Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y22: Quantum Solids - He4 |
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Sponsoring Units: DCMP Chair: Matthias Graf, Los Alamos National Laboratory Room: 324 |
Friday, March 22, 2013 8:00AM - 8:12AM |
Y22.00001: Dissipative superfluid mass flux through solid $^4$He Yegor Vekhov, Robert Hallock The thermo-mechanical effect in superfluid helium is used to create a chemical potential difference, $\Delta \mu$, across a liquid or solid $^4$He sample and induce a mass flux. With an improved technique, measurements of the mass flux, $F$, through a solid-filled sample cell at several fixed helium sample temperatures, $T$, have been done as a function of $\Delta \mu$. And, measurements of $F$ (in the range $100 < T < 550$~mK) have been done as a function of temperature for several fixed values of $\Delta \mu$. The temperature dependence of the flow through solid helium above $100$~mK is confirmed to show a reduction of the flux with increasing temperature, while for liquid helium there is no marked temperature dependence in the temperature range studied. The dependence of $F$ on $\Delta \mu$ documents in some detail the dissipative nature of the flow for the case of a solid helium- filled sample cell. In the case of solid helium we observe $F\sim \Delta \mu^b$ with $b \approx 0.3$, which is consistent with expectations for 1D superfluidity. The relationship between this work and the various torsional oscillator NCRI results is not clear. We may be exploring different phenomena. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y22.00002: Exact Solution for Vortex Dynamics in Temperature Quenches of Two-Dimensional Superfluids Andrew Forrester, Han-Ching Chu, Gary Williams An exact analytic solution for the dynamics of vortex pairs is obtained for rapid temperature quenches of a superfluid film starting from the line of critical points below the critical temperature $T_{KT}$. An approximate solution for quenches at and above above $T_{KT}$ provides insights into the origin of logarithmic transients in the vortex decay, and are in general agreement with recent simulations of the quenched XY model. These results confirm that there is no ``creation" of vortices whose density increases with the quench rate as predicted by the Kibble-Zurek theory, but only monotonic decay of the thermal vortices already present at the initial temperature. The problem in the Kibble-Zurek argument is the artificial restriction to measuring the vortex density only at the ``freezeout'' sampling time, which increases with the quench time. But since the the pairs continually decay, of course this will always result in lower vortex densities for a longer quench time and hence a later sampling time. But in fact the vortex densities can be measured at all times, and it then becomes quite clear that the instantaneous superfluid quench has the lowest vortex density at all times of any quench rate, since it most rapidly gets to the lowest temperature. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y22.00003: Elasticity, Plasticity and Supersolidity in Solid Helium John Beamish, Ariel Haziot, Andrew Fefferman, Xavier Rojas, Sebastien Balibar The frequency of a torsional oscillator containing solid helium depends on the helium's elastic properties, as well as its inertia. Mobile dislocations reduce the helium's shear modulus, but they are pinned at low temperatures. The resulting increase in shear stiffness raises the TO frequency and can mimic mass decoupling in a supersolid. The size of this elastic effect depends on the geometry of the oscillator and on the magnitude of the modulus changes. We recently showed that the elastic effect can be large enough to explain the apparent mass decoupling in some oscillators whose torsion rods have a central hole to admit the helium, suggesting that the apparent supersolidity is an artifact due to elastic changes. We have observed extremely large modulus changes in high quality single crystals. We were able to identify the dislocations responsible for the elastic changes and to show that they were arranged in a network with very large pinning lengths. The large modulus changes reflect the dislocations' extremely high mobility at low temperatures, which produces a ``giant plasticity'' in this quantum crystal. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y22.00004: The giant plasticity in $^4$He crystals Ariel Haziot, Andrew Fefferman, Xavier Rojas, John Beamish, Sebastien Balibar We have applied very small shear stresses (down to 1 nanobar) to oriented single $^4$He crystals, and directly measured their response as a function of temperature (from 15 mK to 1 K), orientation, crystal quality, $^3$He concentration, frequency and shear stress magnitude. For particular orientations, we have found a giant plasticity that is reversible, associated with the elastic coefficient c$_{44}$ which nearly vanishes around 200 mK. Other elastic coefficients show no measurable anomaly. The strong reduction of c$_{44}$ (80\% in high quality crystals with no impurities) shows that dislocations glide in the basal plane of the hexagonal structure with no dissipation. This plasticity disappears as soon as traces of $^3$He impurities bind to the dislocations (at low T) or if their motion is damped by collisions with thermal phonons (at higher T). It has no equivalent in classical crystals. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y22.00005: Dislocation densities and lengths in solid $^4$He from elastic measurements Andrew Fefferman, Ariel Haziot, John Beamish, Sebastien Balibar Elastic measurements on solid $^4$He show large softening of the shear modulus due to motion of dislocations, behavior which has been described as quantum plasticity. Dislocation networks may also be responsible for the unusual behavior seen at low temperatures in torsional oscillator and flow experiments. However, existing estimates of dislocation densities in helium crystals vary by many orders of magnitude. By measuring the temperature and frequency dependence of the elastic dissipation, we have determined dislocation densities and network lengths in both single crystals and polycrystals of $^4$He. The dislocation lengths are much longer than previous estimates, meaning that they are less connected than previously thought. Even in polycrystals, we find no evidence for the large densities of well-connected dislocations which would be needed to explain mass decoupling in torsional oscillators in terms of superfluidity in a dislocation network. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y22.00006: Solid $^4$He probed by both torsional oscillator and ultrasound Harry Kojima, Izumi Iwasa, John Goodkind The interpretation of observed anomalous increases in the frequencies of torsional oscillators (TO) containing solid $^4$He confined in Vycor nanopores as evidence for emergence of a supersolid phase has been met recently by conflicting experiments. Yet questions remain on the origin of the observed TO anomalies in bulk solid $^4$He samples. To search for the origin, we are carrying out simultaneous measurements of 10 MHz longitudinal ultrasound and TOs (250 $\sim$ 1100 Hz) on identical solid $^4$He samples. Temperature dependence of velocity and attenuation of ultrasound and that of amplitude and frequency of TO are measured. At the temperatures, where TO anomalies occur, anomalies in sound velocity and attenuation also appear. When solid $^4$He is doped with 20 ppm $^3$He, the tempeature of TO anomaly tracks that of ultrasound. Interpretation of these observations in terms of the motion of dislocation lines will be presented. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y22.00007: Excitations of Amorphous Solid Helium Jacques Bossy, Jacques Ollivier, Helmut Schober, Henry R. Glyde We present neutron scattering measurements of the dynamic structure factor, $S(Q,\omega)$, of amorphous solid helium confined in 47 \AA~pore diameter MCM-41 at pressure $48.6$ bar. At low temperature, $T$ = 0.05 K, we observe $S(Q,\omega)$ of the confined quantum amorphous solid plus the bulk polycrystalline solid between the MCM-41 powder grains. No liquid-like phonon-roton modes, other sharply defined modes at low energy ($\omega<$ 1.0 meV) or modes unique to a quantum amorphous solid that might suggest superflow are observed. Rather the $S(Q,\omega)$ of confined amorphous and bulk polycrystalline solid appear to be very similar. At higher temperature ($T>$ 1 K), the amorphous solid in the MCM-41 pores melts to a liquid which has a broad $S(Q,\omega)$ peaked near $\omega \simeq$ 0 characteristic of normal liquid $^4$He under pressure. Expressions for the $S(Q,\omega)$ of amorphous and polycrystalline solid helium are presented and compared. In previous measurements of liquid $^4$He confined in MCM-41 at lower pressure the intensity in the liquid roton mode decreases with increasing pressure until the roton vanishes at the solidification pressure (38 bars), consistent with no roton in the solid observed here [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y22.00008: Plasticity and dislocation-induced anomalous softening of solid helium under DC shea Irene Beyerlein, Caizhi Zhou, Jung-Jung Su, Matthias Graf, Charles Reichhardt, Alexander Balatsky The classical motion of gliding dislocation lines in slip planes of crystalline solid helium leads to plastic deformation even at temperatures far below the melting temperature and strongly affects elastic properties. In this work we propose that the gliding of dislocations and plasticity may be the origin of many observed elastic anomalies in solid He-4, which have been argued to be connected to supersolidity. We present and propose a dislocation motion model that describes the stress-strain $\tau $--$\varepsilon $ curves and work-hardening rate $\tau $/d$\varepsilon $ of a DC shear experiment to be performed at constant strain rate in solid helium. The calculated $\tau $/d$\varepsilon $ exhibits strong softening with increasing temperature owing to the motion of dislocations, which mimics anomalous softening of the elastic shear modulus $\mu $. In the same low-temperature region the classical motion of dislocations causes dissipation with a prominent peak [1] [1] C. Zhou et al., Philos. Mag. Lett. 92 (2012) 608 [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y22.00009: Simultaneous measurements of the torsional oscillator and shear modulus of solid 4He diluted with various 3He concentration Jaeho Shin, Wonsuk Choi, Jaewon Choi, Seong Jang, Keiya Shirahama, Eunseong Kim In 2004, Kim and Chan observed the non-classical rotational inertia (NCRI) of solid helium-4 by using a torsional oscillator (TO). Below 200mK, the resonance period of solid helium dropped, which was originally interpreted as the mass decoupling of the fraction of solid helium. Recently, anomalous increase in the shear modulus of solid helium was found and showed striking similarity in temperature, frequency, 3He concentration, and drive dependence to those of the NCRI [2]. To understand the connection between the NCRI and the shear modulus anomaly, we simultaneously measure the change in the resonance frequency and the stiffness of solid helium below 200mK. The torsion cell contains a pair of the concentric piezoelectric transducers (PZT) which defines an annular channel for the simultaneous measurements. We will report the interference between the motion of the TO at resonance and AC motion of the PZT in solid 4He with different 3He concentration. \\[4pt] [1] E.Kim and M.H.W Chan Nature 427, 225-227 (2004)\\[0pt] [2] J. Day and J. Beamish Nature 450, 853-856 (2007) [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y22.00010: $^4$He adsorption on $\alpha$-graphyne Yongkyung Kwon, Hoonkyung Lee, David M. Ceperley Path-integral Monte Carlo calculations have been performed to study $^4$He adsorption on a single $\alpha$-graphyne sheet that is a hexagonal network of $sp$- and $sp^2$-bonded carbon atoms. Using the $^4$He-substrate interaction described by a pairwise sum of the helium-carbon inter-atomic potentials, we have found that each hexagon of a graphyne can accomodate one $^4$He atom at its in-plane center. The first layer of $^4$He atoms adsorbed on this $^4$He-attached graphyne sheet with a composite of C$_8$He$_1$, exhibits various quantum phases depending on the helium coverage. It is found to be in a Mott insulating state at a coverage of 0.0706~\AA$^{-2}$ with three $^4$He atoms occupying each unit cell while the helium atoms form a commensurate triangular solid at 0.0941~\AA$^{-2}$. With the introduction of Ising pseudospins for two degenerate configurations of three $^4$He atoms in a hexagonal cell, the transition from the Mott insulator to the triangular solid can be interpreted as a ferromagnetic transition. In addition we find stable formation of zero-point vacancies in the commensurate triangular solid and their roles in possible realization of supersolidity are under investigation [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y22.00011: Pursuit of the Elusive Supersolid Xiao Mi, John D. Reppy The excitement following the initial report of supersolid behavior for $^{\mathrm{4}}$He embedded in porous Vycor glass has been tempered by the realization that many of the early supersolid observations were contaminated by effects arising from an anomaly in the elastic properties of solid $^{\mathrm{4}}$He. In an attempt to separate dynamic elastic effects from a true supersolid signal, we employed a torsional oscillator with two eigen frequencies to study the $^{\mathrm{4}}$He-Vycor system. We found that frequency dependent elastic signals can entirely account for the observed period shift signals. Although, we conclude that supersolid does not exist for the $^{\mathrm{4}}$He-Vycor case, the question of its presence in bulk samples remains open. In our current experiments we apply the two-frequency test to bulk samples of solid $^{\mathrm{4}}$He. Again we find a frequency dependent contribution arising from elastic effects. However, in some cases we also find a small frequency independent contribution, which may indicate the existence of a remnant supersolid phase. Given the history of this subject such results must be treated with caution. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y22.00012: An Ordered State of Dislocations in Solid Helium Hans Jochen Lauter, Eckhard Krotscheck, Efim Kats, Kenneth Herwig, Andrey Podlesnyak, Diallo Souleymane, Glyde Henry, Andreii Savici An ordered state of dislocations, see e.g. [1], is disclosed from neutron inelastic scattering data taken from solid helium at 40mK and a pressure of about 30bar. A characteristic feature is the phonon gap at the origin of about 0.15 meV, which reveals the non-equilibrium state of stressed helium created by rapid cooling with the blocked-capillary method. Energy gain scattering starts to appear at a temperature of 0.5 K that underlines the non-equilibrium state of stressed helium and the non-applicability of the detailed balance. The increasing thermal occupation of phonon-states observed as increasing intensity in energy gain scattering builds to a phase transition close to 1.4K. The creation of a helium single crystal with hcp-structure in thermal equilibrium [2] is observed at this temperature. This phase transition is in agreement with the vanishing quasi two-dimensional superfluid helium in solid helium confined in aerogel around 1.3K [3]. The event of the ``supersolid'' transition around 100mK is not observed in the two neutron scattering experiments.\\[4pt] [1] G. S\"{o}yler, et. al., Phys. Rev. Lett. {\bf 103}, 175301 (2009)\\[0pt] [2] E. Blackburn, et. al., PRAMANA {\bf 71}, 673 (2008)\\[0pt] [3] H.Lauter, et. al., Phys. Rev. Lett. {\bf 107}, 265301 (2011) [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y22.00013: Stability limit of a metastable state of hcp solid helium-4 Fabien Souris, Jules Grucker, Jacques Dupont-Roc, Philippe Jacquier Solid helium has the unique feature of having an horizontal melting curve in the P,T plane. This offers novel opportunities to study the stability limits of a metastable solid, by using the pressure as a control parameter of the metastability. The metastable state is obtained by focusing a $1$~MHz ultrasonic sound wave inside an helium-4 crystal. Around $4$~bar below the melting pressure, the metastable crystal becomes unstable. Different configurations with one or two ultrasonic emitters have been used and lead to the same stability limit. This happens at much lower depression than predicted by nucleation theory or by quantum Monte Carlo simulations. Repeated experiments show that the instability initially appears as a small defect ($\sim0.2$~mm) located at the maximum isotropic strain. Further studies are performed to understand the underlying mechanism of the instability. Possible scenarios accounting for this unexpected observation are discussed. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y22.00014: Path-Integral Monte Carlo Simulations of Ideal Strength and Peierls Stress in HCP 4He Edgar Josu\'e Landinez Borda, Maurice De Koning The ideal strength of a crystal is defined as the stress required to induce plastic deformation in a defect-free crystal. It is a theoretical upper bound to the strength of real crystals. The Peierls stress, on the other hand, is the minimum stress required to move a lattice dislocation and produce defect-mediated deformation. Here we present results for both quantities in HCP 4He as obtained from a series of Path-integral Monte Carlo simulations and discuss them in terms of its deformation behavior. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y22.00015: Superfluid transition in a correlated dislocation network Hannes Meier, Mats Wallin, Stephen Teitel The search for a supersolid state in He-4 solids has motivated theoretical investigations of 3D connected superfluid dislocation networks. It has usually been assumed that a 3DXY universality class controls the superfluid transition in such systems since the random distance between intersections of the dislocation lines carrying superfluidity appears as uncorrelated disorder which is irrelevant at the 3DXY transition. We consider the possibility that the random disorder instead has long range correlations, and investigate several different models of correlated defects. Analytic arguments and extensive Monte Carlo simulations demonstrate new disordered universality classes for the superfluid transition with a smooth temperature dependence at the transition of the superfluid density and heat capacity. [Preview Abstract] |
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