critical temperature and isotope effect. Dependence on bandwidth, band-filling, and direct Coulomb repulsion

Abstract: We investigate the dependence of the superconducting critical temperature and the isotope coefficient on bandwidth, band-filling, and the direct Coulomb repulsion, within Eliashberg theory. The Migdal approximation is assumed throughout, and the Coulomb repulsion is modelled by the Hubbard U and treated in the simplest approximation. We assume a constant density of states with a finite bandwidth. We find that while, in principle, small isotope coefficients are possible, it is unlikely that the isotope coefficient can ever be negative within this model. Furthermore, it is difficult to achieve small isotope coefficients for realistic parameters. Finally, we discuss a possible means by which large isotope coefficients can occur at low filling.

Nuclear magnetic order in solid 3He

Abstract: Over the past 20 years our understanding of cooperative magnetic phenomena in solid 3He has expanded greatly Starting from the concept of atomic exchange, the author explains how early experiments led to the abandonment of the naive Heisenberg nearest neighbor antiferromagnetic Hamiltonian, in favor of a multiple exchange model He then describes the detailed experimental picture of the spin-ordered phases which has developed over the past decade, and the successes and failures of the multiple exchange model to explain the experimental results Today solid 3He is a powerful model magnetic system for extending our general understanding of magnetic order, and should continue to play an important role for many years to come

Unexpected Hydrodynamic Behavior of Multielectron Bubbles in Liquid Helium I

Abstract: We have investigated multielectron bubbles (with charges Z ~ 106 electrons) in normal liquid 4He. The drift velocity in an electric field and the charge of the bubbles were measured simultaneously. We found the drag coefficient to be over an order of magnitude higher than expected from ordinary hydrodynamics. The velocities did not show a correlation with the charge and could vary up to a factor of 3 for equally charged bubbles.

Specific heat and Curie temperature evaluation on triangular lattice: Exchange frequencies for3He adsorbed on grafoil, evaluated by path integral techniques

Abstract: The authors compute, using path integrals, the cyclic-exchange frequencies of a monolayer of {sup 3}He adsorbed on graphite in the incommensurate solid phase. Interactions between helium atoms are treated with the Aziz potential; between an helium atom and the plan of graphite they use an interaction potential which depends only on the distance above the surface. The specific heat (C{sub v}) and the magnetic susceptibility ({chi}) are evaluated from the calculated multiple-exchange hamiltonian. The C{sub v} value converges with the 2-, 3- and 4-body exchange cycles while larger cycles are needed for the convergence of the {chi} value. For the density 0.0785 atom/{Angstrom}{sup 2}, they find that the variations of C{sub v} is antiferromagnetic, whereas the variations of {chi} is ferromagnetic. 6 refs., 1 fig., 1 tab.

Low-frequency kinetics of high-T c metal oxides

Abstract: A microscopic explanation of the low-frequency kinetic properties of metal oxides is proposed. It is based on a strong electron-phonon interaction, which forms a charged Bose liquid of small bipolarons. The large value, the nonKorringa temperature dependence above T c, and the absence below T c of the coherent peak of the nuclear spin relaxation, as well as an unexpected “coherent peak” of the low frequency dynamic conductivity and the linear T-dependence of the resistivity are explained.

Superfluid-insulator transition in disordered boson systems

Abstract: We review the localization problem in two dimensions for interacting bosons in a random potential This problem is intimately related to the study of 4 He adsorbed in porous media, Josephson junction arrays, disordered superconducting films and vortex glasses Using path integral Monte Carlo techniques, we find a superfluid, a localized or ‘Bose glass’ insulator with gapless excitations, and (at commensurate densities) a Mott insulator with a finite gap to excitations

The superconducting state of YBa2Cu3O7-δ

Abstract: Many low temperature properties of high Tc superconductors deviate significantly from the detailed predictions of BCS theory. Here we discuss whether these effects could be caused by either: (a) an unconventional pairing state, or (b) local randomness in the gap function due to the intrinsic disorder. We review recent experiments pertinent to these questions: Josephson effects in (001) oriented planar junctions between YBa2Cu3O7-δ and classic superconductors and the temperature dependence of the a-b plane electro-magnetic penetration depth at low temperatures. We also calculate the density of states of s-wave superconductors with local quenched disorder in the gap function so as to determine whether s-wave pairing could be consistent with the low energy quasiparticle excitations seen in many experiments.

Investigation of quantum systems with surface plasmons and surface state electrons

Abstract: The authors report on investigations of thin films of the quantum systems hydrogen and helium both in thermodynamic equilibrium and in metastable states. Information about the film thickness and surface roughness is obtained from the excitation of surface plasmons and mobility measurements of surface state electrons. The equilibrium studies show triple-point wetting for H[sub 2] on Au substrates, in agreement with earlier results for similar systems. Unexpectedly, a distinct hysteretic behavior of the saturated film thickness is found near the triple-point. Superfluid [sup 4]He films on as prepared Ag display complete wetting. Investigations of H[sub 2] films in metastable states prepared by quench-condensation at 1.5K reveal pronounced changes in the film structure upon annealing. These changes set in far below the desorption temperature and are ascribed to surface diffusion.

Path integral monte carlo simulations of thin4He films on a H2 surface

Abstract: Atomatically thin4He films of up to three monolayers on molecular hydrogen (1,1,1) surfaces are studied at T = 0.5 K, using path integral Monte Carlo. We compute the binding energy of4He to the H2 substrate as a function of4He coverage and obtain evidence of the prewetting transition. Density profiles perpendicular to the4He-H2 interface are obtained, as well as the zero point motion and effective mass of4He parallel to the substrate surface. The superfluid density of4He vs. coverage is calculated, and the intermediate scattering function is computed, from which we estimate the speed of third sound. Finally, we calculate the vorticity-vorticity correlation function.

Vortex rings and the superfluid λ-transition

Abstract: A vortex-ring formulation of the4He λ-transition provides a clear physical picture of the superfluid transition. The thermally excited rings are dipoles which orient in an applied flow field, and their net backflow cancels part of the applied flow, reducing the superfluid density. At the λ-point rings of infinite size drive the superfluid density to zero as a power law of the reduced temperature, with an exponent of 0.672. By fitting to the experimental superfluid amplitude the core energy of the smallest rings is found to be 6.1K at Tλ and their diameter is 2.3 A. It is proposed that these can be identified as the roton excitations of the Landau model. The vortex theory also yields new insights into topics such as boundary effects at a wall, finite-size effects, and the dynamics of the transition.

Evidence for the absence of 4He adsorption on cesium coated graphite surface

Abstract: In agreement with recent theoretical predictions, volumetric vapor pressure isotherm studies indicate a substantial reduction in the adsorption of 4He at temperature near and below 2K when the graphite surface is coated with Cs.

Nuclear demagnetization cryostat at University of Florida Microkelvin laboratory

Abstract: Two nuclear-demagnetization cryostats, each containing 173 mol of copper coolant in a 8 T magnetic field and based on an Oxford Instruments model 1000 dilution refrigerator, have been under development at the University of Florida Microkelvin Laboratory for studies of nuclear magnetic ordering in metals and in solid3He, properties of superfluid3He at T≈0, and mesoscopic systems, and for development of primary thermometers based on population differences in nuclear quadrupole systems. The performance of the first cryostat is analyzed with emphasis on improvements to be made.

Effects of 3 He impurities on the 4 He solid-liquid interface

Abstract: Using crystallization waves we have measured the interfacial stiffness, $$\bar \alpha$$ , and the temperature dependence of the growth resistance, (Km)−1, of the4He solid-liquid interface in the presence of3He impurities. For the purest4He sample, (Km)−1 is consistent at the lowest temperatures with the assumption that the growth velocity is limited by the scattering of ballistic phonons from the moving interface. At higher concentrations of3He, we observe that (Km)−1 increases exponentially with temperature below 0.25 K. We observe that $$\bar \alpha$$ decreases with increasing3He concentration.

An attempt to observe phonon dimensionality crossover effects in the inelastic scattering rate of thin free-standing aluminum films

Abstract: We compare the inelastic scattering rate in 220-A-thick free-standing films with nearly identical films on substrates. The scattering rate was determined by fitting the magnetoresistance to theories of quantum transport. Although the films are sufficiently thin so as to modify the three-dimensional spectrum of thermal phonons, we find no significant difference between the scattering rates in free-standing and supported films. While experiments on other materials down to lower temperatures are necessary, we conclude that the cubic temperature dependence of the inelastic scattering rate in these thin aluminum films is not strongly affected by the phonon dimensionality.

Experimental approaches to the wetting transition with helium films

Abstract: Alkali metal surfaces have been suggested not to be wetted by helium at zero temperature. Several experiments have been realized to check if these surfaces, as well as other weak binding substrates, indeed exhibits this particularly interesting property at any finite temperature. Although works are still in progress, essential results are (i) substrates non wetted by helium films do exist (ii) prewetting jumps from low to high coverage have been observed for weakly wetted ones (iii) the consequences of substrate roughness and chemical impurities are suspected to be important and remain to be assessed. These results are reviewed and discussed. Applications of those substrates for both3He and4He isotopes are considered.

Calculation of He II flow in tubes

Abstract: The flow of superfluid helium through a tube with different temperatures at the ends differs considerably from that of a Newtonian fluid. The strong dependence of the thermodynamic properties on temperature, the internal convection mechanism and the structure of superfluid turbulence causes unusual flows. The equations for the flow of He II are integrated using a new one-dimensional, steady state model to study the flow in a tube. A wide range of driving conditions is studied. The temperature and pressure profiles along the tube fall into four classes. A dimensionless parameter called σ is defined which determines the progression through the four classes of behavior. The deviation of the flow from Newtonian is measured by σ. Significant maxima of the temperature and pressure can occur between the ends of the tube for large values of σ. The shapes of the profiles and the mass flux depend primarily on σ, the geometry and the boundary conditions. Formulas are presented which relate the variables of interest to the boundary conditions. These formulas result from averaging the equations of motion along the tube. A general and unified approach, based on σ, is presented for analyzing experimental data and designing new experiments. It is shown that the common practice of neglecting the pressure term in the energy equation results in poor prediction for many situations. The occurrence of large maxima of pressure and temperature imply that the interpretation of some of the experimental data of the literature should be reconsidered.