Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A22: Collective Modes and Superflow in Solid Helium |
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Sponsoring Units: DCMP Room: 202A |
Monday, March 2, 2015 8:00AM - 8:12AM |
A22.00001: Gapped collective modes in quantum solids Daniel Arovas, Snir Gazit, Assa Auerbach, Heloise Nonne, Daniel Podolsky The harmonic theory of crystals predicts that the excitation spectrum of a Bravais lattice, i.e. a mono-atomic crystal structure, consists solely of gapless acoustic phonos. Surprisingly, an inelastic Neutron scattering experiment of solid He4 BCC phase has uncovered a zero momentum gapped excitation [1]. Motivated by the large zero point motion in the BCC phase, we describe the crystal through a phenomenological effective Ginzburg-Landau field theory of a charge density wave order parameter. We find that the excitation spectrum contains gapped modes which correspond to fluctuations of the charge density wave order parameter amplitude. We characterize the modes according to their symmetry and compute their visibility in Neutron scattering experiments. To further validate our results, we calculate the scalar susceptibility by means of an ab-inito quantum Monte Carlo simulation. We find a gapped resonance in good agreement with the experimental measurements. Our results motivate future studies of the excitation spectrum of quantum solids. [1] T. Markovich, E. Polturak, J. Bossy, and E. Farhi, Phys. Rev. Lett. 88, 195301 (2002) [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A22.00002: Strong-coupling and the stability of crystalline order in superfluid $^3$He films Joshua Wiman, J. A. Sauls In a film of thickness $D$, weak-coupling theory for p-wave, spin-triplet pairing predicts a ``stripe'' phase that spontaneously breaks translational symmetry in the plane of the film.\footnote{Vorontsov \& Sauls. \emph{Phys. Rev. Lett.} 98, 2007.} NMR on superfluid $^3$He confined in a slab has so far failed to detect any signature of the stripe phase, and the A-B transition is observed at lower temperatures than predicted by weak-coupling theory.\footnote{Levitin et al. \emph{Science} 340, 2013.} We report calculations of the phase diagram for $^3$He films based on Ginzburg-Landau (GL) theory that includes strong-coupling effects via experimentally estimated $\beta$ parameters.\footnote{Choi et al. \emph{Phys. Rev. B} 75, 2007.} At low pressures GL theory predicts the A-stripe phase transition, for small $D$, to be significantly suppressed compared to weak-coupling. For large $D$, the stripe transition is eliminated in favor of an A-B transition at lower temperatures than in weak-coupling. At higher pressures the stripe phase is predicted to be stable only at very low temperatures, outside the expected applicability of the strong-coupling GL theory. Our results suggest that the discrepancy between experiment and weak-coupling theory likely results from strong-coupling effects. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A22.00003: Is there a stable commensurate solid phase in the second $^{4}$He layer on graphite? -- path integral Monte Carlo study Jeonghwan Ahn, Hoonkyung Lee, Yongkyung Kwon Existence of a stable commensurate structure in the second $^{4}$He layer on graphite has been a subject of intensive experimental and theoretical studies because of its implication in the possible realization of two-dimensional supersolidity. Earlier path-integral Monte Carlo (PIMC) calculations of Pierce and Manousakis predicted a stable C$_{4/7}$ commensurate structure above the first-layer $^{4}$He atoms fixed at triangular lattice sites [1], but Corboz \textit{et al}. later showed that no commensurate phase was stable when quantum dynamics of the first-layer $^{4}$He atoms was incorporated in the PIMC calculations [2]. On the other hand, recent heat capacity measurements of Nakamura \textit{et al}. provided a strong evidence for a commensurate solid in the second $^{4}$He layer over an extended density range [3]. Motivated by this, we have performed new PIMC calculations for the second helium layer on graphite. Unlike previous PIMC calculations where a laterally-averaged one-dimensional substrate potential was used, we here employ an anisotropic $^{4}$He-graphite potential described by a sum of the $^{4}$He-C pair potentials. With this fully-corrugated substrate potential we make more accurate description of quantum dynamics of the first-layer $^{4}$He atoms and analyze its effects on the phase diagram of the second layer. \\[4pt] [1] M. Pierce and E. Manousakis, \textit{Phys. Rev. Lett.} \textbf{81}, 156 (1998).\\[0pt] [2] P. Corboz \textit{et al}., \textit{Phys. Rev. B} \textbf{78}, 245414 (2008).\\[0pt] [3] S. Nakamura \textit{et al}., arxiv:1406.4388 (2014). [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A22.00004: High temperature superfluidity in a commensurate phase of adsorbed $^4$He Raina Olsen It is well known that a substrate can have a significant effect on the phase diagram of adsorbed atoms. For instance, $^4$He adsorbed on graphene forms a solid structure commensurate (aligned) with the substrate which has a density much smaller than the density of bulk solid $^4$He. This occurs because the underlying periodic potential stabilizes the solid by opening an energy gap between the commensurate solid and the longest wavelength lattice excitations which would otherwise change the structure. Here we report calculations of superfluidity for $^4$He in a periodic adsorption potential with variable lattice spacing, using a Bogoliubov transformation to calculate the energy spectrum of the excitations. We find a gap in energy between the superfluid state and the longest wavelength excitations. When this superfluid energy gap is large enough, there should be few excitations even at temperatures above the lambda point, where superfluidity is not observed in the bulk. This occurs only when the lattice spacing of the substrate corresponds to the lattice spacing of bulk solid $^4$He. In contrast, when the substrate periodicity is too large, as is the case with graphene, a classical commensurate solid is expected instead. We discuss other possible materials. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A22.00005: Dislocation structure and mobility in hcp $^4$He Maurice de Koning, Edgar Josu\'e Landinez Borda, Wei Cai We present results of Path-integral Monte Carlo simulations of the basal-plane screw dislocation in hcp $^4$ He at temperatures below 1K. First, our results show that, due to the extremely low stacking-fault energy, its core is widely extended, dissociating into a pair of Shockley partials separated by a ribbon of stacking-fault. Second, our findings suggest that the stress required to initiate dislocation motion is different from zero and of the order of 0.1 bar. Finally, we discuss the role of $^3$He impurities. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A22.00006: Shear modulus of solid $^3$He in the bcc and hcp phases John Beamish, Zhigang Cheng, Fabien Souris The shear modulus of solid hcp $^4$He decreases significantly at temperatures above 100 mK [1, 2]. This is due to to dislocations which are localized when pinned by $^3$He impurities at low temperature but become mobile when $^3$He impurities ``evaporate'' at high temperature. The unpinned dislocations move freely in the basal plane of the hcp structure. This produces anisotropic and extraordinarily large softening of the shear elastic constant C$_{44}$, an effect referred to as ``giant plasticity'' [2]. Previous measurements [3] on solid $^3$He showed similar shear modulus changes in the hcp phase but not in the bcc phase. Here, we report new shear modulus measurements in both the bcc and hcp phases.$^3$He. These show a similar shear modulus anomaly in the bcc phase, indicating that dislocation softening is not unique to hcp phase of helium. We compare our results for bcc and hcp $^3$He to those hcp $^4$He, and discuss the roles that lattice structure and quantum statistics play in dislocation motion and impurity pinning. \\[4pt] [1] J. Day and J. R. Beamish, Nature 450, 853 (2007).\\[0pt] [2] A. Haziot et al., Phys. Rev. Lett. 110, 035301 (2013). [3] J. T. West et al., Nature Physics 5, 598 (2009). [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:24AM |
A22.00007: Impact of dislocations on the structure of solid helium Hans Lauter, John Goodkind, Kenneth Herwig, Eckhard Krotscheck, Efim Kats, Andrey Podlesnyak, Andreii Savici, Diallo Souleymane, Justin Caremichael Uncommon phonon spectra were obtained from solid helium below 1.3K and at pressures near 30 bar. Rapid cooling using the blocked capillary method created stressed solid helium in non-equilibrium state. Using inelastic neutron scattering, we disclosed the absence of Bragg-scattering combined with the presence of a phonon-gap, a phenomenon revealing the absence of long-range crystalline order. The energy of the gap is close to the value of a thermal activation energy measured by ultrasonic attenuation in unstrained solid 4He [1] crystals. The dispersion of the phonons shows point-like intensities interpreted as signature of finite-length edge dislocations. The range and shape of the strain field perpendicular to the dislocation line was identified discerning excitations related to the fluttering mechanism [2]. These finding give new input to the discussion of a dislocation network in view of the shear modulus in distorted solid 4He [3,4]. \\{4pt}[1] G.A. Lengua, J.M. Goodkind, J. Low Temp. Phys. {\bf 79}, 251 (1990) \\{0pt}[2] T. Ninomiya, J. Phys. Soc. Jpn. {\bf 36}, 399 (1974) \\{0pt}[3] J. Day, J. Beamish, Nature {\bf 450}, 853 (2007) \\{0pt}[4] A. Haziot, et.al., Phys. Rev. Lett. {\bf 110}, 35301 (2013) [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A22.00008: Interaction of ultrasound and torsional oscillation in solid $^4$He Izumi Iwasa, John Goodkind, Harry Kojima A new cell for studying ultrasound (10 MHz) propagation and torsional oscillation (1013 Hz) in solid He-4 was constructed. Improvements were made in the design of the spacer for the quartz transducers and the diameter of the torsion rod containing helium fill hole to reduce the effects of the shear modulus of the solid He-4 sample on the torsional oscillator response. Sudden shifts in both the sound propagation velocity and attenuation are observed below 100 mK. The detailed response depends on the ultrasound excitation level and thermal history. Increase in torsional oscillator frequency is observed at nearly the same temperature as where the sound propagation property shifts occur. At temperatures below 50 mK, changes in the ultrasound excitation level induce changes in the torsional oscillator frequency. Interpretation of these results in terms of He-3 impurity being trapped on dislocation lines will be discussed. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A22.00009: Frequency-dependent Study of Ultrapure Solid 4He by Using Rigid Double-pendulum Torsional Oscillator Jaewon Choi, Jaeho Shin, Eunseong Kim The physical origin of the period drop found in the torsional oscillator (TO) containing solid 4He was previously interpreted as the appearance of supersolidity.\footnote{E. Kim, M. H. W. Chan, \textbf{Science} 305, 1941 (2004)} The current consensus is that the increase in the shear modulus leads to the period anomaly. Further studies show that the stiffening effect in TO can be amplified if a TO is not properly designed to be ``rigid.'' In this study, we designed a rigid double-pendulum TO. High purity solid 4He sample (0.6ppb) was grown by the block capillary method. The resonant period of TO starts to decrease from the empty cell data at 80mK. The ratio of the resonant period changes to the total mass loading are $3.8\times10^{-5}$ and $2.6\times10^{-4}$ for 1$^{\mathrm{st}}$ and 2$^{\mathrm{nd}}$ mode, respectively. Unlike recent experiment\footnote{X. Mi, J. D. Reppy, \textbf{J. Low. Temp. Phys.} 175, 104 (2014)}, we could not found a frequency-independent period drop. The upper bound for the putative supersolid fraction is less than $4\times10^{-6}$. The dissipation peak accompanied with the period drop was also analyzed with Cole-Cole plot and $\omega \tau$ plot. We conclude that major contribution for the anomalous TO responses comes from the elastic effect. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A22.00010: Probing helium mass flow through a solid-liquid-solid double junction Zhigang Cheng, John Beamish, Andrew Fefferman, Fabien Souris, Sebastien Balibar Recent experiments by Hallock and coworkers [1] observed mass transport through solid $^4$He and suggested it was due to flow along dislocation lines. In those measurements, helium was injected and removed through Vycor ``electrodes'' filled with superfluid $^4$He. Here, we report the results of a related experiment: a Vycor rod filled with superfluid $^4$He is sandwiched between two bulk solid regions. By compressing solid $^4$He on one side and measuring pressure changes on the other, we can detect flow through the Vycor, without necessarily having flow through the solid. In high pressure crystals we saw no flow below 1 K but in samples below 28 bar we observed flow down to the lowest temperatures (below 20 mK). The temperature dependence of this flow was very similar to that of the flow seen in previous experiments [1]: it began around 600 mK, increased as the temperature was reduced, then decreased dramatically at a temperature which depended on $^3$He impurity concentration (around 75 mK for standard isotopic purity samples). We suggest that flow in solid $^4$He experiments is limited by mass transfer through the solid-liquid interface at the Vycor ends. \\[4pt] [1] Phys. Rev. Lett. 105 145301 (2010); Phys. Rev. Lett. 113, 035302 (2014). [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A22.00011: Mass Superflux in Solid Helium: Dependence on Temperature, Density and $^3$He Impurity Concentration Yegor Vekhov, Robert Hallock The mass flux, $F$, induced to flow through solid $^4$He by means chemical potential differences imposed by the fountain effect in the range $25.6 < P< 26.4$~bar rises with falling temperature below ~650 mK. At a low temperature, $T_d$, the flux drops sharply. The behavior of the flux above $T_d$ is consistent with the presence of a bosonic Luttinger liquid. We report a study $F$ as a function of $^3$He concentration, $\chi$ $(0.17 - 220)$~ppm, and explore the effect of level of $^3$He impurities on $T_d$. We find a strong reversible reduction of the flux, typically complete within a few mK. We find that $T_d$ is an increasing function of $\chi$ and the $T_d(\chi)$ dependence differs somewhat from the predictions for bulk phase separation. It is possible that the cores of edge dislocations carry the flux. In such a case the flux may be extinguished by the decoration of the cores or dislocation intersections by $^3$He. We find that $F$ is sample-dependent, but that the temperature dependence of $F$ above $T_d$ is universal; data for all samples scale and collapse to a universal temperature dependence, independent of $^3$He concentration but with a weak pressure dependence. [Work supported by NSF DMR 12-05217.] [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A22.00012: Mass Superflux in Solid Helium: What Limits the Flux? Robert Hallock, Yegor Vekhov The thermo-mechanical effect in superfluid helium is used to create a chemical potential difference, $\Delta \mu$, across a superfluid-filled vertical Vycor rod. This rod separates a bulk liquid superfluid helium reservoir, $R1$, on the top of the Vycor at $T1 = 1.46 - 1.51$~K and solid hcp $^4$He on the bottom at $TC = 0.1 - 0.8$~K. Two \textit{in situ} capacitance pressure gauges, $C1$ and $C2$, are placed at the ends of the horizontal cylindrical solid helium sample (1.84~cm$^3$, $25.9 - 26.4$~bar) and located at different distances from the position of the Vycor rod in the solid helium, 10 and 31~mm, respectively. A $T1$ decrease/increase changes $\Delta \mu$ and leads to a solid helium pressure increase/decrease detected by both $C$s. The rate of pressure change is slower at the further gauge, $C2$, than at the nearer one, $C1$. This behavior is interpreted as due to the presence of a mass flux bottleneck inside the solid helium sample. We believe, e.g. in the case of a $T1$ decrease, that helium atoms emerge from the Vycor rod, perhaps migrate along the superfluid core of edge dislocations in solid helium and adsorb on them. This is the so-called ``syringe-effect'' or superclimb of edge dislocations. The dependence on temperature will be discussed. [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A22.00013: Quantum Plasticity and Supersolid Response in Helium-4 Anatoly Kuklov, Lode Pollet, Nikolay Prokof'ev, Boris Svistunov We argue that the three key phenomena recently observed in solid $^4$He ---mass supertransport, anomalous isochoric compressibility (syringe effect), and giant plasticity---are closely linked to each other through the physics of an interconnected network of tilted quantum-rough gliding and superclimbing dislocations. Such roughness is guaranteed, on one hand, by tilting of dislocations in Peierls barrier, and, on the other, by fast tunneling of kinks and jogs. Quantum rough gliding or superclimbing dislocation features 1D quantum liquid of kinks or jogs, respectively. As immediate implications of this connection several predictions follow: In the absence of $^3$He impurities, the syringe effect and giant plasticity persist down to $T=0$; the dynamical low-frequency syringe and giant-plasticity responses are dispersionless; and similarly to giant plasticity but without direct relationship to the supertransport along the dislocation cores, $^3$He impurities should suppress the syringe effect partially or completely at appropriately low temperatures. [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A22.00014: Helium-4 superfluid density; action-at-a-distance effects Stephen R.D. Thomson, Francis M. Gasparini We report results from experiments with $^4$He confined in two concentric 277 nm thick planar regions that are connected across a ring which forms a thin film weak link 33~nm thick. Measurements of the superfluid fraction within rings of varying widths have shown that the planar regions affect the film in the ring over distances much longer than the correlation length $\xi$. These results are analogous to those reported for a different geometry of both the specific heat and superfluid fraction [1]. To investigate the width dependence of this proximity effect we have performed measurements with rings that are 8, 17, 40 and 100 $\mu$m wide. We will discuss our method of measurement and a possible mechanism for the long range action-at-a-distance effect suggested in [2] for the 2D Ising system. \\[4pt] [1] Perron J~K et. al. 2013 {\em Phys. Rev. B\/} {\bf 87} 094507 \newline [2] Abraham D~B et. al. 2014 {\em Phys. Rev. Lett.\/} {\bf 113} 077204 [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A22.00015: Mass transport in micrometer size solid He-4 Ariel Haziot, Duk Young Kim, Moses Chan We have studied the transport of $^4$He atoms through a thin solid $^4$He slab of $\sim$50 $\mu$m thick sandwiched between two superfluid 'electrodes' of liquid helium filled Vycor rods. The geometry of the experiment is similar to the configuration used by Hallock and collaborators at the University of Mass [1,2] however the thickness of our solid sample is about 8000 times thinner than the UMass solid sample. The observed mass flow through the solid slab shows the characteristic of a superflow and the rate is more than a 1000 times higher than the UMass experiment. The mass flow rate decreases strongly with the pressure to vanish around 31 bar and it shows an hysteresis loop as a function of pressure. In contrast to the UMass experiment, the mass flow rate in our experiment decreases weakly and smoothly with increasing temperature between 30 mK and 1.2 K. In addition, we found no dependence on the $^3$He concentration from 3 ppm to 1\%. \newline \newline [1] M. Ray and R. B. Hallock, \textit{Physical Review Letter} \textbf{100}, 235301 (2008) \newline [2] Ye. Vekhov, W. J. Mullin and R. B. Hallock, \textit{Physical Review Letter} \textbf{113}, 035302 (2014) [Preview Abstract] |
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