Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T47: Solid He4 and Other Quantum Solids |
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Sponsoring Units: DCMP Chair: Moses Chan, Pennsylvania State University Room: Mile High Ballroom 4F |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T47.00001: The properties of dislocations in $^4$He crystals Sebastien Balibar, Beamish John, Andrew Fefferman, Ariel Haziot, Fabien Souris We have measured (1,2) the response of oriented $^4$He crystals to an ac-driving strain as a function of temperature, strain amplitude, frequency, and $^3$He content. The very large softening of these crystals around 0.2~K is due to the free motion of dislocations parallel to the basal planes in the absence of dissipation from collisions with thermal phonons or from the binding of $^3$He impurities. We have built a complete model for the mechanical properties of $^4$He crystals, which is in full quantitative agreement with all experimental results so that most of the properties of these dislocations are now well established. These properties are incompatible with the two scenarios that had been proposed for supersolidity in $^4$He. Dislocations have a density between 10$^4$ and 10$^6$ cm$^{-2}$. They move like free strings down to 20~mK, meaning that the kink energy is negligible. They have a large distribution in length and a small connectedness. $^3$He impurities bind to dislocations with an energy distributed around 0.67~K and move with them below 45~$\mu$m/s.\\ 1- A. Haziot et al., Phys. Rev. Lett. 110, 035301 (2013), Phys. Rev. B 87, 060509(R) (2013), and Phys. Rev. B 88, 014106 (2013).\\ 2- A. D. Fefferman et al., submitted to Phys. Rev. B, Nov. 2013.\\ [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T47.00002: Critical dislocation speed in $^4$He crystals John Beamish, Ariel Haziot, Andrew Fefferman, Fabien Souris, Sebastien Balibar, Humphrey Maris Our experiments show that in $^4$He crystals, the binding of $^3$He impurities to dislocations does not necessarily imply their pinning. In these crystals, there are two different regimes in the motion of dislocations with impurities bound to them. At low driving strain $\varepsilon$ and frequency $\omega$, where the dislocation speed is less than a critical value (45 $\mu$m/s), dislocations and impurities apparently move together. Impurities really pin the dislocations only at higher values of $\varepsilon \omega$. The critical speed separating the two regimes is two orders of magnitude smaller than the speed of free $^3$He impurities in the bulk crystal lattice. We obtained this result by studying the dissipation of dislocation motion as a function of the frequency and amplitude of a driving strain applied to a crystal at low temperature. Our results resolve an apparent contradiction between experiments that showed a frequency-dependent transition temperature from a soft to a stiff state, and other experiments or models where this temperature was assumed to be independent of frequency. The impurity pinning mechanism for dislocations appears to be more complicated than previously assumed. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T47.00003: Dislocation networks in helium-4 crystals Andrew Fefferman, Fabien Souris, Ariel Haziot, John Beamish, Sebastien Balibar The mechanical behavior of crystals is dominated by dislocation networks, their structure and their interactions with impurities or thermal phonons. However, in classical crystals, networks are usually random with impurities often forming non-equilibrium clusters when their motion freezes at low temperature. Helium provides unique advantages for the study of dislocations: crystals are free of all but isotopic impurities, the concentration of these can be reduced to the ppb level, and the impurities are mobile at all temperatures and therefore remain in equilibrium with the dislocations. We have achieved a comprehensive study of the mechanical response of $^{4}$He crystals to a driving strain as a function of temperature, frequency and strain amplitude. The quality of our fits to the complete set of data strongly supports our assumption of string-like vibrating dislocations. It leads to a precise determination of the distribution of dislocation network lengths and to detailed information about the interaction between dislocations and both thermal phonons and $^{3}$He impurities. The width of the dissipation peak associated with impurity binding is larger than predicted by a simple Debye model, and much of this broadening is due to the distribution of network lengths. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T47.00004: Search for dislocation free $^4$He crystals Fabien Souris, Andrew Fefferman, Ariel Haziot, John Beamish, Sebastien Balibar The elastic anomaly of $^4$He crystals is known to be a consequence of the motion of their dislocations. We have built an acoustic cell in order to grow and study crystals with the smallest possible density of dislocations. It has a polished inner surface to avoid pinning sites for the liquid-solid interface. Piezoelectric transducers are placed outside the cell volume, in order to drive and detect acoustical resonances through built-in copper membranes.\\ We expect dislocation free crystals to behave rather differently from the usual ones (1,2). For example, they should not show any anomalous softening. Preliminary results show that crystals grown in this particular cell have longer dislocation lengths than in those studied in previous experiments (1,2). Centimeter long dislocations should resonate below $20$~kHz.\\\\ 1- A. Haziot \emph{et al.}, Phys. Rev. Lett. 110, 035301 (2013), Phys. Rev. B 87, 060509(R) (2013), and Phys. Rev. B 88, 014106 (2013).\\ 2- A. D. Fefferman \emph{et al.}, submitted to Phys. Rev. B, Nov. 2013.\\ [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T47.00005: Is Supersolid still out there? Xiao Mi, Anna Eyal, John D. Reppy After almost a decade of experiments attempting to display a superfluid-like behavior in solid $^4$He, it now seems that this ``super-solid'' state may not exist. Although, the results of some of the experiments reporting a supersolid behavior can be interpreted by other means, there exist others for which a plausible alternative explanation is lacking. Currently we are performing experiments employing a double torsional oscillator (TO), which can discriminate between two scenarios - that of signals arising from the acceleration of the sample involving elastic effects, which depend on the square of the frequency, and that of a supersolid condensate indicated by a frequency-independent term. We see a small frequency-independent term for our bulk samples contained in both cylindrical and annular geometries. This term represents a fraction of about 10$^{-4}$ of the total moment of inertia of the solid sample. The observation of such small signals requires high stability for the TO and in our most recent measurements, we have been able to improve the stability and signal to noise ratio by an order of magnitude over our previous works. The small remaining frequency-independent signals, we observe, are inexplicable by elastic effects alone and may be indicative of a true supersolid. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T47.00006: Ultrasound propagation in polycrystalline solid $^4$He Harry Kojima, Izumi Iwasa, John Goodkind We are carrying out measurements of 10 MHz longitudinal ultrasound propagation in polycrystalline solid $^4$He samples grown by the blocked capillary method. Temperature dependence of the velocity and attenuation of ultrasound are measured. The observed temperature dependence during cooling runs at $T >$ 200 mK can be described qualitatively in terms of the effects of the motion of dislocation lines present in the samples. At $T <$ 100 mK significant deviations from the higher temperature behavior are observed. Sharp anomalous changes in the velocity and attenuation appear near 70 mK. At the end of a cooling run at 20 mK, if the ultrasound excitation pulse amplitude is decreased below a threshold level, the attenuation decreases to a minimum and it remains constant at the minimum as the pulse amplitude is increased back up. The anomalous temperature dependence and the hysteretic behavior are discussed as possible consequences of $^3$He impurity atoms being ``condensed" onto dislocation lines. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T47.00007: Torsional Oscillator Studies on Solid Helium Duk Y. Kim, Moses H.W. Chan In 2004, the series of torsional oscillator (TO) experiments by Kim and Chan initiated considerable research activities on the supersolidity of helium. However, recent experiments in rigid torsional oscillators which reduce the effect of stiffening of bulk solid helium at low temepratures showed very small or negligible changes in the resonant period. A new TO experiment of solid helium confined in porous Vycor glass with no bulk solid helium in the sample cell show no evidence of supersolidity [1]. Moreover, we have repeated an earlier experiment [2] on hcp $^3$He solid, which shows similar low temperature stiffening like hcp $^4$He. We found that the small drop of the resonant period measured in the hcp $^3$He samples is comparable to that measured in the hcp $^4$He samples. These results confirm that the resonant period drops in torsional oscillators are consequence of the shear modulus stiffening effect in solid helium. Remaining issues and open questions on the supersolidity will be discussed. \\[4pt] [1] D. Y. Kim and M. H. W. Chan, Phys. Rev. Lett. 109, 155301 (2012)\\[0pt] [2] J. T.West, O. Syshchenko, J. Beamish, and M. H.W. Chan, Nature Phys. 5, 598 (2009) [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T47.00008: Hysteretic behavior in torsional oscillator experiments {\&} de Gennes, Bean and Livingston effect in hcp $^{4}$He Minoru Kubota, Hiroaki Ueda, Masahiko Yagi, Nobutaka Shimizu, Kris. Rogacki, Kazuo Inoue Recent reports on the absence of supersolid signal in $^{4}$He in Vycor as well as reports on effects of the sample elasticity to torsional oscillator (TO) experiments caused people to ask if supersolid may not exist. There are recent activities to check such questions more quantitatively. We revisit our TO study, which was performed on relatively small number of bulk hcp $^{4}$He samples, but under quite different conditions as under DC rotation as well as under wide range of AC excitation V$_{ac}$ with extremely high stability. We proposed a transition at $T_{c} =$75($\sim$ 60) mK well below the onset temperature of the anomaly around 500 mK in the same sample. The transition at $T_{c}$ was detected by three independent methods. Namely, the hysteresis appears below this $T_{c}$ when AC excitation was changed under a certain sequence. We analyzed the maximum period shift across the hysteretic loop as a function of $V_{ac}$. This quantity appears abruptly below $T_{c}$ and surprisingly its $T$ dependence coincides with that of the extra energy dissipation rotational velocity Omega linear slope under DC rotation, also below $T_{c}$. We discuss that the maximum is caused by de Gennes, Bean and Livingston effect, which is a quantized vortices effect known for superconductors. The third $T_{c}$ detection is given by a jump in the log $V_{ac}$ linear dependence of the period shift. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T47.00009: Dynamic Structure Factor in BCC Helium from Quantum Monte Carlo Daniel Arovas, Snir Gazit, Daniel Podolsky, Assa Auerbach, Heloise Nonne An unexpected optic-like mode has been observed by inelastic neutron scattering in BCC Helium-4. We report on worm algorithm quantum Monte Carlo calculations of the dynamic structure factor in order to compare with experiment. A theoretical model based on a dynamical Landau-Ginzburg action is also analyzed. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T47.00010: A Phonon Gap in Solid Helium Hans Jochen Lauter, John Goodkind, Kenneth Herwig, Eckhard Krotscheck, Efim Kats, Andrey Podlesnyak, Andreii Savicii, Diallo Souleymane, Justin Carmichael Using inelastic neutron scattering, we have found an energy gap of about 0.15 meV in a phonon-like spectrum of solid 4He at temperatures below about 0.5 K and pressures near 30 bar. The solid He sample was formed in a stressed, non-equilibrium state, using rapid cooling with the blocked-capillary method. We interpret the gap as evidence for excitations along the edge dislocations according to the Frenkel-Kontorova [1] model. The energy of the excitations is a linear function of q above the gap and can be related to the stress distribution around the dislocation line. Other interpretations are possible e.g. the creation of kinks on dislocation [2]. The energy of the gap is close to the value of a thermal activation energy measured by ultrasonic attenuation in unstrained solid 4He [3] crystals. If the two are measuring the same excitations, they constrain possible models for the cause. The gap may also be related to dislocations in quantum crystals [4]. \\[4pt] [1] T. Kontorova, Y.I. Frenkel, Zh. Eksp. Teor. Fiz. {\bf 8}, 1349 (1938) \\[0pt] [2] I. Iwasa, H. Suzuki, J. Phys. Soc. Jpn. {\bf 49}, 1722 (1980)\\[0pt] [3] G.A. Lengua, J.M. Goodkind, J. Low Temp. Phys. {\bf 79}, 251 (1990)\\[0pt] [4] D. Aleinikava, E. Dedits, A. B. Kuklov, D. Schmelzer, EPL {\bf 89}, 46002 (2010) [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T47.00011: Effect of $^3$He on the extinction of mass flux in solid helium Ye. Vekhov, Robert Hallock The flux, $F$, carried by solid $^4$He , with nominal 300 ppb $^3$He concentration, $\chi$, in the range 25.6 - 26.3 bar rises with falling temperature and at a temperature $T_d$ the flux decreases toward zero [1]. The behavior of the flux above $T_d$ demonstrates the presence of a bosonic Luttinger liquid [2]. We study $F$ as a function of $^3$He concentration $\chi$ to explore the effect of $^3$He on $T_d$. We find that the extinction of the flux is a sharp transition, typically complete within a few mK change in temperature. We find that $T_d$ is an increasing function of $\chi$ and we compare ($T_d,\chi$) with predictions for homogeneous phase separation. We conclude that phase separation plays an important role in the flux extinction. It is possible that the cores of edge dislocations carry the flux, and the flux is extinguished by the decoration by $^3$He of the cores or dislocation intersections. \\[4pt] [1] M.Ray and R.B. Hallock, PRL 105, 145301 (2010); PRB 84, 144512 (2011).\\[0pt] [2] Ye. Vekhov and R.B. Hallock, PRL 109, 045303 (2012). [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T47.00012: Universal temperature dependence of the mass flux in solid helium Robert Hallock, Ye. Vekhov The flux, $F$, carried by solid $^4$He, with nominal 300 ppb $^3$He concentration, $\chi$, in the range 25.6 - 26.3 bar rises with falling temperature and at a temperature $T_d$ the flux decreases toward zero [1]. The behavior of the flux above $T_d$ demonstrates the presence of a bosonic Luttinger liquid [2]. We study $F$ as a function of $^3$He concentration $\chi$ for $T > $ $T_d$ to explore the effect of $^3$He on the temperature dependence of $F$. We find that $F$ is sample-dependent and that the temperature dependence of $F$ above $T_d$ is universal; data for all samples scales to collapse on a universal curve. The universal behavior extrapolates to zero flux in the vicinity of $T_h$ $\approx$ 610 mK. With increases in temperature, an activated process degrades the flux. One possibility is the presence of kinks on dislocation cores, which would introduce disorder and introduce phase slips. \\[4pt] [1] M.Ray and R.B. Hallock, PRL 105, 145301 (2010); PRB 84, 144512 (2011).\\[0pt] [2] Ye. Vekhov and R.B. Hallock, PRL 109, 045303 (2012). [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T47.00013: Commensurate-incommensurate solid transition in the $^{4}$He monolayer on a single $\gamma $-graphyne sheet Yongkyung Kwon, Jeonghwan Ahn We have performed path-integral Monte Carlo calculations to study $^{4}$He adsorption on $\gamma $-graphyne. Assuming the $^{4}$He-substrate interaction described by a pairwise sum of empirical helium-carbon interatomic potentials, we find that unlike $\alpha $-graphyne [1], a single sheet of $\gamma $-graphyne is not permeable to $^{4}$He atoms despite its large surface area. One-dimensional density distribution shows layer-by-layer growth of $^{4}$He on $\gamma $-graphyne. Partially-filled $^{4}$He monolayers are found to exhibit different commensurate structures depending on the helium coverage; it shows a C$_{2/2}$ commensurate structure at the areal density of 0.0491 {\AA}$^{-2}$, a C$_{3/2}$ structure at 0.0736 {\AA}$^{-2}$, and a C$_{4/2}$ structure at 0.0982 {\AA}$^{-2}$. After going through various domain structures, the $^{4}$He monolayer is completed at the areal density of 0.115 {\AA}$^{-2}$ where $^{4}$He adatoms form an incommensurate triangular solid. Possible superfluid response of the $^{4}$He monolayer on $\gamma $-graphyne is now under investigation. \\[4pt] [1] Y. Kwon, H. Shin, and H. Lee, Phys. Rev. B \textbf{88}, 201403(R) (2013). [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T47.00014: Nature of Defects and Their Energies in the Lowest Landau Crystal States Alex Archer, Jainendra Jain The observed activation energies in the insulating phase in the lowest Landau level, believed to be a crystal, are as much as one order of magnitude smaller than interstitial defect energies in a Hartree-Fock crystal. Additionally, the melting temperature is significantly lower than expected. We have modeled the lowest Landau level insulating phase as a series of composite fermion crystals and evaluated their phase diagram [1]. We now investigate several different types of defects of the composite fermion crystals, including interstitials, vacancies, dislocations, grain boundaries, and also inherently quantum defects with no classical analog. We show that significantly lower energy defects appear in the composite fermion crystals, thus bringing theory into closer agreement with experiments. [1] A. C. Archer, K. Park and J. K. Jain, PRL {\bf 111}, 146804 (2013). [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T47.00015: ABSTRACT WITHDRAWN |
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