Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S2b: Supersolidity and Bose-Einstein Condensation in Solid Helium |
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Sponsoring Units: DCMP Chair: Moses Chan, Penn State University Room: LACC 151 |
Wednesday, March 23, 2005 4:18PM - 4:54PM |
S2b.00001: Observation of superflow in solid $^4$He Invited Speaker: At temperatures below 2.176 K, liquid $^{4}$He enters into a superfluid state and flows without any friction. There is strong evidence that Bose-Einstein condensates in dilute gases also exhibit superfluidity. Perhaps it is counter to intuition, but superfluid like behavior is thought possible even in solid helium. We employed a torsional oscillator technique and found evidence of superflow in bulk solid $^{4}$He (1) and solid $^{4}$He confined in porous Vycor glass (2). The effect appears as a drop in the resonant oscillation period as the sample cell is cooled below about 0.2 K. A series of control experiments reveals that the effect is due to irrotational superflow as in superfluid helium. The supersolid fraction in the low temperature limit is about 1{\%}. It has a ``universal'' temperature dependence that is different from that of the superfluid transition in liquid and different from that of a weakly interacting Bose gas. The supersolid fraction is strongly attenuated with increasing oscillation speed, indicating that the critical velocity is extremely low. (1) E. Kim and M. H. W. Chan, \textit{Science} \textbf{305}, 1941 (2004) (2) E. Kim and M. H. W. Chan, \textit{Nature} \textbf{425}, 227 (2004) [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:30PM |
S2b.00002: Quantum Phase Transition of $^4$He Confined in a Nanoporous Material Invited Speaker: Confinement of $^4$He in porous medium such as Vycor glass results in suppressions of freezing and superfluidity. The suppressions can be enhanced as the pore size decreases to nanometer scale. From torsional oscillator [1] and pressure studies we have revealed the $P-T$ phase diagram of $^4$He confined in a porous Gelsil glass which has nanopores of 2.5 nm in diameter. We have found that the superfluid transition temperature approaches 0 K at 3.4 MPa, and the freezing pressure shifts from the bulk one to 3.5 MPa. The solid - nonsuperfluid phase boundary is independent of temperature below 1 K, suggesting that the nonsuperfluid phase has low entropy as well as solid. The features indicate that the confined $^4$He undergoes a superfluid - nonsuperfluid - solid quantum phase transition at 0 K. The low - entropy nonsuperfluid phase may be a localized Bose - condensed state, in which global phase coherence is destroyed by strong correlation between $^4$He atoms or by random potential. [1] K. Yamamoto, H. Nakashima, Y. Shibayama, K. Shirahama, Phys. Rev. Lett. 93, 075302 (2004). [Preview Abstract] |
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