Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W7: Novel Phenomena in Quantum Fluid He-3 |
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Sponsoring Units: DCMP Chair: Gary Ihas, University of Florida Room: Colorado Convention Center Korbel 4A-4B |
Thursday, March 8, 2007 2:30PM - 3:06PM |
W7.00001: Transport in highly spin-polarized normal liquid 3He Invited Speaker: Normal liquid Helium 3 is an ideal system to study the role of correlations in fermions physics. It is characterized by strong interactions between particles, the range of which is comparable to the inter-atomic distance. As such, it represents an intermediate case of complexity, halfway between the electronic systems and the ultra-cold Fermi gases. In particular, transport in degenerate Helium 3 involves not only s-wave scattering, but also partial waves with non-zero orbital angular momentum. Studying the polarization dependence of transport allows to directly probe this fact. We will report on transport experiments in highly spin-polarized, degenerate, liquid 3He, obtained by melting spin polarized solid 3He and rapidly cooling the resulting liquid down to about 60 mK. While the polarization dependence of viscosity is unexpectedly close to that predicted for a free fermion gas, the thermal conductivity increases much less with polarization than expected in that case. We will discuss the possible reasons for this difference. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W7.00002: Magnetic Relaxation and Minority Spin Condensate in Spin-Polarized Superfluid $^3$He A$_1$ Invited Speaker: The magnetic relaxation phenomena in superfluid $^3$He A$_1$ phase are studied using a magnetic fountain pressure detector in which a large reservoir is connected to a small sensor chamber through two superleak channels of height 18 $\mu$m. Superflow in simultaneous mass/spin current is driven by an externally applied magnetic field. Measurements of the relaxation of the induced fountain pressure are carried out under a variety of conditions including pressure(3 - 29 bar), temperature, static field(up to 8 T) and $^4$He(5 monolayers) coverage. The relaxation of the fountain pressure arises from the time dependent spin density in the sensor chamber. The observed relaxation time $\tau$ varies from 80 s near the upper transition temperature, T$_{c1}$, to less than 0.1 s near the lower transition temperature, T$_{c2}$. The measured relaxation rate increases starting near the middle of A$_1$ phase and more rapidly as the T$_{c2}$ is approached. The $^4$He coverage is observed not to affect the measured spin relaxation rate and this indicates that the relaxation is a bulk liquid effect. The rapid increase in relaxation rate is interpreted in terms of the Leggett-Takagi$^1$ mechanism of intrinsic spin relaxation arising from a small but increasing presence of minority spin pair condensate$^2$(with pair magnetic moment aligned in the opposite direction to the applied field) in A$_1$ phase as T$_{c2}$ is approached. It is concluded that the conventional view of the superfluid A$_1$ phase being composed of condensate pairs with magnetic moment aligned strictly along the applied field is inadequate. \newline $^1$ A.J. Leggett and S. Takagi, Ann. Phys. \textbf{106}, 79(1977). \newline $^2$ H. Monien and L. Tewordt, J. Low Temp. Phys. \textbf{60}, 323(1985). [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W7.00003: Specific Heat of Superfluid $^{3}$He and Andreev Bound States Invited Speaker: The specific heat at the normal to superfluid transition gives a clear thermodynamic signature for onset of the superfluid state marked by a discontinuity which has been accurately determined by Greywall[1]. We have measured the effect of a silver surface on the specific heat at this transition and we have found a temperature dependent suppression of the specific heat in the superfluid state which we have studied as a function of temperature and pressure[2]. This result can be understood in terms of the contribution to the Free energy from surface Andreev bound states which have a range of half of a superfluid coherence length. For the case of very large surface-to-volume ratio, as can be achieved with high porosity silica aerogel, the superfluid transition is suppressed. We have measured the specific heat anomaly at the transition temperature for this case[3] and interpret our measurements in terms of scattering theory. At the lowest temperatures a band of Andreev surface bound states dominate the specific heat of the superfluid $^{3}$He/aerogel system. This work was performed in collaboration with H. Choi, J.P. Davis, and J. Pollanen at Northwestern University, supported by the NSF grant DMR-0244099. [1] D.S. Greywall, Phys. Rev. B. 33, 7520, (1986). [2] H. Choi, J.P. Davis, J. Pollanen, and W.P. Halperin, Phys. Rev. Lett. 96, 125301 (2006). [3] H. Choi, K. Yawata, T.M. Haard, J.P. Davis, G. Gervais, N. Mulders, P. Sharma, J.A. Sauls, and W.P. Halperin, Phys. Rev. Lett. 93, 145301 (2004). [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:54PM |
W7.00004: Surface Andreev Bound States of $^3$He-B by Transverse Acoustic Impedance Measurements Invited Speaker: Complex transverse acoustic impedance of the superfluid $^3$He was measured at the frequencies of 10 to 80 MHz from 6 up to 25 bar by a CW bridge method. The observed temperature dependence of it was well explained by the quasi-classical theory with random $S$-matrix model for a diffusive surface. The impedance was influenced by pair breaking and by quasi-particle density of states at the surface, which was drastically modified from the bulk one by the formation of surface Andreev bound states. In B phase, an additional gap in SDOS opened between the upper energy edge $\Delta^*$ of the surface Andreev bound states band and the bulk energy gap $\Delta$. Temperature dependence of $\Delta^*$ was measured and was about 30\% smaller than theoretical values. In A phase, flat and gapless SDOS was confirmed experimentally for the first time. It is demonstrated that the present spectroscopic method is a good tool to investigate the surface microscopic state, which has not been possible for the charge neutral {\it P}-wave superfluid. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W7.00005: Pure quantum turbulence in superfluid $^{3}$He. Invited Speaker: Turbulence in classical fluids has far reaching technological implications but is poorly understood. A better understanding might be gained from studying turbulence in quantum systems. In a pure superfluid (at low temperatures), there is no viscosity and vortex lines are quantised. Quantum turbulence consists of a tangle of quantised vortex lines which interact via their self induced flow. We have recently developed techniques for detecting quantum turbulence in superfluid $^{3}$He-B in the low temperature limit. We find that the transition to turbulence from a moving grid occurs by the entanglement of emitted vortex rings. At low grid velocities, ballistic vortex rings are emitted which become entangled at higher grid velocities leading to turbulence. The quantum turbulence decays in a manner very similar to classical turbulence, suggesting that energy is transferred down a broad range of length scales. The decay mechanism, in the absence of any normal fluid, is a very interesting, and currently unsolved, theoretical problem. Experimentally, it appears that the quantum of circulation plays the role of viscosity in governing the decay rate. We hope to gain more detailed information by measuring fluctuations in the turbulent flow field. We discuss current and future experiments. [Preview Abstract] |
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