Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W1: Supersolids |
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Sponsoring Units: DCMP Chair: Moses Chan, Pennsylvania State University Room: Spirit of Pittsburgh Ballroom A |
Thursday, March 19, 2009 11:15AM - 11:51AM |
W1.00001: The role of dislocations in supersolid Helium-4 Invited Speaker: Disorder and crystallographic defects play a major role in the supersolid phase of ${}^4$He. Monte Carlo simulations addressed the physics of vacancies [1], grain boundaries [2] and dislocations [3], which are the focus of this talk. We find that certain types of edge and screw dislocations are superfluid while other remain insulating, depending on their orientation, Burgers vector and possible splitting of the core. The mechanism for superfluidity is provided by the strain near the core of the defect exceeding a threshold value [4]. Superfluid dislocations can build a network of phase coherent tubes (the so-called Shevchenko state) [3], which might lead to an observable mass decoupling in experiment. I will also look at the interactions between a Helium-3 impurity atom and a screw dislocation [5] and make contact with recent experiments. \\[4pt] [1] M. Boninsegni, A. Kuklov, L. Pollet, N. Prokof'ev, B. Svistunov and M. Troyer, {\it The fate of vacancy-induced supersolidity in ${}^4{\rm He}$}, Phys. Rev. Lett. {\bf 97}, 080401 (2006).\\[0pt] [2] L. Pollet, M. Boninsegni, A. B. Kuklov, N. V. Prokof'ev, B. V. Svistunov, and M. Troyer, {\it Superfluidity of Grain Boundaries in solid $^4$He}, Phys. Rev. Lett. {\bf 98}, 135301 (2007).\\[0pt] [3] M. Boninsegni, A. B. Kuklov, L. Pollet, N. V. Prokof'ev, B. V. Svistunov, and M. Troyer, {\it Luttinger Liquid in the Core of Screw Dislocation in Helium-4}, Phys. Rev. Lett. {\bf 99}, 035301 (2007).\\[0pt] [4] L. Pollet, M. Boninsegni, A. B. Kuklov, N. V. Prokofev, B. V. Svistunov, and M. Troyer, {\it Local stress and superfluid properties of solid Helium-4}, Phys. Rev. Lett. {\bf 101}, 097202 (2008).\\[0pt] [5] P. Corboz, L. Pollet, N. V. Prokof'ev, and M. Troyer, {\it Binding of a $^3$He impurity to a screw dislocation in solid $^4$He}, Phys. Rev. Lett. {\bf 101}, 155302 (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W1.00002: Elastic Behavior and Dislocations in Solid Helium Invited Speaker: Torsional oscillator experiments show decoupling of solid $^4$He below 200 mK, the signature of the ``non-classical rotational inertia'' which would characterize a supersolid phase of matter. Solids are distinguished from liquids by their non-zero elastic shear modulus and we have developed a new technique to measure this modulus at very low frequencies and amplitudes. Our measurements show a large and unexpected stiffening in the shear modulus of $^4$He below 200 mK. It has the same dependence on temperature, frequency, amplitude and $^3$He impurity concentration as the decoupling seen in torsional oscillator measurements and the two phenomena are clearly related. The elastic behavior is explained in terms of the motion of dislocations and their pinning by impurities, suggesting that these defects play an important role in supersolidity. We have now extended our elastic measurements to both bcc and hcp $^3$He. These measurements, and comparisons to new torsional oscillator results, clarify the roles of quantum statistics and crystal structure in the behavior of solid helium. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W1.00003: The effect of order, disorder, and confinement on the NCRI response in solid $^{4}$He Invited Speaker: After a multitude of experimental and theoretical efforts over the past few years attempting to explain the microscopic origin of non-classical rotational inertia (NCRI) signals seen in torsional oscillator (TO) experiments, disorder has emerged as a crucial factor for determining the supersolid behavior. In an attempt to discover the type of disorder relevant to the NCRI effect we have performed TO experiments on solid $^{4}$He samples grown at constant pressure or temperature from the superfluid [1], a method known to reliably produce large helium crystals. We show that similar NCRI signals are seen for samples with grain boundary areas that differ by orders of magnitude, indicating that grain boundaries are largely irrelevant to the supersolid phenomenon. In addition, we probe the effect of confining the helium crystals within several restricted geometries. [1] A.C. Clark, J.T. West, and M.H.W. Chan, Phys. Rev. Lett. \textbf{99}, 135302 (2007). [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W1.00004: X-ray scattering experiments on solid helium Invited Speaker: Using x-ray synchrotron radiation, we have studied the nature of crystals of solid $^{4}$He at temperatures down to 50 mK. Measurements of peak intensities and lattice parameters do not show indications of the supersolid transition. Between 50 mK and 0.6K the relative change in the lattice parameters is less than 2$\times $10$^{-5}$ and that in $\left\langle {u^2} \right\rangle $ less than 4$\times $10$^{-3}$. Scanning with a small (down to 10 x 10 $\mu $m$^{2})$ beam, we resolve a mosaic structure within these crystals consistent with numerous small angle grain boundaries. The mosaic shows significant motion even at temperatures far from melting. When grown in aerogel, solid $^{4}$He polycrystalline, with an hcp crystal structure (as in bulk) and a crystallite size of approximately 100 nm. In contrast to the expectation that the highly disordered solid will have a large supersolid fraction, torsional oscillator measurements show a behavior that is strikingly similar to high quality crystals grown from the superfluid phase. The low temperature supersolid fraction is only $\sim $3x10$^{-4}$ and the onset temperature is $\sim $~100~mK. Work done in collaboration with C.A. Burns, M.H.W. Chan, C.N. Kodituwakku, L.B. Lurio, A. Said and J.T. West. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 2:15PM |
W1.00005: Torsional oscillator experiments on helium films on graphite; the search for a two dimensional supersolid Invited Speaker: $^{4}$He films adsorbed on graphite have been investigated by torsional oscillator methods in the temperature range 2.5mK to 3.5K, focussing primarily on the behaviour of the second layer. The second layer atoms move in a $^{4}$He lattice potential created by the compressed first solid layer. In our experiments, we observe an anomalous response consistent with that previously discussed by Crowell and Reppy [1]. We have made precise measurements of the film decoupling and its contribution to the dissipation over this wide temperature range. These allow us to infer $\rho _{s}(T$ = 0) as a function of second layer density. The results indicate a supersolid response of the second layer in the vicinity of the putative $\surd $7 $\times \surd $7 triangular superlattice phase. The triangular lattice, unlike the square lattice, is predicted to support a supersolid phase. Cold atoms in a triangular lattice provide a candidate system to stabilise this new phase of matter, but this has yet to be realized experimentally. The ``super''-response of the second $^{4}$He layer as a function of filling of the underlying lattice potential will be discussed. \\[4pt] [1] P A Crowell and J D Reppy, Phys. Rev. B53, 2701 (1996) [Preview Abstract] |
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