Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N33: Quantum Fluid and Solids III |
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Sponsoring Units: DCMP Chair: Chris Gould, University of Southern California Room: LACC 511C |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N33.00001: Liquid helium in disorder and boson localization Francesco Albergamo, Jonathan Pearce, Henry Glyde, David Daughton, Norbert Mulders, Jacques Bossy, Helmut Schober Neutron scattering measurements of the excitations of liquid $^4$He confined in three porous media focusing on temperatures around the superfluid-normal fluid critical temperature $T_c$ are presented and discussed. The three porous media are Vycor ($T_c = 2.05$ K at SVP), 44 {\AA} pore diameter gelsil ($T_c = 1.92$ K at SVP) and 25 {\AA} pore diameter gelsil ($T_c \approx 1.0$ K at SVP)~${^{[1,2]}}$. In all these media, liquid $^4$He supports well-defined phonon-roton excitations above $T_c$, in the "normal" phase (up $T_\lambda = 2.17$ K at SVP). Since well-defined excitations are associated with Bose-Einstein condensation (BEC), this suggests that there is BEC in the "normal" phase. Also, since there is no superflow, this BEC is apparently localized in the media separated by regions of normal fluid. In this picture, the superfluid-normal transition in disorder is associated with an extended-localized BEC crossover with localized BEC remaining above $T_c$~${^{[3]}}$. \\${^{[1]}}$F. Albergamo et al., Phys. Rev. B 69, 014514 (2004) \\${^{[2]}}$J. V. Pearce et al., Phys. Rev. Lett. 93, 145303 (2004)) \\ ${^{[3]}}$H. R. Glyde et al., Phys. Rev. Lett. 84, 2646 (2000) [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N33.00002: Excitations of metastable superfluid $^4$He at pressures up to 40 bars Jonathan Pearce, Jacques Bossy, Helmut Schober, Henry Glyde, David Daughton, Norbert Mulders We have performed neutron scattering measurements~${^{[1]}}$ of
the fundamental excitations
of liquid $^4$He confined in 44 {\AA} pore diameter gelsil glass
at pressures up to 40 bars in the wave vector range 0.4 $ |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N33.00003: $^4$He Films Adsorbed in Porous MCM--41 Ceramic: Crossover to One-dimensional Superfluidity Han-Ching Chu, Gary A. Williams The superfluid density of $^4$He films adsorbed in MCM-41 ceramic having long \hbox{40-\AA}-diameter cylindrical pores is studied with a torsion oscillator technique. Finite-size Kosterlitz-Thouless transitions are observed in the data, with the broadening of the transitions increasing considerably as the film thickness decreases. The data are in good agreement with the Machta-Guyer theory of the KT transition in a cylindrical channel, but fits to the broadening require an increasing vortex core size. In the thinnest films a crossover to the one-dimensional behavior predicted theoretically is observed, where the vortex core radius limits at the cylinder radius and the $T = 0$ superfluid density approaches zero. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N33.00004: Structure of He-4 adsorbed on single-wall carbon nanotube bundles Oscar Vilches, Jonathan Pearce, Mark Adams, Mark Johnson, Henry Glyde The structure and dynamics of $^4He$ adsorbed on single-wall carbon nanotubes (SWNT) has for a long time been the subject of intensive theoretical investigation. Here we present the first experimental measurements of the structure of $^4He$ adsorbed on SWNTs, obtained using neutron diffraction techniques. The structure of this highly quantum system has been measured as a function of the $^4He$ coverage, up to one monolayer, and appears to be qualitatively similar to that of more classical systems. By combining the current data with our existing measurements of the heat capacity and isosteric heat of adsorption, we are able to identify the contributions from the linear chains in the outer groove sites, and the two- dimensional patches on the curved outer walls. In conjunction with molecular dynamics simulations, we are able to draw some conclusions about the population mechanism of $^4He$ on SWNTs based on direct observations of the adsorbate structure. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N33.00005: Ellipsometery to Probe prewetting and superfluid transitions of thin liquid helium films on rubidium Timothy McMillan, James Rutledge, Peter Taborek Helium films adsorbed on intermediate binding strength substrates, like rubidium, exhibit markedly different behavior than on more common strongly binding substrates, or weask substrates like cesium. Previous experiments on Rb using a quartz crystal microbalance(QCM) have shown that the prewetting and superfluid transitions occur at virtually the same chemical potential. The superfluid transition seems to be coupled to the prewetting transition and hysteretic, which would be an apparent disagreement with Kosterlitz-Thouless paradigm. By using a modulated null Ellipsometer in conjunction with the QCM we are able to simultaneously measure both the total coverage and the normal component of the helium film. This allows us to separate the prewetting and superfluid transitions and document how the binding potential of the substrate may lead to deviations from a standard Kosterlitz-Thouless transition. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N33.00006: Simultaneous Measurement of Casimir Film Thinning and Superfluid Density in $^4$He Films Near the $\lambda$-Transition John Abraham, Gary A. Williams, Konstantin Penanen, Talso Chui Experiments are being undertaken to measure simultaneously the Casimir film thinning effect and the superfluidity density of $^4$He films near $T_\lambda$. A silicon substrate with nanometer-scale roughness will be employed to minimize any effects of capillary condensation. The superfluid density and superfluid onset point will be monitored by third-sound propagation at resonant frequencies below 5 Hz. Metallic capacitor plates for both the thickness measurement and third sound detection will be evaporated on the back sides of the silicon wafers to avoid perturbing the film being measured. High-resolution thermometry will be employed to minimize heat input from temperature regulation, which can lead to fountain pressures and nonuniform film thickness. A precise location of the KT transition onset point relative to the film thinning may help to resolve existing discrepancies between theory and experiment. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N33.00007: Superfluidity and Capillary Condensation in Porous Materials R.J. Lazarowich, P. Taborek, J.E. Rutledge We have studied superfluid onset and capillary condensation in liquid helium in porous gold using the quartz crystal microbalance (QCM) technique. Au/Ag alloy films were sputtered onto QCM surfaces, and used to form porous gold substrates of various thicknesses and pore size. Helium isotherms on these samples show that at low temperature where the superfluid transition occurs prior to capillary condensation, superfluid onset has the conventional features of a KT transition, including mass decoupling and dissipation. At higher temperatures, the superfluid transition occurs within the capillary condensation hysteresis region where the film thickness is a multivalued function of the chemical potential. In this regime, the only signature of superfluid onset is a peak in the dissipation; there is no discernable mass unloading. This behavior persists even at temperatures near Tlambda, where onset occurs when the pores are full of liquid. To determine how universal this behavior is, we have attempted to make a porous material with uniform pore size by electrochemical anodization of aluminum films on a QCM. We will present preliminary isotherms on these porous alumina films. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N33.00008: Capillary Condensation of Helium in Aerogels of Different Porosity John Beamish, James Day, Tobias Herman When fluids are adsorbed in small pores, capillary forces are large and usually result in hysteresis between adsorption isotherms taken during filling and emptying. However, previous measurements with fluids in high porosity silica aerogels showed non-hysteretic behavior which could be interpreted as an equilibrium liquid-vapor coexistence curve. This curve was much narrower than in bulk fluid and the critical temperature, T$_c$, was suppressed. We have made direct capacitive pressure-density measurements near the critical point of helium confined in aerogels with porosities between 95\% and 98\%. We see hysteresis in isotherms in both aerogels and, although the aerogels fill over very narrow pressure ranges, we never see a true two-phase coexistence region. The hysteresis loops shrink and eventually disappear at about 5.155 K (in the 95\% porosity aerogel) and 5.175 K (in the 98\% porosity aerogel). We compare the shapes of the hysteresis loops in the two aerogels and how they evolve near the critical point. This work was supported by a grant from NSERC. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N33.00009: Measurements of $^3$He magnetiztion on ZYX graphite using SQUID NMR Jinshan Zhang, Lei Guo, Yuliang Du, C.M. Gould, H.M. Bozler $^3$He films on ZYX graphite are more nearly ideal 2D systems than is the case with $^3$He on more commonly used grafoil substrates, given the larger platelet size (100-200 nm) and smaller spread angle ($\sim$5 degrees) of ZYX. We have begun a study of these films using our SQUID NMR techniques. We are currently surveying a range of coverage from a little less than first layer completion to greater than third layer promotion in order to map out differences between the magnetism of $^3$He on ZYX and grafoil. The primary objective of this study is to sort out the size effects on the apparent finite temperature ordering of these films in the ferromagnetic coverage region. The small surface area of ZYX graphite (1-1.5 m$^2$/g) poses an experimental challenge. We will present details of our wide bandwidth two-stage SQUID system used in these experiments. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N33.00010: Longitudinal and transverse NMR study of superfluid $^3$He in aerogel Yuliang Du, Lei Guo, Jinshan Zhang, H.M. Bozler, C.M. Gould The comparison of longitudinal and transverse resonances in the superfluid phases of $^3$He was the most decisive confirmation of the superfluid order parameters and of Leggett's theoretical predictions. Recently, the study of the impact of impurities on superfluid $^3$He has been made possible by using low-density aerogel as a filler in the liquid. Predictions of an altered superfluid order parameter in aerogel can be tested by comparing longitudinal and transverse resonances. Previously, longitudinal resonance has been measured using conventional NMR techniques with a high Q coil. One characteristic of the longitudinal NMR line is its short effective T$_2$ making conventional pulsed NMR techniques difficult. We expect that the presence of aerogel would make T$_2$ even shorter. SQUID NMR offers wide bandwidth at low frequencies. We will discuss our efforts to observe longitudinal resonance in both bulk superfluid and aerogel using SQUID techniques. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N33.00011: Impurity Scattering in Superfluid $^3$He: A New Phase James Baumgardner We present continuous-wave NMR measurements of superfluid $^3$He confined to two low-density silica aerogels, with porosities of 99.3\% and 98.6\%. We find within the aerogel two superfluid phases of differing symmetries separated by a first-order transition. The transition between these two phases is broadened by strong interfacial pinning, even near $T_c$. This should preclude determination of the equilibrium transition temperature between the two phases, but we have developed a method for determining this temperature, despite the pinning. We find that the thermodynamic transition temperature versus sample pressure within the aerogel differs greatly from the bulk, yet is independent of aerogel density. We conclude that the presence of aerogel has stabilized a new phase, not known to exist previously. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N33.00012: Bose-Einstein Condensation and atomic kinetic energies in liquid ${^3}$He-${^4}$He Souleymane Diallo, Jonathan Pearce, Richard Azuah, Henry Glyde We present Deep Inelastic Neutron Scattering (DINS) measurements of mixtures of liquid ${^3}$He-${^4}$He in both the superfluid and normal phases. The measurements were performed on the MARI time-of-flight spectrometer at the ISIS pulsed spallation neutron source, at wavevectors $26\le$ Q $\le29$ \AA$^{-1}$ for four different $^{3}$He concentrations $x$. From the data, we extract both the condensate fraction $n_0$ and the single particle kinetic energies $\langle K_3\rangle$ and $\langle K_4\rangle$ of each isotope. We find a relative increase in $n_0$ from 7.25\% for the bulk data ($x=0\%$) to $\sim$12\% for $x=20\%$, in agreement with theoretical calculations but less than that found in the only other DINS measurement of $n_0$. The measured values of $\langle K_3\rangle$ ($\sim$11 K for $x=10\%$) fall in the range of previous DINS measurements. Surprisingly, $\langle K_4\rangle$ is found to be somewhat independent of $x$, in contrast to most calculations and measurements. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N33.00013: Structure in the Condensate Occupation of Trapped Hard Sphere Bosons Jonathan L. DuBois, K. Birgitta Whaley We have performed detailed variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) calculations of the ground state and condensate properties for small numbers $(1 < N < 100)$ of hard sphere bosons in a harmonic trap. Condensate properties are obtained by evaluating the eigenvectors (Natural Orbitals) and eigenvalues (Occupation numbers) of the one body density matrix. For macroscopic systems with weak interactions, $na^3 \ll 1$, the OBDM description of the condensate in terms of a single large eigenvalue with large occupation and the ``condensate wave-function" of Gross-Pitaevskii (GP) theory are equivalent. Unlike GP theory, however, condensate properties obtained within the OBDM formulation are a property of the full ground state wave-function of the many-body Hamiltonian and are equally valid for any number of particles and interaction strength. We find that in the mesoscopic regime, $N < 30$ \& $na^3 > 10^{-3}$, the $N$ dependence of the fraction of particles in the condensate orbital $n_0(N)$ is not a simple monotonically decreasing function of $N$. Quite remarkably, we find that there are instead ``magic" particle numbers for which the condensate experiences anomalous depletion. This structure in the condensate fraction will be described fully and possible implications for experiment will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N33.00014: Structure Factor of Superfluid He II about Dispersion Minimum J.X. Zheng-Johansson, P-I. Johansson The SHM-RSB/$\Delta_b$-dynamic scheme of He II has within the framework of the well established condensed matter theory facilitated the predictions of superfluidity, critical velocity, circulation quantization, and other key properties of He II in overall good agreement with experiments[1]. In relevance to neutron scattering this scheme leads to that at the dispersion minimum $(q_b, \Delta_s)$, the structure factor of He II contains an elastic and inelastic component: $S(q)=S^{{\rm el}} (q)+ S^{{\rm inel}}(q)$. Here $S^{{\rm el}}(q)=\frac{N}{2\pi \hbar}e^{-2W}[1+\int g(R) e^{i {\bf q}_b\cdot {\bf R}} d R] f_0 \delta(\omega-0) $ probes the instantaneous configuration of the disordered superfluid atoms and is a broad function. $S^{{\rm inel}}(q)=\frac{1}{2\pi\hbar N}\delta(q-q_b) f_b \delta(\omega- \frac{\Delta_s}{\hbar}) $ is due to scattering by the excitations of superfluid bond $\Delta_b$, at an energy cost $\Delta_s$. (Definitions for other variables are given e.g. in [1]2004b.) $\Delta_b$ has its origin in many-quantum-atom correlation and thus has a well defined value through this many- atoms averaging operation. Accordingly $S^{{\rm inel}}(q)$ is a sharp function; it is related to the dynamic structure factor here as: $S^{\rm inel}_b(q)|_{q=q_b}= S_b(q,\omega)|_ {\omega=\frac{\Delta_s}{\hbar}}$.\\[0cm] [1] J.X. Zheng- Johansson and P-I. Johansson, in "New Developments in Superconductivity Research", R.W. Stevens Editor, Nova Science, 2003, ISBN 1-59033-862-6; "The Microscopic Theory of Superfluid He II", Nova Science, 2004a, ISBN 1-59033-974-6; arXi:cond- mat/0410442, 2004b; arXi:cond-mat/0410485, 2004c. [Preview Abstract] |
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N33.00015: Magnetism of a multielectron bubble in liquid helium Jacques Tempere, Isaac F. Silvera, Jozef T. Devreese Multielectron bubbles are cavities in liquid helium containing electrons. A typical N=10000 electron bubble is forced open by the Coulomb repulsion of the electrons, balanced by the surface tension of the helium, leading to a typical radius of 1 micron. The electrons in the bubble form a spherical two-dimensional electron gas (S2DEG): they collect in a nanometer thin layer anchored to the in inner surface of the bubble. We investigate the properties of this S2DEG, both for weak and strong magnetic fields. Already at a few tens of gauss, typical multielectron bubbles enter the strong-field regime where the single-particle energy levels arrange themselves in Landau bands of doubly degenerate levels in stead of highly-degenerate Landau levels. [Preview Abstract] |
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