Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W47: Superfluid He4 and Other Quantum Liquids |
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Sponsoring Units: DCMP Chair: Shaun Fisher, Lancaster University Room: Mile High Ballroom 4F |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W47.00001: Pressure driven flows of superfluid helium-4 through a single nanopipe Angel Velasco, Crystal Yang, Zuzanna Siwy, Peter Taborek We have measured flow rates of helium-4 through single etched nanopores of 72 nm and sub-20 nm diameter in PET and mica respectively with a mass spectrometer. Flow rates were measured as a function of pressure at constant temperature and at saturated vapor pressures along the coexistence curve between 0.5 K and 3.0 K. Due to the constraint of the mass spectrometer the low pressure side was maintained at P$=$0 creating an intrinsic superfluid/vapor interface which forms inside the pipe or at its exit. We observed flow velocities in the range of 2-4 m/s in the low temperature region which is consistent with Feynman's critical velocity. Near the lambda point our temperature dependent critical velocity did not agree with the thermal vortex nucleation theory. The superfluid transition temperature was measured to be suppressed by 2-3 mK in the 72 nm nanopore. We have also measured flow rates in the normal state and found rates in exact agreement with conventional viscous theory with zero slip length. The results were also consistent with previous nanofluidic studies. Supported by NSF DMR-0907495. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W47.00002: The dynamic pair distribution function of superfluid $^{4}$He Souleymane Diallo, Wojciech Dmowski, Kostantin Lokshin, Georg Ehlers, Takeshi Egami We present precision neutron scattering measurements of the spatial and time correlations between atoms in liquid $^{4}$He using dynamic pair distribution methods. As the Bose-Einstein condensate (BEC) sets in below the superfluid transition temperature $T_{\lambda}$, we observe clear changes in the local environment of the atoms. These local changes are investigated beyond the first coordination shell. We also test our observations against recent classical and ab-initio molecular dynamics simulations in which the local configurational excitations in the atomic connectivity network was found to be the elementary excitation in some liquid metals. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W47.00003: Towards the in-situ detection of a single He2* excimer in superfluid helium Faustin Carter, Scott Hertel, Michael Rooks, Daniel Prober, Daniel McKinsey Incident radiation can excite superfluid helium into a diatomic He2* excimer, which decays through the emission of a 15 eV photon. Such excimers have been used as tracers to measure the superfluid's quantum turbulence, thanks partly to the long half-life of the He2* triplet state (13 seconds). However, the efficient detection of these excimers remains a challenge. We present a detector capable of in-situ detection of the He2* excimers either directly (the excimer collides with the detector), or by collecting the 15 eV photon emission upon decay. This detector is based on a tungsten superconducting transition edge sensor and is designed to operate near 100 mK in a dilution refrigerator. We will discuss operating characteristics and present preliminary data with an aim towards the detection of a single excimer. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W47.00004: Flow visualization in superfluid helium-4 using a thin line of He$_{2}$ excimer tracers Alex Marakov, Jian Gao, Wei Guo, Steven Van Sciver, Gary Ihas, Daniel McKinsey, William Vinen Cryogenic flow visualization techniques have been proven in recent years to be a very powerful experimental method to study turbulence in superfluid helium-4 (He II). In order to extract quantitative information of the flow field, we developed a new technique based on the generation of a thin line of He$_{2}$ excimer tracers via femtosecond-laser field ionization. These tracers move solely with the normal-fluid component in He II and can be imaged using a laser-induce fluorescence technique. Studying the drift and distortion of the tracer line in a turbulent flow shall allow us to measure the instantaneous flow velocity field and hence determine the structure functions and the energy spectrum of the turbulence. We discuss the preliminary results obtained that for the first time visually reveal the existence of a laminar-to-turbulent transition in the normal fluid in thermal counterflow. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W47.00005: Localized Bose-Einstein Condensation in Liquid $^4$He in Disorder Henry R. Glyde, Jacques Bossy, Jacques Ollivier, Helmut Schober Liquid $^4$He in porous media is an example of Bosons confined to nanoscales and in disorder. In porous media, the superfluid phase is suppressed to low temperature. The smaller the pore size, the further the transition temperature to the superfluid phase, $T_c$, is suppressed below the bulk value, $T_\lambda$. In 28 A pore diameter FSM, for example, $T_c$ = 0.8 K compared with $T_\lambda$ = 2.17 K at saturated vapor pressure[1]. We present measurements of the phonon-roton (P-R) modes and of Bose-Einstein condensation (BEC) of liquid $^4$He in porous media. These measurements show that the ``critical" temperature of BEC in porous media, $T_{BEC}$, lies close to $T_\lambda$ largely independent of the porous media investigated. There is a temperature range, $T_c$ $< T <$ $T_{BEC}$, in which there is BEC but no superflow. This is interpreted as a ``localized" BEC (LBEC) region. The model of the LBEC liquid is isolated islands or blobs of BEC separated by otherwise normal liquid. The phase of each island is independent so that there is no phase coherence across the sample as needed for superflow. The P-R data and LBEC phase will be discussed.\\[4pt] [1] J. Taniguchi, Y. Aoki and M. Suzuki, Phys. Rev. B{\bf 82}, 104509 (2010). [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W47.00006: Localized Bose-Einstein Condensation in films of Liquid $^4$He in Disorder Jacques Bossy, Jacques Ollivier, Helmut Schober, Henry R. Glyde When porous media is only partially filled, the helium is deposited as films on the porous media walls. The initial helium is tightly bound on the media walls, denoted ``dead layers". In subsequent fillings there is a transition to a superfluid phase below a critical temperature $T_c$ and $T_c$ increases with increasing filling. We present measurements of phonon-roton modes in liquid $^4$He films on 25 A diameter gelsil at fillings greater than 70\%. P-R modes are observed at low temperature and up to a maximum temperature denoted $T_{PR}$ which also increases with filling. Above $T_{PR}$ well-defined P-R modes are not observed. Since well-defined P-R modes exist where there is Bose-Einstein condensation (BEC), $T_{PR}$ is associated with $T_{BEC}$, the critical temperature for BEC in films. $T_{PR}$ lies above $T_c$ observed by Yamamoto et al. (Phys. Rev. Lett. 93, 075302 (2004)) in the same gelsil. There is a temperature range $T_c$ $ < T <$ $T_{BEC}$ where there is BEC but no superflow. This is identified as a ``localized" BEC region in which the BEC exists in islands or blobs separated by normal liquid. The P-R mode data and LBEC region will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W47.00007: Third Sound Measurements of Superfluid $^4$He Films on Multiwall Carbon Nanotubes Below 1 K Emin Menachekanian, Vito Iaia, Andrew Li, Bob Chen, Gary Williams Third sound is studied for superfluid films of $^4$He adsorbed on multiwall carbon nanotubes of average diameter 12 Angstroms packed into an annular resonator. The third sound is generated with mechanical oscillation of the cell, and detected with carbon bolometers. A filling curve at temperatures near 250 mK shows oscillations in the third sound velocity, with maxima at the completion of the third and fourth atomic layers. The ``dead'' layer appears to be close to two atomic layers, about one layer thinner than previously found for flat graphite surfaces. We attribute this weaker binding to the effect of the cylindrical geometry on the van der Waals potential, the repulsive surface tension forces from the high curvature, and the lower density of the tubes compared to graphite. At the completion of the third layer there is a sudden reduction of the superfluid onset temperature, and then a recovery back to the Kosterlitz-Thouless linear dependence, forming re-entrant superfluidity. In a small region around 2.5 layers there is very anomalous behavior in the low-temperature variation of the third sound velocity, which is found to increase linearly with temperature. This could be related to changes in the gas-liquid coexistence at this intermediate fill. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W47.00008: Non-universal Casimir Effect in Saturated Superfluid $^4$He Films at T$_\lambda$ John Abraham, Gary Williams, Konstantin Penanen Measurements of Casimir effects in $^4$He films in the vicinity of the bulk superfluid transition temperature $T_\lambda$ have been carried out, where changes in the film thickness and the superfluid density are both monitored as a function of temperature. A new Casimir film-thickening effect is observed precisely at $T_\lambda$ when the temperature is swept extremely slowly. We believe this arises from the viscous suppression of any second sound modes in the superfluid film, while thermally excited second sound still propagates in the bulk superfluid to within microkelvins of $T_\lambda$, giving rise to a free energy difference between the bulk and film. At $T_\lambda$ this difference drops abruptly to zero, leading to a step increase in the film thickness that we have observed. The magnitude of the step increases rapidly with the equilibrium film thickness, in agreement with a calculation of the Casimir energy balance. From the amplitude of the increase we can extract the first measurement of the second-sound free energy at $T_\lambda$, found to be about 2.6 ergs/cc. This is at least roughly consistent with a Debye-type calculation of the free energy. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W47.00009: Dimensional crossover in quantum fluids Adrian Del Maestro, Bohdan Kulchytskyy, Guillaume Gervais In one spatial dimension it is not possible to break a continuous symmetry due to strong fluctuations, even at zero temperature. Although fermionic examples of quasi-one-dimensional systems abound, including carbon nanotubes and quantum wires, it is much more difficult to reach this limit in high density bosonic fluids due to a short coherence length on the atomic scale. Recent advances in nanofabrication techniques now allow for the confinement of helium-4 inside pores with nanometer radius, but unequivocal experimental evidence of one-dimensional behavior is still lacking. We have performed large scale quantum Monte Carlo simulations near the superfluid transition and have investigated how the signatures of dimensional crossover are reflected in the thermodynamic properties of nano-confined helium-4. The results hint at the emergence of quantum hydrodynamics inside the pore at length scales that can be probed in the laboratory. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W47.00010: Observation of quantum decay of homogeneous, isotropic (grid) turbulence Gary Ihas, Lydia Munday, Jihee Yang, Kyle Thompson, Wei Guo, Roman Chapurin, Shaun Fisher, Peter McClintock, W.F. Vinen In classical grid turbulence fluid is forced through a stationary grid. In the quantum case a grid moves through an initially stationary superfluid driven by a linear motor. We have developed a motor using superconducting drive coils and bearings, moving a grid at constant speed (0 and 15 cm/s). Stalp \textit{et al}\footnote{S. R. Stalp, L. Skrbek, and R. J. Donnelly, \textit{Phys. Rev. Lett}. \textbf{\textit{82}}, \textit{4831} (\textit{1999}).} report the decay of vortex-line density $L$ in the grid's wake measured by 2$^{\mathrm{nd}}$ sound attenuation. $L$ decayed at large times as $t^{-3/2}$, interpreted as a quasi-classical Richardson cascade of energy-containing eddies size limited by channel width, associated with a Kolmogorov energy spectrum. It is assumed eddies produced on a scale of the grid mesh grow through the classical fluids mechanism.\footnote{P.A. Davidson, \textit{Turbulence}, Oxford Univ. Press., UK (2004).} We can now test a semi-quantitative theory with different mesh grids or channel sizes, relating to the possible existence of inverse turbulent cascades. Our 2$^{\mathrm{nd}}$ sound system is conventional, but with a novel phase and amplitude feedback loop making stringent constant temperature unnecessary. Both $t^{-3/2}$ and non-$ t^{-3/2}$ decays have been observed with 2 mesh sizes. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W47.00011: Fluid Flow and Depinning of a Metastable Vortex Rena Zieve, Emily Hemingway, Ingrid Neumann We observe a vortex line pinned around a straight wire in superfluid helium, in the absence of external rotation. Unperturbed, the vortex line remains pinned indefinitely, but we can partially detach the vortex by heating the fluid in brief stints. Here we show that a key property of the heating cycle is the maximum rate of change of the temperature, which suggests that the fluid velocity generated by a temperature gradient plays an important role in the depinning. Our measurements of how depinning depends on both maximum temperature and velocity show a crossover from a velocity-dependent depinning temperature at slow heating rates to a velocity-independent temperature near 1.2 K for faster heating. We discuss how vortex loops generated by the thermal fluid flow may be responsible for the depinning. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W47.00012: Incompressible Quantum Glass State of Bosons in Two Dimensions Wenan Guo, Yancheng Wang, Anders Sandvik We study the quantum glass state intervening between the conventional superfluid and Mott-insulator states of lattice bosons with random potentials at average filling $\rho=1$. Its properties at temperature $T=0$ are controlled by rare large regions of superfluid surrounded by Mott insulator. These regions make the state gapless although it is insulating. Contrary to the commonly accepted theory of this state in two dimensions, we show here that a vanishing gap does not necessarily imply nonzero compressibility. Using quantum Monte Carlo simulations of the Bose-Hubbard model and a percolation theory, we show that the compressibility $\kappa$ follows the form $\kappa \sim {\rm exp}(-b/T^\alpha)$ with $\alpha <1$. In addition, the dynamic exponent of the superfluid-quantum glass transition is found to be smaller than 2. The system is, thus, incompressible at $T=0$ and should be classified as a Mott glass. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W47.00013: Dynamics of the Direct Vortex Cascade in 2D Quantum Turbulence Gary Williams, Andrew Forrester The growth and decay of the direct vortex cascade in 2D quantum turbulence is studied for different rates of injection of vortex pairs of a given separation (the stirring scale), with the system in contact with a heat bath at low temperature. At low injection rates the vortices have no effect on the superfluidity, and the distribution of pair separations spreads out until reaching the steady-state distribution of the direct cascade, where pairs annihilate at the same rate they are injected. This cascade has a k$^-3$ energy spectrum, the same spectrum as the direct enstrophy cascade in 2D classical turbulence. On switching off the injection, the pair distribution first decays starting from the initial stirring scale, with the total vortex density decreasing linearly in time. As pairs at smaller scales decay, the vortex density then falls off as a power law, the same power law found in recent exact solutions of quenched 2D superfluids. At high injection rates the large density of vortices drives the superfluid density to zero at long length scales, and the growth and decay of the cascade is found to be much slower for this case. [Preview Abstract] |
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