Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z33: Quantum Fluids and Solids II |
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Sponsoring Units: DCMP Chair: John Davis, University of Alberta Room: C143/149 |
Friday, March 25, 2011 11:15AM - 11:27AM |
Z33.00001: Dissipation of compound torsional oscillator loaded with solid $^4$He containing $^3$He impurity level from 0.3 to 25 ppm Patryk Gumann, Michael Keiderling, David Ruffner, Harry Kojima High sensitivity of supersolid phenomenon in solid $^4$He to low levels of $^3$He impurity($x_3$) is a puzzle not yet understood. We have analyzed the data taken using our compound torsional oscillator on the variation of dissipation as $x_3$ was varied between 0.3 and 25 ppm. The compound oscillator allows studies of the dissipation at two oscillator mode frequencies(0.5 and 1.2 kHz). Arrhenius plots of temperatures, where peaks in dissipation occur, vs. frequency allow extracting the activation energy and the characteristic time. The data are consistent with distributions of activation energy whose widths increase with $x_3$ but the mean value of $\sim$ 430 mK independent of $x_3$. The characteristic time varies approximately as $\propto x_3^{2/3}$. Temperature dependence of the dissipation is consistent with Debye model but frequency dependence is not. We give an interpretation of the characteristic time in terms of diffusion of $^3$He along dislocation lines. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z33.00002: The rotation anomaly of high quality $^{4}$He single crystals Xavier Rojas, Ariel Haziot, Joshua T. West, Moses H.W. Chan, Humprey Maris, S\'ebastien Balibar We have built a transparent torsional oscillator in order to monitor the growth of $^{4}$He crystals near 20 mK. It allows us to measure the rotational inertia of high quality oriented single crystals and compare it with low quality crystals or polycrystals grown at constant volume. It is also possible to vary the $^{3}$He concentration from 0 to 0.3 ppm (natural purity). Since the change in TO period associated with the change in shear modulus of the He sample could be calculated, we could see if supersolidity is really due to superflow along dislocation lines. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z33.00003: Shear Modulus of Solid Helium Confined in Aerogel Arif Rabbani, John Beamish Torsional oscillator experiments on $^4$He show supersolid behavior which appears to be associated with disorder. However, confining helium in the pores of an aerogel does not enhance the supersolid decoupling, even though x-ray measurements confirm that the crystals are highly disordered. Solid helium's shear modulus also shows anomalous behavior below 150 mK, stiffening as mobile dislocations are pinned by $^3$He impurities at low temperatures. A highly porous material such as aerogel should also provide effective pinning sites for dislocations. We have made shear modulus measurements on solid $^4$He grown in a 95\% porosity aerogel. We see large modulus decreases as the samples are warmed but these occur at much higher temperatures and over a broader range than in bulk $^4$He. The frequency dependence of the modulus and dissipation are consistent with a thermally activated process. The activation energies are roughly 10 to 15 K and may be associated with vacancy motion. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z33.00004: Possible glass anomalies in the shear modulus and dielectric function of solid helium Jung-Jung Su, Matthias J. Graf, Alexander V. Balatsky The shear modulus of solid $^4$He exhibits an anomalous change at low temperature that is qualitatively similar to a frequency change in torsional oscillator experiments. We propose that in solid $^4$He the stiffening of the shear modulus with decreasing temperature can be described with a generalized susceptibility including a glassy backaction by assuming a distribution of temperature-dependent relaxation times $\tau(T)$. The glass susceptibility captures the freezing out of glassy degrees of freedom below a characteristic crossover temperature $T_X$, when the dynamic response of the solid satisfies $\omega \tau(T_X) \sim 1$, thus leading to a viscous response. We predict that the maximum change of the amplitude of the shear modulus and the height of the dissipation peak are independent of the applied frequency $\omega$. Recent measurements of the dielectric function $\epsilon(\omega)$ by the UFL group show a similar amplitude increase. We propose that changes in $\epsilon(\omega)$ are due to the glassy dynamics of low-lying excitations and are related to the shear modulus through acousto-optical coupling. We predict a dissipation peak in the imaginary part of the dielectric function, where the change in the real part is largest. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z33.00005: Stress induced roughening of superclimbing dislocation in solid $^4$He Darya Aleinikava, Anatoly Kuklov We investigate numerically superclimb [1] of dislocation in solid $^4$He biased by externally imposed chemical potential $\mu$. The effective action takes into account quantum phase slips in the core superfluid as well as the core displacement in Peierls potential within the Granato-L\"{u}cke string model. The bias produces stress on the core and this can result in dislocation roughening. Such roughening is characterized by hysteretic behavior at temperatures (T) below some threshold $T_{\rm hyst}$. At $T>T_{\rm hyst}$ strong resonant peaks develop in the dislocation differential response. These peaks exhibit periodic behavior vs $\mu$, with the period determined by Peierls potential and dislocation length. We explain these effects by thermally assisted tunneling of jog-antijog pairs across the barrier created by Peierls potential and the bias. Since superclimbing is controlled by core superflow, speed of sound along the superfluid core exhibits dip-like features at the peak positions. We propose that this effect is seen in the mass transport experiment [2].\\[4pt] [1] S. G. S\"{o}yler, et al, Phys. Rev. Lett. {\bf 103}, 175301 (2009).\\[0pt] [2] M. W. Ray and R. B. Hallock, Phys. Rev. Lett. {\bf 105}, 145301 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z33.00006: Andreev-Lifshitz Theory Applied to Normal Solids under Pressure Matthew Sears, Wayne Saslow On letting the superfluid density go to zero, the Andreev-Lifshitz hydrodynamic theory of supersolids becomes applicable to an ordinary solid.\footnote{A. F. Andreev and I. M. Lifshitz, Sov. Phys. JETP 29, 1107 (1969).} Under applied pressure $P_{a}$, needed to produce solid He$^3$ and He$^4$ or to be of geophysical relevance, the system has both an elastic stress $\lambda_{ik}$ and an internal pressure $P$, with $P\delta_{ik}=P_{a}\delta_{ik}+\lambda_{ik}$ in equilibrium. $P$ may be thought of as being due to a vacancy fluid. For $P_a$ small compared to the bulk modulus, Maxwell relations give $P\sim P_{a}^{2}$. The dynamical equations lead to three sets of propagating elastic modes (longitudinal and transverse sound) and two diffusive modes (one largely of entropy density and one largely of vacancy density -- or, more generally, defect density), all of which we study for non-zero $P_{a}$.\footnote{M. R. Sears and W. M. Saslow, Phys. Rev. B 82, 134304 (2010).} The vacancy diffusion mode has diffusion constant $D_{L}\sim P_{a}^{2}$, and is diffusive because its associated internal pressure fluctuation $P'$ nearly cancels its lattice stress fluctuation $\lambda'_{ik}$. This mode permits the system to respond differently to transducers with different surface treatments. We specifically have in mind solid $^4$He, which requires $P_a \sim 25$ bars to solidify; however, the results should apply to any solid under pressure. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z33.00007: Quantum phases of grain boundaries in solid $^4$He Debajit Goswami, Kinjal Dasbiswas, Alan Dorsey First-principles Monte Carlo simulations show that the core of a dislocation in solid $^4$He is superfluid and has Luttinger-liquid like properties [Phys. Rev. Lett. {\bf 99}, 035301 (2008), Phys. Rev. Lett. {\bf 103}, 175301 (2009)]. Low angle grain boundaries in crystals can be thought of as a linear array of dislocations, suggesting that a grain boundary in solid $^4$He can be modeled as an array of coupled Luttinger liquids. By exploiting this analogy, in this work we study the quantum phases of the grain boundary, as well as analogies with dipolar condensates in one dimensional optical lattices. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z33.00008: Supersolid $^4$He monolayer Massimo Boninsegni The conditions for the existence of a low temperature supersolid $^4$He monolayer are investigated by Monte Carlo simulations. The crystalline film considered here is one that not registered with the underlying substrate crystal structure. Its superfluid response is underlain by large zero-point motion of atoms in the direction perpendicular to the substrate. It is proposed that the physics described here might be observable in a helium film adsorbed on a Ni substrate. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z33.00009: U(1) $\times$ U(1) Kosterlitz-Thouless transition of the Larkin-Ovchinnikov phase in an anisotropic two-dimensional system Chungwei Lin, Xiaopeng Li, W. Vincent Liu We study Kosterlitz-Thouless (KT) transitions of the Larkin-Ovchinnikov (LO) phase for a two-dimensional system composed of coupled one-dimensional tubes. The main character of LO phase is a stripe structure (periodic in only one direction) in the order parameter. The low energy excitations involve the deformation of the stripe configuration and the fluctuation of the phase which can be described by two anisotropic XY model. We compute from a microscopic model the coefficients of XY model from which the KT transition temperatures are determined. We found the $T^{KT} \propto t_{\perp}$ for small intertube tunneling $t_{\perp}$. As $t_{\perp}$ increases the system undergoes a first-order transition to normal phase at zero temperature. Our method can be used to determine the Goldstone excitations of any stripe order involving charge or spin degrees of freedom. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z33.00010: Small-angle Neutron Scattering Measurements of Liquid Helium Mixtures Confined in MCM-41 Helmut Kaiser, Timothy Prisk, Paul Sokol, Ian Steward, Claudia Pantalei Small-angle neutron scattering (SANS) was used to study the isotopic distribution of liquid helium mixtures confined in MCM- 41, a silica glass with a 2D hexagonal net of monodisperse cylindrical pores, as a function of filling and He$^{3}$ concentration. The ordered pore array of MCM-41 gives rise to Bragg reflections with intensities determined by both how the liquid fills the pores and how the isotopes are distributed within the pores. The modulation in peak intensity can be modeled by writing down a form factors for cylindrical objects with varying scattering length density. Comparison will be made with small-angle X-ray (SAXS) scattering measurements performed with synchrotron light on liquid helium mixtures confined in aerogel. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z33.00011: Deep Inelastic Neutron Scattering Study of Nanoconfined Liquid Helium Mixtures Paul Sokol, Timothy Prisk, Narayan Das The single-particle momentum distribution $n(p)$ plays a central role in the contemporary understanding of quantum many- body systems, especially the helium liquids. The superfluid behavior of liquid He$^{4}$ below the famous lambda-point temperature is associated with the Bose condensation of a macroscopic fraction of the He$^{4}$ atoms to the zero momentum state. This manifests itself in $n(p)$ as a $\delta$-function singularity at $p = 0$. Similarly, the Fermi liquid character of He$^{3}$ is associated with a sharp discontinuity in the Fermi surface at the Fermi momentum $p_{F}$. Using the Wide Angular Chopper Spectrometer at the Spallation Neutron Source, we recently carried out a deep inelastic neutrons scattering study of dilute He$^{3}$ + He$^{4}$ solutions confined in mesoporous MCM-41 in order to investigate the effects of confinement on the non-classical momentum distribution of an isotopic helium solution. The Bose condensate fraction, Fermi surface, average isotopic kinetic energies, and related work in the literature will be discussed. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z33.00012: A Luttinger liquid core inside helium-4 filled nanopores Adrian Del Maestro We study the effects of confining helium-4 at low temperatures inside long narrow pores with nanometer radii using worm algorithm path integral quantum Monte Carlo. The results show that even in the phase with a finite superfluid response, the interaction between helium atoms and the surface of the pore induces radial density oscillations which decay as the center of the pore is approached. For some special radii, an ``inner tube'' of helium atoms exists, and a careful scaling analysis confirms that it behaves like a one dimensional Luttinger liquid at low energies. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z33.00013: $^4$He Adsorption on a Single Graphene Sheet: Path-integral Monte Carlo Study Yongkyung Kwon, David Ceperley We have performed path-integral Monte Carlo calculations to study $^4$He adsorption on a single graphene sheet, where the $^4$He-substrate interaction is described by the sum of the helium-carbon pair potentials. Among those proposed to account for helium scattering data on the graphite surface, we employ three different types of the inter-atomic pair potentials; a spherical 6-12 potential, an anisotropic 6-12 potential, and an anisotropic Yukawa-6 potential. Regardless of the choice of the pair potential, a first $^4$He monolayer is found to show the C$_{1/3}$ commensurate structure at a surface density of 0.0636~\AA$^{-2}$ and to go through the domain wall phases for densities above the commensurate one before crystallizing into an incommensurate triangular solid. Below the commensurate density, however, the low-temperature phase of this helium adlayer varies depending on the choice of the $^4$He-substrate interaction. The calculation based on the spherical pair potentials suggests a superfluid liquid phase at lower densities while incorporation of anisotropy into the helium-carbon pair potential results in a low-density state of a solid with clustered vacancies. Finally we observe van der Waals correlation between the upper monolayer and the one below the graphene sheet. The effects of this interlayer correlation on a possible formation of stable vacancies will be discussed. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z33.00014: Commensurate-Incommensurate Transition in $^{4}$He Monolayer Adsorbed on a C$_{60}$ Molecule Hyeondeok Shin, Yongkyung Kwon Path-integral Monte Carlo calculations have been performed to study adsorption of $^{4}$He on a single C$_{60}$ fullerene molecule. In order to account for helium corrugations on the molecular surface, the sum of all interatomic pair potentials between a carbon atom and a $^{4}$He atom is used for the $^{4}$He-C$_{60}$ interaction. The radial density distributions reveal a layer-by-layer growth of $^{4}$He with the first adlayer being located at a distance of $\sim$ 6.2 $\AA$ from the center of a C$_{60}$ molecule. This first layer is found to exhibit various quantum states as the number of adsorbed $^{4}$He atoms $N$ varies. For $N$=32 the helium layer shows a commensurate solid structure with twenty helium atoms being localized on the tops of the hexagon centers of the C$_ {60}$ surface and the other twelve atoms above the pentagon centers. As more $^{4}$He atoms are added, a commensurate-incommensurate transition is observed. After going through various domain wall states the first layer is crystallized into an incommensurate solid for $N \sim 52$. We find that solid states observed for $N$=32,44, and 48 do not show any superfluid response even below 0.2 K while domain-wall fluids formed with 45 to 47 $^4$He atoms show significant superfluid fractions below 0.6 K. Finally different quantum states oberseved in the first $^4$He layer around a C$_{60}$ are compared with phase diagrams determined for the helium monolayer on a graphite surface. [Preview Abstract] |
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