Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S31: Quantum Fluids |
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Sponsoring Units: DCMP Chair: Yoonseok Lee, University of Florida Room: Colorado Convention Center 401 |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S31.00001: High Frequency Transverse Acoustics in Superfluid $^{3}$He John P. Davis, Hyoungsoon Choi, Johannes Pollanen, William P. Halperin Acoustic studies of superfluid $^{3}$He have a rich history and have contributed significantly to our understanding. Recently we have made technical advances that have allowed us to extend transverse acoustic measurements to higher frequencies. We will discuss these techniques, as they are applicable to a broad range of acoustic measurements. The relevant frequency in superfluid $^{3}$He is associated with the pair energy, which varies from 69 to 194 MHz in the range of 0 to 34 bar. With our improved acoustic response, up to the 200 MHz range, we can probe the structure of the order parameter deep into the superfluid at higher pressures than before. This has allowed us to make precision measurements of one of the collective modes of superfluid $^{3}$He, the Imaginary Squashing Mode [1]. From these measurements we have extracted values for the strength of $f$-wave pairing interactions in this dominantly $p$-wave superfluid. We will discuss our results in comparison with earlier measurements from the Acoustic Faraday Effect [2]. [1] J.P. Davis, H.Choi, J. Pollanen, and W.P. Halperin, Phys. Rev. Lett. \textbf{97}, 115301 (2006). [2] Y. Lee, T.M. Haard, W.P. Halperin and J.A. Sauls, Nature \textbf{400}, 431 (1999). [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S31.00002: High Porosity Silica Aerogels Engineered for Superfluid $^{3}$He Research J. Pollanen, S. Blinstein, H. Choi, J.P. Davis, T.M. Lippman, K.R. Shirer, W.P. Halperin, L.B. Lurio Silica aerogel is a network of strands with a diameter of 3 nm and average separation \textit{$\xi $}$_{a}\approx $ 30 -- 100 nm. Low-density aerogel can be used to introduce disorder in superfluid $^{3}$He because the superfluid coherence length is of the same order as \textit{$\xi $}$_{a}$. We have developed novel sample growth and preparation techniques for producing aerogels for a variety of measurements on superfluid $^{3}$He. In particular, it has been proposed that anisotropic aerogels can be used to understand the stability of the A-like superfluid $^{3}$He phases [1, 2]. We can introduce anisotropy in aerogel on length scales relevant to superfluid $^{3}$He. Anisotropy can be induced with uniaxial strain, or alternatively, during growth and drying stages. We have performed small angle x-ray scattering to probe these two types of anisotropy and find that uniaxial strain can be used to tune between them. [1] C.L. Vicente, \textit{et al.}, \textit{Phys. Rev. B.} \textbf{72}, 094519 (2005). [2] K. Aoyama and R. Ikeda, \textit{Phys. Rev. B} \textbf{73}, 060504(R) (2006). [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S31.00003: Understanding Superfluid $^{3}$He by Determining \textit{$\beta $}-Coefficients of Ginzburg-Landau Theory H. Choi, J.P. Davis, J. Pollanen, W.P. Halperin The Ginzburg-Landau (GL) theory is a phenomenological theory that is used to characterize thermodynamic properties of a system near a phase transition. The free energy is expressed as an expansion of the order parameter and for superfluid $^{3}$He there is one second order term and five fourth order terms. Since the GL theory is a phenomenological theory, one can determine the coefficients to these terms empirically; however, existing experiments are unable to determine all five fourth order coefficients, the \textit{$\beta $}'s. To date, only four different combinations of \textit{$\beta $}'s are known [1]. In the case of supeprfluid $^{3}$He, using quasiclassical theory, the coefficients have been calculated [2]. We used the calculation as a guide to construct a model to define all five \textit{$\beta $}'s independently. The model provides us with the full understanding of the GL theory for $^{3}$He, which is useful in understanding various superfluid phases of both bulk $^{3}$He and disordered $^{3}$He in aerogel. \newline [1] H. Choi \textit{et al}., J. Low Temp. Phys., submitted; http://arxiv.org/abs/cond-mat/0606786. \newline [2] J.A. Sauls and J.W. Serene, Phys. Rev. B \textbf{24}, 183 (1981). [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S31.00004: Anisotropic properties of superfluid $^3$He near free surface studied by surface electrons Kimitoshi Kono, Hiroki Ikegami We performed conductivity measurements of the Wigner solid on the surface of superfluid $^3$He-A and B phases under magnetic fields. The $^3$He-A phase has a nodal point of energy gap at North and South Poles of the Fermi sphere and is anisotropic. A unit vector directing from the South Pole to the North Pole is refered to as the $\hat{\mbox{\boldmath$\ell$}}$ vector. The $\hat{\mbox{\boldmath$\ell$}}$ vector tends to align parallel to a surface normal. The conductivity of the Wigner solid is sensitive to the quasiparticle distribution, and hence, sensitive to the alignment of the $\hat{\mbox{\boldmath$\ell$}}$ vector. Our observation is in good agreement with the abovementioned picture of the $\hat{\mbox{\boldmath$\ell$}}$ vector alignment. In the B-phase the situation is more subtle. Nevertheless, our observation gives a strong support for the present understanding of the magnetic-field-induced anisotropy of the B-phase. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S31.00005: Ultrasound Propagation in the Normal State of Liquid $^{3}$He/ 98{\%} Aerogel. H.C. Choi, N. Masuhara, B.H. Moon, P. Bhupathi, M.W. Meisel, Y. Lee, N. Mulders We studied the propagation of longitudinal sound in the normal state of liquid $^{3}$He/ 98{\%} aerogel at 9.5 MHz. The absolute attenuation and sound velocity were determined by direct propagation of sound pulses through the medium. Our measurements cover a wide range of temperatures from 2 mK to 200 mK at three different pressures (10, 21 and 29 bars). As reported by Nomura \textit{et al}., the sound mode remains in the hydrodynamic limit down to 2 mK due to the impurity scattering off the aerogel. However, we observed a new feature in the high temperature range that the attenuation shows a minimum and increases at high temperature. The minimum (T$_{M})$ occurs around 60 mK at 10 bars and moves to 40 mK at 29 bars. For T $>>$ T$_{M}$, the attenuation at high temperature shows a T$^{0.7}$ dependence for all pressures. We will discuss our observations in the framework of theories proposed by Higashitani \textit{et al.} and Biot. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S31.00006: Mass of a quantized vortex David Thouless, James Anglin There have been many discussions of the mass of quantized vortices in superfluids, but different conclusions have been reached. There is a consensus that vortex mass diverges in compressible superfluids. We have studied the vortex mass in an incompressible quantum fluid by considering a vortex driven slowly round a circular orbit, treating frequency and speed as small parameters. The centrifugal force measures the vortex mass. If a vortex is driven by a large-radius repulsive potential its mass is close to the mass of fluid displaced, as in classical hydrodynamics, but for small pinning radius the mass diverges as the logarithm of the pinning radius. It can be argued that this logarithmic dependence on the pinning radius is a general feature of models of quantized vortices. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S31.00007: Vortex stability influenced by surface topology I. Neumann, P. Voll, N. ApRoberts-Warren, R.J. Zieve We examine the stability of a pinned superfluid helium vortex line. The vortex pins around a thin wire, which terminates at each end at either a rounded bump, a conical indentation, or a flat surface. With the cryostat stationary, we measure the persistence of the vortex. With no external disturbance, it remains indefinitely. We briefly heat the cell and find the temperature at which the vortex depins. By observing the vortex motion after it partially detaches from the wire, we can determine at which terminus it detached. We find that pinning terminating at a bump is generally the easiest to overcome thermally, and pinning at a flat end is the hardest. This pattern would not be expected from considerations of vortex line energy alone. We take the observations as evidence of an additional contribution to the pinning energetics. One possibility is an interaction of the vortex with the curvature of the containers surface which favors pinning at points of negative Gaussian curvature, making the bump terminus a less advantageous pin site. The combined effects of vortex line energy and this surface curvature interaction may explain the observed vortex depinning behavior. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S31.00008: Energy Loss from Reconnection with a Vortex Mesh Rena Zieve, Ingrid Neumann We have observed experimentally that a vortex moving in a cylindrical cell loses energy up to eight orders of magnitude more rapidly than expected from bulk mutual friction alone. Here we investigate the possibility that reconnections with a mesh of small vortices pinned to the cell wall dominate the energy loss. Such pinned vortices may be an unavoidable consequence of rotating the cryostat. Once rotation ceases, most vortices move to the cell wall and annihilate, leaving behind fragments pinned at any microscopic wall roughness. We simulate the situation by requiring the free vortex to move at the local superfluid velocity, and by allowing for reconnections when two vortices approach closely. To keep the simulation run time practical, we use an artificially high friction coefficient. We find that as the vortex moves, reconnections with pinned vortices can reduce its length, and hence its line energy. The energy dissipation of the vortex moving through the mesh can exceed the loss rate from mutual friction by 50\% to 100\%. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S31.00009: Phase-slip avalanches in the superflow of $^4$He through arrays of nano-apertures David Pekker, Roman Barankov, Paul M. Goldbart Recent experiments have explored the dynamics of $^4$He superflow through an array of nano-apertures~[1]. These experiments have found that, as the temperature is lowered, phase-slippage in the apertures changes its character from synchronous to asynchronous. Here, we construct a model~[2] of the superflow that incorporates two basic ingredients: (a)~disorder associated with each aperture having its own random critical velocity, and (b)~an effective inter-aperture coupling, mediated through the bulk superfluid, which stimulates the apertures in the neighborhood of an aperture that has already phase-slipped also to slip. We find that at lower temperatures the synchronicity is lost, due to broadening of the distribution of the critical velocities associated with the reduction of the superfluid healing length. We also observe that as the disorder becomes weak, compared to the inter-aperture coupling, there is a non-equilibrium transition from a regime of small phase-slip avalanches to a regime in which interactions between phase-slips in nearby apertures lead to system-wide phase-slip avalanches. \newline [1] Y. Sato, E. Hoskinson, and R. E. Packard, Phys. Rev. B {\bf 74}, 144502 (2006).\newline [2] D. Pekker, R. Barankov, and P. M. Goldbart, cond-mat/0606560. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S31.00010: Thinning of superfluid films: critical effects immediately below the $\lambda$ point Aviva Shackell, Roya Zandi, Joseph Rudnick, Mehran Kardar, Lincoln Chayes Experiments on $^4$He films reveal the presence of an attractive Casimir-like force at the bulk $\lambda$-point and in the superfluid regime. We address the unexpectedly large magnitude of that force in the regime immediately below the $\lambda$ point. A simple mean field calculation incorporating the appropriate boundary conditions and adjusted for the renormalizing effects of critical fluctuations points to the source of this dramatic behavior. We find that the location of the minimum of the scaling function is at $x=tL^{1/\nu}=-\pi^2$ in excellent agreement with the experimental finding of $x=tL^{1/\nu}=-9.7 \pm 0.8 $. Other aspects of the effective force induced by critical fluctuations will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S31.00011: Superfluid p-H$_{2}$ Monolayer in Carbon Nanostructures Milen Kostov A fluid of para-hydrogen (p-H$_{2})$ molecules is a prime candidate for potential superfluid, due to the light mass (half the mass of helium) and the existence of a compound boson ground state. In bulk p-H$_{2}$ superfluidity is not observed because, unlike helium, molecular hydrogen solidifies at a temperature (triple point T=13.8 K) significantly higher than that (T$\sim $2K) at which such phenomena as Bose Condensation and, possibly, superfluidity (SF) might occur. This is due to the fact that H$_{2}$- H$_{2}$ interaction is significantly stronger than the He-He one (more than a factor of three in the well depth). One way to attain a liquid ground state at low T is to reduce the effective attraction between the H$_{2}$ molecules. Here a novel solution to the problem is proposed, which implies that a SF monolayer p-H$_{2}$ can be achieved in a carbon slit-pore with height $H\sim $5.8 {\AA}, where the alignment of the graphitic planes corresponds exactly to the AB stacking sequence in a pristine hexagonal graphite crystal. Our approach is based on the idea to attain a liquid ground state of p-H$_{2}$ monolayer at low T (T$\sim $2K), through a substantial renormalization of the pair interaction of p-H$_{2}$ molecules due to their interaction with the surface electrons of the carbon slit pore. In this environment, the resulting \textit{de Boer quantum parameter }\textit{$\eta $} for the adsorbed p-H$_{2}$ film lies in the vicinity of the threshold value for zero-temperature Bose liquid. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S31.00012: Properties of Helium-4 In and Near the Self-Organized Critical State S.T.P. Boyd, D.A. Sergatskov, R.V. Duncan If a downward heat flux is imposed on a sample of $^{4}$He near $T_{\lambda}$, the sample can self-organize so that its temperature tracks the variation of $T_{\lambda}$ induced by the hydrostatic pressure head. This ``Self-Organized-Critical'' (SOC) state is the only means by which a uniform reduced temperature very close to $T_{\lambda}$ can be achieved on Earth in $^{4}$He. We recently reported preliminary analysis of extensive new measurements of the SOC state showing three new results: strong nonlinearity in the upward-going wave under high drive levels, the qualitative form of the breakdown of the SOC state with increasing downward heat flux greater than $\sim $12$\mu $W/cm$^{2}$, and, most intriguingly, we have corroborated and extended the as-yet unexplained result of Lee \textit{et al.} that the thermal resistivity of helium-II near $T_{\lambda }$ is larger under downward heat flux than it is under upward heat flux of equal magnitude (upward resistivity measured previously by Baddar \textit{et al.}). We find that the ``downward'' resistivity exceeds the ``upward'' by factors ranging from 18X at 20$\mu $W/cm$^{2}$ to 12X at 80$\mu $W/cm$^{2}$. Here we report results of further analysis which help to quantify and flesh out this intriguing picture. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S31.00013: High Tc Magnet Leads for Research Cryostats Yuko Shiroyanagi, Gokul Gopalakrishnan, Sanghun An, Thomas Gramila The incorporation of high temperature superconducting wires in cryogenic systems has almost exclusively been in those systems with active cryocoolers, or when very high currents are necessary. Despite their obvious advantages, however, various properties of the wires have precluded their use in typical liquid Helium research cryostats. We report here the successful implementation of these wires into a research cryostat magnet lead design, and will discuss design features, aspects of assembly, and characterization of the lead system. The overall design is based on a baffle cooled approach [1] for removing heats from the leads, whose development involved careful numerical modeling. The design approach used for the Hi-Tc magnet lead system leverages this capability to address the various problems associated with superconducting wires, permitting their incorporation. \newline [1] Y.Shiroyanagi, G. Gopalakrishnan, S.An and T.J. Gramila, ``Novel Approach for Magnet Leads,'' submitted to JLTP. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S31.00014: Wave-function approach for a rotating fermionic superfluid Victor Vakaryuk Rotation of a neutral fermionic superfluid in annular geometry is considered using Gross-Pitaevskii ansatz for the wave function of the system. It is shown that, in the thermodynamic equilibrium, the rotation frequency at which transition between different total angular momentum states occurs is independent on interparticle interactions assuming they are central. The question of whether or not the equilibrium state of a superconductor in a magnetic field corresponds to rotation is also addressed. [Preview Abstract] |
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