Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W23: Solid Helium II |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Josh West, Pennsylvania State University Room: C125-C126 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W23.00001: Nanoscale Supersolidity in $^4$He Adsorbed on a C$_{20}$ Molecule Yongkyung Kwon, Hyeondeok Shin, Soomin Shim We have studied adsorption of $^4$He on the surface of a single C$_{20}$ fullerene molecule using the path-integral Monte Carlo method. For a full incorporation of the surface corrugations on the molecular surface the $^4$He-C$_{20}$ interaction is treated with a sum of empirical helium-carbon interatomic pair potentials. Radial density distributions show layer-by-layer growth of $^4 $He, and a detailed analysis of energetics and angular density distributions reveals that the strongly-bound first layer, located at a distance of $\sim 4.9$~\AA~from the center of the C$_{20}$ molecule, is in various quantum states as the number of $^4$He atoms changes. This layer, when completed with 32 atoms, is found to be a commensurate solid with an icosahedral lattice structure. We observe that near the completion of the first layer, mobile vacancies can be activated at a low temperature of $T=0.31$ K, which results in a finite superfluid fraction as well as a crystalline order. This is a manifestation of vacancy-based supersolidity on a nanometer scale. Finally we analyze the effects of $^3$He impurities on the superfluidity of the $^4$He adlayer on a C$_ {20}$. [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W23.00002: Supersolid-like Behavior in Thin Solid $^4$He Films Adsorbed on a Nanoporous Glass Keiya Shirahama, Takayuki Kogure, Rama Higashino, Hitomi Yoshimura, Yoshiyuki Shibayama Two-dimensional $^4$He solid is a prospective system for observing supersolidity. We study thin solid $^4$He films adsorbed on a porous glass with 2.5 nm pore size. Torsional oscillator (TO) measurements are carried out for coverage $n$ from 6 to 30 $\mu$mol/m$^2$. Even in the solid films ($n < 21 \mu$mol/m$^2$) we have observed an increase in the TO frequency associated with a dissipation peak; i.e. the supersolid - like behavior. The onset temperature of the frequency shift shows an interesting coverage dependence: It is 1 K at 6 $\mu$mol/m$^2$ and approaches 0 K near the critical coverage $n_c = 21 \mu$mol/m$^2$, above which liquid film superfluidity is observed. The overall behaviors might be interpreted as a quantum critical phenomenon around $n_c$. Further studies including oscillation velocity dependence and measurements for solid $^3$He films are underway. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W23.00003: Torsional Oscillator Measurements of 2D Amorphous Solid $^4$He Sangil Kwon, Duk Young Kim, Eunseong Kim Superfluidity in two dimensional (2D) helium-4 film adsorbed on various amorphous substrates is understood by Berezinskii- Kosterlitz-Thouless transition. The first few layers of helium are strongly localized to the substrates due to the van der Waals interaction and do not participate in superflow. However, these inert layers were proposed to be another candidate of supersolid.\footnote{J. Sarfatt, Phys. Lett. A \textbf{30}, 300 (1969)} We used a torsional oscillator technique to search for a possible 2D amorphous supersolid state. We investigated the drive dependence of helium film, assuming that the critical velocity is very small. We will present preliminary results on helium films adsorbed on a bare Vycor substrate and a hydrogen preplated Vycor substrate. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W23.00004: Mutual effect of $^3$He impurities and Peierls potential on shear modulus softening in solid $^4$He D. Aleinikava, E. Dedits, A.B. Kuklov, D. Schmeltzer We investigate numerically dislocation crossover from quantum smooth to classically rough state in solid $^4$He in presence of both - Peierls potential and $^3$He impurities as pinning centers providing gaussian trapping potential. Monte Carlo simulations have been performed within the formalism [1]. $^3$He is modeled as classical particles localized on dislocations according to thermal equilibrium with the bulk at some activation energy $E_0$, the $^3$He total fraction $x_3$ as well as the dimensionless dislocation density $x_d<<1$ (in units of interatomic distance). It is shown that the softening of the shear modulus $\mu(T)$ observed in Ref.[2] cannot be explained within the simple $^3$He evaporation model under the assumption of zero Peierls potential: for realistic $E_0$, $x_3$ and $x_d$ the temperature range over which the softening occurs is much narrower, $\approx E_0/\ln(x_d/x_3^2)$ (where $x_3 \sim x_d$), than the one observed in [2]. Inclusion of the Peierls potential smoothens out the crossover and allows good fit of the data [2]. \\[4pt] [1] D. Aleinikava, E. Dedits, A. B. Kuklov, D. Schmeltzer, arXiv:0812.0983 \\[0pt] [2] J. Day and J. Beamish, Nature {\bf 450}, 853(2007). [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W23.00005: Dislocation Mobility in a Quantum Crystal: the Case of Solid $^{4}\mathrm{He}$ Maurice de Koning, Renato Pessoa, S. A. Vitiello We investigate the structure and mobility of dislocations in hcp $^{4}\mathrm{He}$ crystals. In addition to fully characterizing the five elastic constants of this system, we obtain direct insight into dislocation core structures on the basal plane, which demonstrate a tendency toward dissociation into partial dislocations. Moreover, our results suggest that intrinsic lattice resistance is an essential factor in the mobility of these dislocations. This insight sheds new light on the possible correlation between dislocation mobility and the observed macroscopic behavior of crystalline $^{4}\mathrm{He}$. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W23.00006: Grain boundary roughening transitions S.T. Chui We consider the roughening of small angle grain boundaries consisting of arrays of dislocations and found two transitions, corresponding to fluctuations of the dislocations along and perpendicular to the boundaries. The latter contributes to a large scale fluctuation of the orientation of the crystal but the former does not. The transition temperatures of these transitions are very different, with the latter occuring at a much higher temperature. Order of magnitude estimates of these temperatures are consistent with recent experimental results from elasticity and X-ray measurements in solid $^4$He. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W23.00007: Giant isochoric compressibility of solid He4 due to superclimb of dislocations A.B. Kuklov, \c{S}.G. S\"{o}yler, L. Pollet, N.V. Prokof'ev, B.V. Svistunov In the experiment on superfluid transport in solid He4 [PRL {\bf 100}, 235301 (2008)], Ray and Hallock observed an {\it anomalously large isochoric compressibility}: the supersolid samples demonstrated a significant and apparently spatially uniform response of density and pressure to chemical potential $\mu$, applied locally through Vycor ``electrodes.'' We propose that the effect is due to {\it superclimb}: edge dislocations can climb because of mass transport along superfluid cores and, as a result, the crystal can accumulate extra matter which is, practically, independent of the dislocation density, provided it is uniform. We corroborate this scenario by {\it ab initio} simulations of an edge dislocation in solid He4 at $T=0.5K$ with Burgers vector along the C-axis: its superfluid core (split into partials) climbed in response to changes of $\mu$ [1]. At low $T$ the effect must be suppressed due to a crossover to the smooth dislocation, with the temperature scale determined by the energy of jog-antijog pair. \\[4pt] [1] \c{S}.G. S\"{o}yler, A. B. Kuklov, L. Pollet, N. V. Prokof'ev, and B. V. Svistunov , Phys. Rev. Lett. {\bf 103}, 175301 (2009). [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W23.00008: Dislocation-induced superfluidity in a model supersolid Kinjal Dasbiswas, Debajit Goswami, Chi-Deuk Yoo, Alan Dorsey The effect of an edge dislocation in inducing superfluidity is explored by coupling the elastic strain field of the dislocation to the superfluid density, and solving the corresponding Ginzburg-Landau theory. It is shown that superfluid density is induced along a single dislocation below a critical temperature determined by the ground state solution of a 2D Schr\"odinger equation with a dipolar potential. This superfluid behavior can be described by a 1D Ginzburg-Landau equation obtained through a weakly nonlinear analysis. We then extend our analysis to a network of dislocation lines considered before by Shevchenko and Toner, which could serve as a model for superflow through solid $^4$He. The effect of fluctuations and dynamics are included through a full time dependent Ginzburg-Landau theory. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W23.00009: Dynamics of Topological Defects in Supersolids Chi-Deuk Yoo, Alan T. Dorsey There has been a growing consensus that topological defects present in supersolids, such as vortices and dislocations, may play an important role in explaining the experimental results of solid helium-4 at low temperatures. In this work we study the dynamics of topological defects in isotropic supersolid films. First, we derive the effective action for vortices and dislocations by integrating out the environmental degrees of freedom from both the superfluid and the lattice. This provides us with effective frequency-dependent inertial masses of vortex and dislocation. Second, we investigate the classical and quantum dissipative creep of a vortex and a dislocation in the presence of a model pinning potential by using the derived action. This work is supported by the NSF. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W23.00010: Bound states of edge dislocations Debajit Goswami, Kinjal Dasbiswas, Chi-Deuk Yoo, Alan Dorsey We investigate bound state solutions of the 2D Schr\"odinger equation with a dipole potential originating from the elastic effects of a single edge dislocation. These solutions are important for several physical systems, including the binding of $^3$He impurities to dislocations in solid $^4$He, and the nucleation of superfluidity on dislocations in $^4$He. We present a variational estimate for the binding energy and numerically solve the eigenvalue problem to obtain several bound states. In our nondimensionalized units, we find a ground state energy of -0.139. The energy spectrum obtained matches with that from semiclassical considerations. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W23.00011: Quantum Monte Carlo Study of Anomalous Excitations in BCC Helium-4 Jaron Krogel, Saad Khairallah, David Ceperley Crystalline Helium displays an intriguing set of non-classical behavior. In 2002, inelastic neutron scattering experiments\footnote{T. Markovich, E. Polturak, J. Bossy, and E. Farhi, Phys. Rev. Lett. 88, 195301 (2002)} revealed anomalous optic-like branches, which were attributed to localized excitations. The nature of these excitations is explored with Correlation Function Quantum Monte Carlo, in which a basis of many-body trial wave functions is projected onto the exact set of excitations. We review the general form of the method, its adaptation to solid Helium, and results for the excitation spectrum. Preliminary results indicate that p-like site excitations can partially recover the spectrum. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W23.00012: A Superglass Phase of Interacting Bosons Roger Melko, Ka-Ming Tam, Scott Geraedts, Stephen Inglis, Michel Gingras Motivated by recent experimental suggestions of glassy behavior in supersolid Helium, we introduce a Bose-Hubbard Hamiltonian with random disordered interactions as a model to study the interplay of superfluidity and glassiness in a system of three-dimensional bosons. Solving the model using large-scale quantum Monte Carlo simulations, we show that these disordered interactions promote a stable superglass phase, where superflow and glassy density localization coexist in equilibrium without exhibiting phase separation. The robustness of the superglass phase is underlined by its existence in a replica mean-field calculation on the infinite-dimensional Hamiltonian. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W23.00013: Heat capacity reveals a possible glass state in hcp $^4$He solids Jung-Jung Su, Matthias J. Graf, Alexander V. Balatsky We model the low-temperature specific heat of solid $^4$He by invoking two-level tunneling states in addition to the usual phonon contribution of a Debye crystal for temperatures far below the Debye temperature, $T < \Theta_D/50$. By introducing a cutoff energy in the two-level tunneling density of states, we can describe the excess specific heat observed in solid hcp $^4$He, as well as the low-temperature linear term in the specific heat. Agreement is found with recent measurements of the temperature behavior of both specific heat and pressure. These results suggest the presence of a small fraction, at the parts-per-million(ppm) order, of two-level tunneling systems in solid $^4$He, irrespective of a possible existence of supersolidity. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W23.00014: Mass Injection and Flow in Solid $^4$He Michael Ray, Robert Hallock Our experiments, which utilize a cell filled with solid helium that is pierced by two superfluid-filled Vycor electrodes, have been interpreted to show evidence for the flow of mass though solid $^4$He [1]. When flow is observed, there is also a change in the pressure recorded on capacitive pressure gauges directly in contact with the solid at each end of the cell. When no evidence for flow is seen we typically see no change in the pressure of the solid. Both features are explained by a theory of the isochoric compressibility of solid $^4$He [2]. The theory, supported by simulations, proposes that flow through the system can be achieved by the ``super-climb" of edge dislocations, fed by superflow along dislocation cores, and this can only take place in a finite temperature range low enough so that the dislocation cores are superfluid, but high enough so that the dislocations are rough. We will report continuing experiments to investigate these phenomena. Our experiments are supported by the NSF via DMR 08-55954. \\ \\ $[1]$ M.W. Ray and R.B. Hallock, Phys. Rev. Lett. \textbf{100}, 235301 (2008); Phys. Rev. B \textbf{79}, 224302, (2009).\\ $[2]$ \c{S} G. S\"{o}yler {\it et al.} Phys. Rev. Lett. \textbf{103}, 175301 (2009). [Preview Abstract] |
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