Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y3: Recent Developments in Solid 4He |
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Sponsoring Units: DCMP Chair: Moses Chan, Pennsylvania State University Room: Ballroom A3 |
Friday, March 25, 2011 8:00AM - 8:36AM |
Y3.00001: Dynamics, Defects and Deformation in Solid Helium Invited Speaker: The shear modulus of solid $^4$He shows remarkable softening above 100 mK, the same temperature range in which the apparent supersolid disappears in torsional oscillator experiments. We have measured helium's shear modulus and dissipation at frequencies from 0.5 to 8500 Hz. The onset temperature for softening/stiffening is broad, frequency dependent, and is accompanied by a dissipation peak - features typical of a dynamical crossover in a disordered system rather than a true phase transition. This behavior can be qualitatively explained if dislocations are mobile at high temperatures but are pinned by $^3$He impurities below 100 mK. To better understand the role of dislocations, we have plastically deformed crystals by rapid thermal quenching and used pressure gradient measurements to study subsequent annealing. In our most recent experiments we have sheared solid helium mechanically and looked at the effect of large deformations on the helium's elastic properties. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 9:12AM |
Y3.00002: New evidence of supersolidity in rotating solid helium Invited Speaker: The irrotationality of superfluid causes it to decouple form the container, leading to a reduction in the rotational inertia. This is more technically known as non-classical rotational inertia (NCRI). Although it is intuitively most natural to associate superflow only with the liquid phase, a decrease in the resonant period of a torsional oscillator (TO) was detected in solid helium below about 200 mK and interpreted as the appearance of NCRI. However, the resonant period may be also reduced for reasons other than supersolidity, such as the temperature dependence of the elastic modulus of solid helium. Unusual increase in the shear modulus with striking resemblance to those of NCRI supports the non-superfluid explanations. We superimposed dc rotation onto oscillatory measurements to distinguish between the supersolidity and classical elastic modulus change effects. We performed such simultaneous measurements of the TO and the shear modulus, and observed substantial change in the resonant period with rotational speed where the modulus remained unchanged. This contrasting behavior suggests that the decrease in the TO period is a result of supersolidity. This work is performed by collaboration with H. Choi, D. Takahashi, and K. Kono. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:48AM |
Y3.00003: Quantum plasticity and supersolidity Invited Speaker: We have discovered that, in the total absence of impurities, helium 4 crystals are anomalously soft [1]. In our opinion, this is a consequence of the quantum properties of their dislocation lines which are able to move macroscopic distances (typically a fraction of a millimeter) at high speed (several meters per second) as a response to very small applied stresses (one microbar). Moreover, this quantum plasticity appears to be closely related to another astonishing property of quantum crystals, namely their ``supersolidity,'' that is the possible superflow of a fraction of the crystal mass through the rest which remains elastic, actually more rigid than in the normal state [2]. Very tiny traces of helium 3 impurities are sufficient to pin the dislocations below about 100 mK and destroy the quantum plasticity. By studying rotational and elastic properties of crystals with various qualities and variable helium 3 content, we are now checking that supersolidity is a consequence of matter flowing along dislocation lines but only if these dislocations are pinned by impurities. \\[4pt] [1] X. Rojas, A. Haziot, V. Bapst, H.J. Maris, and S. Balibar, Anomalous softening of helium 4 crystals, Phys. Rev. Lett. 105, 145302 (2010). \\[0pt] [2] S. Balibar, The enigma of supersolidity, Nature 464, 176 (2010). [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:24AM |
Y3.00004: Mass Flow in Solid $^4$He as Observed by Fountain Effect Measurements Invited Speaker: We have created an experimental cell in which solid helium is sandwiched between two Vycor rods which are each in turn in contact with reservoirs of superfluid $^4$He [1]. Application of a temperature difference between the two reservoirs creates a thermo-mechanical effect, which causes a flux of atoms from one reservoir to the other through the solid helium, which is off the melting curve. The flux is measured to increase with falling temperature below about 650 mK, fall precipitously near 80 mK and then rise again at lower temperatures [2]. Results of these experiments as well as the behavior of solid growth will be presented and discussed in the context of recent theoretical work. \\[4pt] [1] M. Ray and R.B. Hallock, Phys. Rev. Letters 100, 235301 (2008); Phys. Rev. B 79, 224302 (2009).\\[0pt] [2] M. Ray and R.B. Hallock, Phys. Rev. Letters 105, 145301 (2010). [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 11:00AM |
Y3.00005: Superclimb of Dislocations in Solid $^4$He Invited Speaker: Edge dislocation with superfluid core can perform {\it superclimb} -- non-conservative motion (climb) assisted by superflow along its core. Such dislocation, with Burgers vector along the C-axis, has been found in {\it ab initio} simulations of {\it hcp} solid $^4$He [1]. Uniform network of superclimbing dislocations can induce {\it isochoric compressibility} $\chi = dN/d\mu $ which is finite (in contrast to ideal solid where it vanishes) and, practically, independent of the network density. Here $N$ is total number of atoms and $\mu$ is chemical potential [1]. Such giant response has been observed by Ray and Hallock during superfluid flow events through solid He4 [2]. Study [3] of superclimbing dislocation within the model of Granato-L\"ucke string, subjected to Peierls potential and to vanishing bias by $\mu$, has found that $\chi$ exhibits wide peak in the intermediate range of temperatures (T) - above some $T_p$ determined by Peierls energy and below $T_s \sim 0.5$K above which superfluidity of the core essentially vanishes. Non-Luttinger type behavior characterized by $\chi \sim L^b$ scaling as some power $1< b \leq 2$ of dislocation length $L$ is observed in the wide peak region. Biasing superclimbing dislocation by finite $\mu$ (due to a contact with liquid $^4$He through vycor electrodes [2],[4]) can induce core roughening caused by thermally assisted tunneling of jog-antijog pairs through the barrier produced by combination of Peierls potential and the bias [5]. The threshold for this effect scales as $\mu_c\sim 1/L^a$ with some power $a\approx 1.7$. The roughening is found to be hysteretic below some temperature $T_{\rm hyst}$. At $T_{\rm hyst}< T < T_R$, with $T_R$ determining temperature of thermal roughening, $\chi$ exhibits strong and narrow resonant peak leading to a dip in the core superfluid sound velocity. This mechanism is proposed as an explanation for a strong and narrow dip observed in critical superflow rate [4]. It is found that the dip characteristics are sensitive to the bias by $\mu$ and, therefore, this can be used as a test for the proposed mechanism. It is also predicted that the dip depth at given $T$ should be periodic in $\mu$ with the period $\sim \mu_c$. \\[4pt] [1] S. G. S\"oyler, et. al., PRL {bf 103}, 175301 (2009).\\[0pt] [2] M. W. Ray and R. B. Hallock, PRL {\bf 100}, 235301 (2008) ; PRB {\bf 79}, 224302 (2009); PRB {\bf 81}, 214523 (2010); Phys. Rev. {\bf B82}, 012502 (2010);\\[0pt] [3] D. Aleinikava, et al., JLTP, to be published;\\[0pt] [4] M. W. Ray and R. B. Hallock , Phys. Rev. Lett. {\bf 105}, 145301 (2010); \\[0pt] [5] D. Aleinikava and A.B. Kuklov, unpublished. [Preview Abstract] |
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