Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V23: Solid Helium I |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Boris Svistunov, University of Massachusetts Amherst Room: C125-C126 |
Thursday, March 18, 2010 8:00AM - 8:12AM |
V23.00001: Dynamics of ${ }^3$He Impurities in Solid ${ }^4$He near the Apparent Supersolid Transition Chao Huan, Sung Su Kim, Liang Yin, Jian-Sheng Xia, Neil Sullivan, Donald Candela We report measurements of the nuclear spin-spin and nuclear spin-lattice relaxation times for ${ }^3$He impurities in solid hcp ${ }^4$He for a molar volume of 21 cm$^3$. The ${ }^3$He concentrations range from 20 to 2000 ppm and the relaxation times were measured for temperatures 1.4 K$>$T$>$0.010 K. The results are discussed in terms of the dynamics of the ${ }^3$He impurities and their scattering by other impurities and by other defects. In addition we report new results for the phase separation of the ${ }^3$He from the ${ }^4$He and the formation of ${ }^3$He nanodroplets in the solid ${ }^4$He matrix. [Preview Abstract] |
Thursday, March 18, 2010 8:12AM - 8:24AM |
V23.00002: Simultaneous Measurement of Torsional Oscillator and NMR of Dilute $^3$He in Solid $^4$He Ryo Toda, Patryk Gumann, Kei Kosaka, Masatomo Kanemoto, Yutaka Sasaki We have carried out simultaneous measurements of on $^3$He-concentrations in solid $^4$He using state-of-the-art NMR technique and a torsional oscillator (TO) method. Our NMR results from a sample with a relatively high concentration ($\sim$ hundred ppm of $^3$He), suggest existence of three different ``states'' of $^3$He which depend on temperature. The first one is characterized by almost invisibly long spin-lattice relaxation T$_{1}$. This would imply that each $^3$He atom behaves as gaseous in a solid $^4$He-matrix. Another characterized by a reasonably long component in T$_{1}$ is a cluster of $^3$He, which appears below the isotopic phase separation temperature. The third one is characterized by a relatively short spin-lattice relaxation time T$_{1}$, which also shows up below the isotropic phase separation temperature. This state may correspond to $^3$He clusters in the disordered part of the crystal, since it does not disappear right away even above the phase separation temperature. As found by other studies, broadening out of the $\Delta$Q$^{-1}$-peak as well as the NCRIf were found in TO. In addition we have also observed a TO response for 0.3 ppm sample. Further studies to clarify this issue are in progress. [Preview Abstract] |
Thursday, March 18, 2010 8:24AM - 8:36AM |
V23.00003: Non-classical Response and Shear Modulus of Solid 4He D.Y. Kim, S. Kwon, H. Choi, W.S. Choi, E. Kim, H.C. Kim The relation between the nonclassical rotational inertia (NCRI) and the shear modulus increase of solid 4He was investigated. A pair of piezoelectric transducers is positioned in the center of a torsional oscillator to measure the shear modulus. We observe the NCRI and the shear modulus increases below 200mK with similar temperature, measurement drive, and frequency dependence. The drive and frequency dependence in the shear modulus increase can be understood by the model of thermally assisted unpinning of dislocations from impurities. However, the shear modulus increase is found to be insufficient to explain the magnitude of the NCRI. In addition, no linear correlation between the magnitude of the NCRI and the shear modulus increase is found and the magnitude of shear modulus shows a rather negative correlation with that of NCRI. [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 8:48AM |
V23.00004: Unveiling Hidden Phases of Solid 4He H. Choi, S. Kwon, D.Y. Kim, E. Kim The discovery of non-classical rotational inertia fraction (NCRIF) of solid 4He in a torsional oscillator (TO) was immediately followed by various attempts to identify the underlying physics of the phenomenon. Despite the efforts the true nature of the non-classical response is still not fully understood. We have investigated dynamic response of solid 4He by changing oscillation rim velocity at fixed temperatures. Main discovery of our experiment is that there are hidden phases of solid 4He that are only accessible by cooling the sample down to some temperature with zero to small rim velocity first, and then subsequently raising the velocity at that temperature. The different phases are identified with two distinct features. One is that the amount of NCRIF is different from that of a typical high-velocity cooled 4He. The other is that the dynamics of the TO changes across these different phases. We believe these phases are distinguished by different pinning mechanisms of excitations present in 4He. [Preview Abstract] |
Thursday, March 18, 2010 8:48AM - 9:00AM |
V23.00005: Resonant ultrasound spectroscopy for supersolid helium studies Guoxing Liu, J. D. Maynard Recent experiments have indicated that the onset of the supersolid state and an increase in the shear modulus of solid helium may be related through a common origin. We have been adapting resonant ultrasound spectroscopy (RUS), which can be used to measure all of a solid's elastic moduli, for use with solid helium. In the RUS technique, a cell with known geometry is fitted with ultrasound drive and receive transducers so that a number (10 - 30) of the cell's natural frequencies may be measured; by analyzing the natural frequencies, all of the elastic moduli of the cell's contents (solid helium) may be determined. For RUS to work, it is essential that the normal modes of the cell be well understood. We have been developing a cell which will maintain robust normal modes when the cell is cycled in temperature and pressure. An important feature is minimizing the amount of epoxy needed for forming a hollow cell for containing the solid helium, since epoxy may not cycle well. Other important features include transducer attachment and the use of an invertable finite element calculation for determining elastic moduli from the measured natural frequencies of a complicated cell. [Preview Abstract] |
Thursday, March 18, 2010 9:00AM - 9:12AM |
V23.00006: Dependence of Hysteresis Effect in Torsionally Oscillated Solid $^4$He on $^3$He Impurity Concentration Michael Keiderling, Patryk Gumann, David Ruffner , Harry Kojima We studied the effects of $^3$He-impurity on the hysteresis phenomenon in solid $^4$He grown in an annular container as measured by a compound torsional oscillator (TO). The sample is cooled while oscillating at an initial high ($\sim$ 500 $\mu$m/s) velocity from a high temperature to a regulated target temperature where the Non-Classical Rotational Inertia fraction(NCRIf) is measured. The velocity is decreased to below a critical velocity of about 15 $\mu$m/s and then increased back to the original velocity where the NCRIf is measured again. The difference in the two NCRIf represents hysteresis. The difference is found to vanish (i.e. reversible) when the temperature is above a temperature $T_h(x_3)$ which depends on the $^3$He impurity concentration $x_3$. $T_h$ was measured for $x_3$ = 0.3, 6, 25 ppm. Surprisingly, $T_h$ coincides with the phase separation temperature evaluated according to Edwards and Balibar theory [1]. [1] D. O. Edwards and S. Balibar, Phys. Rev. B 39, 4083 (1989) [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:24AM |
V23.00007: Study of Energy Dissipation in Solid $^4$He Samples as Measured by Compound Torsion Pendulum Patryk Gumann, Michael Keiderling, David Ruffner, Harry Kojima We have carried out a series of torsional oscillator experiments on solid $^4$He containing $^3$He-concentrations (with concentration x$_3$ = 0.3, 3, 6, 12, 25 ppm) using our compound torsional oscillator (TO). Our studies of the supersolid phenomenon focused on the energy dissipation change ($\sim\Delta$ Q$^{-1}$) as x$_3$ was varied. The unique compound TO design allows observations of identical samples at two distinct frequencies (f$_1$=493 and f$_2$=1165 Hz). When normalized as $\Delta Q^{-1}/\Delta Q^{-1}_{max}(x_3)$ the data collapse onto a universal bell-shaped function of $T/T_{max}(x_3)$ except for the f$_1$ mode in the 12 and 25 ppm samples and the f$_2$ mode in the 25 ppm sample. A clear deviation of $\Delta Q^{-1}$ in both modes from the other values of x$_3$ suggests some additional mechanism for the extra dissipation present in the 25 ppm sample. [Preview Abstract] |
Thursday, March 18, 2010 9:24AM - 9:36AM |
V23.00008: Variation in supersolid signal amplitude: possible cell dependence John Reppy The aim of the present experiment is to test a hypothesis that plastic deformation of solid TO samples may occur during cooling and is the source of disorder in supersolid samples showing large NCRIFs. Cells with a rigid design tend to exhibit rather small supersolid signals even when the surface to volume ratio is large, while the largest signals were observed in cells where one end wall of the cell consists of a flexible diaphragm. The displacement of such a diaphragm by an applied force allows the possibility of a controlled deformation of the solid within the cell. In the first attempt at an experiment of this type, the initial signals were rather large even without any special efforts. Subsequent deformation the solid at a temperature of 200 mK did result in a 20{\%} increase in the NCRIF. This cell may have been sufficiently flexible so that the unavoidable pressure changes upon cooling were sufficient to produce changes in the cell shape large enough to produce a plastic deformation of the solid. The next cell will have a stiffer diaphragm to test this idea. This work has been supported under NSF DMR-0605864. [Preview Abstract] |
Thursday, March 18, 2010 9:36AM - 9:48AM |
V23.00009: Temperature-dependent characteristic velocity and excitations of solid helium-4 Ethan Pratt, Benjamin Hunt, Vikram Gadagkar, Minoru Yamashita, Alexander Balatsky, J.C. Seamus Davis Excitations that allow shear velocity to inhibit superflow are well known in superfluids, and typically generate temperature-dependent critical velocities that depend sensitively on the physics of these microscopic dissipative mechanisms. By contrast, the nature of the microscopic excitations associated with the low-temperature {}``supersolid'' inertial anomaly and shear stiffening of solid $^{4}$He remain unknown, and its temperature-dependent characteristic velocity curve $v_{\star}\left(T\right)$ has not yet been observed. Using a SQUID-based torsion oscillator to map the complete complex rotational susceptibility of solid $^{4}$He, we observed that the internal dissipative excitation rates obey power laws of temperature and velocity, and we acquired the full temperature-dependent characteristic velocity function $v_{\star}\left(T\right)$. We compare these observations to the predicted $v_{\star}\left(T\right)$ curves of several microscopic models, including those of a thin-film vortex unbinding and a two-level-system (TLS) momentum deficit. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:00AM |
V23.00010: Low Frequency Dynamics of Solid Helium John Beamish, James Day, Oleksandr Syshchenko Torsional oscillator measurements involving solid $^{4}$He show a frequency increase at temperatures below 200 mK which suggests that mass is decoupling from the oscillator, the ``non-classical rotational inertia'' (NCRI) which characterizes a supersolid. A dissipation peak accompanies the onset of decoupling but its origin is unclear. The elastic shear modulus stiffens in the same temperature range and is also accompanied by a dissipation peak. It is clear from the similarities between the shear modulus and the torsional oscillator NCRI that the two phenomena are closely related. We have measured the shear modulus and the dissipation (phase angle) in solid $^{4}$He at frequencies from 0.5 to 8000 Hz. The onset of stiffening and the dissipation peak shift to higher temperatures with increasing frequency. The stiffening and dissipation can be described as the real and imaginary parts of a thermally activated Debye relaxation process. However, a broad range of activation energies is needed to describe the elastic response. [Preview Abstract] |
Thursday, March 18, 2010 10:00AM - 10:12AM |
V23.00011: Characterization of solid helium crystals and non-classical rotational inertia measurements Joshua T. West, Moses H. W. Chan Non-classical rotational inertia (NCRI) measurements taken by several groups over the past few years all reproduce the same essential features. However, the magnitudes of the observed NCRI signals vary by three orders of magnitude! Many experiments indicate that disorder in some form (be it point defects, grain boundaries, dislocations, or a glassy phase) is either responsible for, or enhances NCRI. We will summarize the efforts to characterize the disorder in solid helium crystals. X-ray scattering experiments have been performed to look at the effects of rapid quenching. In-situ optical characterization of crystals within a torsional oscillator will be discussed. [Preview Abstract] |
Thursday, March 18, 2010 10:12AM - 10:24AM |
V23.00012: The anomalous stiffness of $^4$He crystals Sebastien Balibar, Xavier Rojas, Humphrey Maris In 2007, Day and Beamish have discovered that the supersolid transition of $^4$He crystals is accompanied by an increase in their shear modulus $\mu$. They proposed that this stiffening is due to the pinning of dislocations by $^3$He impurities. However, we have measured $\mu$ in single crystals which are free of any impurity. In such ultrapure crystals, we have found a stiffening which calls for a mechanism independent of $^3$He content. This new mechanism might be Kuklov's roughening transition of dislocations or some interaction between Anderson's vortex liquid and the lattice. In order to discriminate between these two scenarios, we have built a cell which allows acoustic measurements in ideal $^4$He crystals where there are no impurities and no dislocations. We will analyze the stiffness of single crystals in the presence of variable disorder. [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 10:36AM |
V23.00013: Heat capacity measurements of solid $^{4}$He confined in porous Vycor Z. G. Cheng, X. Lin, J. T. West, M. H. W. Chan Non-classical rotational inertia (NCRI), a possible indication of a supersolid state, has been observed in solid $^{4}$He in bulk samples and in samples confined within porous media. In addition to the expected Debye-T$^{3}$ term, a heat capacity peak has been observed in bulk solid $^{4}$He. The peak temperature of $\sim $60 mK is in the vicinity of the onset of NCRI. The heat capacity peak also shows similar dependence on sample preparation to the NCRI signals. Many of the theories regarding supersolid behavior involve extended defects in the bulk crystal. These theories break down when considering solids confined within porous media. We will present preliminary heat capacity measurements of solid $^{4}$He confined in Vycor glass with a characteristic pore size of 70 angstroms. [Preview Abstract] |
Thursday, March 18, 2010 10:36AM - 10:48AM |
V23.00014: Amorphous solid helium in porous media Jacques Bossy, Thomas Hansen, Henry Glyde Neutron scattering measurements of the static structure factor, $S(Q)$, of helium confined in the porous media MCM-41 of pore diameter 47 $\pm$ 1.5 \AA are presented. Beginning in the liquid phase, as temperature is decreased at constant pressure the $S(Q)$ shows a transition from the liquid to an amorphous solid. No Bragg peaks are observed in the solid and the $S(Q)$ of the solid differs little from that of the liquid. On freezing, a small additional intensity in \sq near the main peak of the liquid $S(Q)$ (Q $\simeq$ 2.2 \AA$^{-1}$) is observed. The $S(Q)$ is compared with simulations of freezing and melting in porous media which support the interpretation of freezing to an amorphous solid. From the measurements of freezing, an approximate phase diagram is determined. A similar amorphous solid $S(Q)$ is observed in 34 \AA~gelsil. [Preview Abstract] |
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