Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P31: Supersolid Experiments |
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Sponsoring Units: DCMP Chair: Norbert Mulders, University of Delaware Room: Colorado Convention Center 401 |
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P31.00001: Effect of $^{3}$He impurity on the supersolid transition of $^{4}$He E. Kim, J. S. Xia, J. T. West, X. Lin, M. H. W. Chan The supersolid phase of $^{4}$He was reported by a series of torsional oscillator experiments [1]. One of the most striking features of the supersolid transition is the intriguing $^{3}$He impurity effect. The addition of an extremely small amount of $^{3}$He impurity broadens the transition and enhances the transition temperature T$_{c}$. This effect is very different from that in helium film and that in `bulk' superfluid helium. We have studied the influence of $^{3}$He impurity on the supersolid transition systematically by progressively diluting isotopically-pure $^{4}$He ($^{3}$He impurity less than 2ppb) with $^{3}$He. The transition temperature is monotonically enhanced with increasing $^{3}$He concentration and the supersolid fraction shows a broad maximum around 0.2 ppm. [1] E. Kim and M. H. W. Chan, \textit{Science} \textbf{305}, 1941 (2004); \textit{Nature} \textbf{425}, 227 (2004); \textit{J. Low Temp. Phys.} \textbf{138}, 859 (2005); \textit{Phys. Rev. Lett. }\textbf{97}, 115302 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P31.00002: NCRI in Helium Crystals Grown Under Constant Pressure A.C. Clark, M.H.W. Chan A prominent issue concerning supersolidity in $^4$He is crystal quality. Several theoretical studies have demonstrated that a perfect crystal is insulating. Apparent experimental discrepancies between different laboratories, while very interesting, have not resolved the matter. In the torsional oscillator experiments, all solid samples previously studied were grown under constant volume. A decrease in pressure occurs during growth so that crystals are forced to expand, possibly resulting in highly strained crystals. There has also been no attempt to seed a single crystal, presumably leading to polycrystallinity. It is known that crystals carefully grown under constant pressure are of high quality. We report on new torsional oscillator measurements of isotopically pure solid $^4$He grown under constant pressure. We detect non-classical rotational inertia (NCRI) in all samples grown to date. Comparisons will be made to earlier studies. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P31.00003: Characterization of $^4$He Samples Exhibiting NCRI M.J. Bowne, Z. Cheng, J.T. West, A.C. Clark, M.H.W. Chan We plan to carry out sound measurements on solid $^4$He samples contained in a torsional oscillator. We believe \textit{in situ} characterization of samples demonstrating non-classical rotational inertia (NCRI) can lead to a better understanding of the microscopic mechanism behind supersolidity. Sound pulses will be generated and detected with a single quartz transducer housed within the torsion cell. The velocity of sound and attenuation will be extracted from the pulse echoes. This information conveys the relative quality of samples, as well as the orientation of the c-axis for single crystals. Preliminary measurements are under way in a test apparatus. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P31.00004: Excess specific heat of solid $^4$He Xi Lin, A. C. Clark, M. H. W. Chan An experimental challenge on heat capacity measurement is discerning the small specific heat of solid helium from that of the metallic cells typically used to confine the high-pressure solid. We report on heat capacity measurements of solid $^4$He contained in a silicon cell, in our search of a thermodynamic signature of the supersolid phase. Data will be presented for several solid samples around 26 bar, where the heat capacity is at least 10 times larger than that of the silicon cell. Below 200mK we observe a heat capacity in excess of that predicted by Debye theory. It is unclear if our observations are directly associated with the supersolid $^4$He phase. In the hope to elucidate whether this phenomenon is connected to the supersolid phase, we are currently investigating the effect of $^3$He impurities. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P31.00005: Shear measurements of bulk solid $^{4}$He James Day, John Beamish Recent torsional oscillator experiments indicate that the non-classical rotational inertia (NCRI) fraction depends on isotopic purity and on the details of crystal growth and annealing, suggesting that defects may be involved. While solid helium does not flow in response to pressure gradients at low temperatures, plastic deformation of solid helium closer to melting creates defects and pressure gradients which are not easily eliminated by thermal annealing. Similar defects must be created during crystal growth by the blocked capillary method or by large thermal gradients. Given the theoretical arguments against supersolidity in defect-free crystals and the preliminary experimental evidence linking NCRI to annealing, it is important to control and study defects in solid helium more directly. To that effect, we have begun to study the static and low frequency shear deformation of crystals grown by different methods. This is a direct measure of the shear modulus of the crystal and should allow us to separate elastic from inertial effects. We can also compare the elastic to the plastic deformation response by increasing the magnitude of the shear stress applied to the crystal. We will describe our experimental design and present preliminary results. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P31.00006: Flow mechanisms of solid $^{4}$He near melting John Beamish, James Day In our recent experiments, we saw no evidence of pressure induced flow of solid helium, neither in the pores of Vycor nor in bulk, in the temperature range where non-classical rotational inertia (NCRI) has been observed; however, we did observe mass flow close to the melting points of our samples. Mass can be transported in crystals through vacancy movement or via the motion of extended defects like dislocations. The high temperature flow of helium confined in Vycor is quite different from that of bulk helium; different mechanisms appear to be involved. In bulk helium the flow is irreversible and is consistent with the creation of defects like dislocations during plastic flow. Plastic flow and dislocation creation cannot occur in nanometer scale channels, and so it is not surprising that we see different behavior for helium in the pores of Vycor. The thermally activated mass flow in Vycor must be due to motion of vacancies or similar point defects. In this talk we will describe the nature of observed flow and possible mechanisms, and discuss its relevance to the NCRI experiments. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P31.00007: Effect of Crystal Growth Velocity on ``Supersolidity'' of $^4$He Keiya Shirahama, Motoshi Kondo, Shunichi Takada, Yoshiyuki Shibayama One of the most important issues of supersolid studies is to elucidate the relationship between the supersolid behaviors and quality of $^4$He crystal. Recent observation of the annealing effect by Rittner and Reppy suggests that supersolidity is strongly dependent on the sample history. We have examined the effect of crystal growth velocity and crystal annealing on supersolidity of solid $^4$He formed in a cylinder torsional oscillator[1]. Solid samples at various pressures are grown by cooling liquid $^4$He under isochoric conditions (blocked capillary method). When the cooling velocity is high (0.1K/min) during crystal growth, the supersolid fraction is 3 times as large as that of slowly grown samples. This supersolid fraction decreased to 1/3 by sample annealings for 1 day near the melting point. On the other hand, no annealing effect is observed in the slowly grown samples. These behaviors strongly suggest that lattice defects formed in the crystal growth process play a crucial role on supersolidity of $^4$He. [1] M.Kondo et al., J.Low.Temp.Phys., to be published [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P31.00008: Influence of sample geometry on the supersolid signal Ann Sophie C. Rittner, John D. Reppy We have used a torsional oscillator with an annular geometry in order to study the correlation between sample volume and supersolid signals systematically. We varied the width of the annulus in the cell with cylindrical magnesium inserts of different radii. In preliminary measurements on an open cylinder cell, we have found an apparent supersolid fraction $\frac{\rho_s}{\rho}$ of 0.04 \% at 26 bar, a maximum velocity of 23 $\mu m/s$ and a sample volume of 2 cc. In an annular cell with a width of 0.635 mm we measured a supersolid fraction of about 0.33 \% at a velocity of 16 $\mu m/s$ and a pressure of 32 bar. The increase of the supersolid fraction in restricted geometries suggests that defects cause the NCRI behavior and could explain the different results of previous torsional oscillator measurements. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P31.00009: Torsional Oscillator for Studying Supersolid $^4$He at Two Resonant Frequencies Joseph Graves, Yuki Aoki, Harry Kojima In order to observe supersolid behavior in solid $^4$He at multiple frequencies while keeping all other parameters constant, an oscillator with two torsional modes has been constructed. The torsion rod is made of beryllium copper and the cylindrical sample chamber is made of Stycast 1266. The two modes have resonant frequencies of 500 and 1200 Hz. Preliminary studies have shown fairly high quality factors of 10$^4$ at 300 K and 7$\times$10$^4$ at 77 K. We plan to measure the changes, at the two frequencies in the identical solid $^4$He sample, of the resonant frequency, dissipation and critical velocity associated with the supersolid phase at temperatures below 200 mK. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P31.00010: Search for new evidence of superfluidity in solid $^4$He by phonon propagation Yuki Aoki, Harry Kojima The phonon propagation generated by heat pulse has been studied in solid $^4$He under pressure between 25 and 56 bar to search for a sign of supersolid behavior at temperatures down to 40 mK. Response to input heat pulses are detected by a titanium film superconducting edge bolometer separated by a 4.5 mm thick solid $^4$He from the heater. According to theoretical studies, a new fourth sound-like mode is expected to emerge in the supersolid state. The sensitivity of our bolometer has been improved from an earlier version by an order of magnitude by changing the film structure for the purpose of searching for a small temperature deviation signal accompanying the fourth sound-like propagation mode. The response of the bolometer to heat pulse was measured in different quality solid samples which had been grown with different cooling rates during solidification. The detected response signal has not revealed any identifiable signature of a new mode within a temperature excursion of about $\Delta T$ = 5 $\mu$K from the background signal shape. An estimated superflow velocity corresponding to the temperature excursion is greater than the critical velocity observed by Kim and Chan. Our detection sensitivity must be further increased before a definitive conclusion on the fourth sound-like mode can be made. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P31.00011: Debye-Waller factor in solid He-4 at sub-Kelvin temperatures Elizabeth Blackburn, John M. Goodkind, Sunil K. Sinha, Jacob Hudis, Collin Broholm, Joost van Duijn, Richard Down, Oleg Kirichek, Chris D. Frost The recent observation by Kim and Chan [Science 305 (2204) 1941] of a transition at low temperatures ($\sim $ 200 mK) in the hcp-phase of solid helium has re-opened interest in the old question of supersolidity. The nature of the low-temperature phase remains in question, and to investigate this in more detail, we have measured the density distribution of He-4 nuclei in crystals of He-4 with a molar volume of 21.3 cm$^{3}$ down to 140 mK. We find no evidence for any changes in the vicinity of the transition. Treating the material as a traditional crystal, we have extracted the mean square displacement for the nuclei and find anisotropy between the in- and out-of-plane motions. Our values are in agreement with previous work at higher temperatures. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P31.00012: Precise neutron diffraction study of hcp and bcc $^{4}$He Ralph Simmons, Robert Blasdell Precise lattice parameter measurements are reported for $^{4}$He in both bcc and hcp phases at low density and low temperature. The results can be used to set limits on a proposed incommensurate equilibrium state of solid $^{4}$He near $T$ = 0. ``Incommensurate" means a net difference between atomic sites and atoms. The relative difference is defined as $\epsilon$. Present measurements were made by carefully calibrated neutron diffraction. The value established at melting, by comparison with published bulk density values, is $\epsilon = 0.4\pm 0.4 \%$. Much of the uncertainty comes from uncertainties in the bulk values. These neutron results on hcp $^{4}$He are also consistent with previous precise x-ray diffraction work on bcc $^{4}$He and, at higher densities, on both $^{4}$He and $^{3}$He. Published isochoric measurements of changes in x-ray lattice parameters as $T$ is reduced from melting can be used to extrapolate $\epsilon$ toward zero $T$, where its most probable value is zero, with the same uncertainty. The present neutron work on hcp phase agrees with published high-resolution synchrotron x-ray work in showing that the $(c/a)$ ratio is slightly smaller than that corresponding to ideal close-packing. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P31.00013: New excitations in bcc $^{4}$He - an inelastic neutron scattering study Oshri Pelleg, Jacques Bossy, Emmanuel Farhi, Meni Shay, Slava Sorkin, Emil Polturak We report results of inelastic neutron scattering experiments on bcc solid $^{4}$He (Pelleg et al. \textit{Phys. Rev. B.} \textbf{73}, 180301(R)(2006)). In the experiments, we studied the excitation spectrum of the solid, including the phonon branches and the recently discovered ``optic-like'' branch ( T.Markovich et al., \textit{Phys. Rev. Lett.} \textbf{88}, 195301(2002)). We were able to determine that the new ``optic-like'' branch has an intrinsic dispersion, hence it is a propagating mode. This excitation also couples to the usual phonons. In addition, in the new experiments we discovered another ``optic-like'' branch. The second ``optic-like'' branch is dispersionless, with an energy around 1 meV ($\sim$~11K). This excitation does not seem to couple to phonons. Hence, the properties of the two ``optic-like'' branches seem different. Since one expects only 3 acoustic phonon branches in a monoatomic cubic crystal, these two new branches must represent some different type of excitations. Some potential interpretations, based on Path Integral Monte Carlo simulations, will be presented. [Preview Abstract] |
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