Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A39: Invited Session: Order from Disorder in Superfluid 3He in Aerogel |
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Sponsoring Units: DCMP Chair: William Halperin, Northwestern University Room: Mile High Ballroom 2A-3A |
Monday, March 3, 2014 8:00AM - 8:36AM |
A39.00001: Superfluid $^3$He in ``nematically ordered'' aerogel Invited Speaker: Vladimir Dmitriev Liquid $^3$He immersed in aerogel allows investigation of the influence of impurities on unconventional superfluidity. In most of such experiments silica aerogels are used. These aerogels consist of thin strands which form a ``wisp.'' Although it is established that superfluid phases of $^3$He in silica aerogels (A-like and B-like) have the same order parameters as A and B phases of bulk $^3$He, many new phenomena were observed. In particular, it was found that global anisotropy of aerogel (e.g. caused by squeezing or stretching) can orient the order parameter. Depending on prehistory and on the type of the anisotropy the A-like phase may be homogeneous or in a state with random orbital part of the order parameter. Theory predicts that a large stretching anisotropy may even influence the order parameter structure: polar phase (or A phase with polar distortion), which are not realized in bulk $^3$He, may become more favorable than pure A phase [1]. Large stretching anisotropy is hardly achievable in silica aerogel. Therefore in experiments described in the talk we used a new type of aerogel, consisting of Al$_2$O$_3\cdot$H$_2$O strands which are parallel to each other [2], i.e. this aerogel may be considered as infinitely stretched. We found that the superfluid phase diagram of $^3$He in such ``nematically ordered'' aerogel is different from the case of $^3$He in silica aerogel and that both observed A and B phases have large polar distortion. This distortion is larger at low pressures and grows on warming. There are indications that a pure polar phase appears near the superfluid transition temperature [3]. Recent results will be also presented.\\[4pt] [1] K. Aoyama and R. Ikeda, Phys.Rev.{\bf B}, {\bf 73}, 060504 (2006).\\[0pt] [2] R.Sh. Askhadullin, P.N. Martynov, P.A. Yudintsev et al., J. Phys.: Conf. Ser. {\bf 98}, 072012 (2008).\\[0pt] [3] R.Sh. Askhadullin, V.V. Dmitriev, D.A. Krasnikhin et al., JETP Lett., {\bf 95}, 326 (2012). [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 9:12AM |
A39.00002: Half - Quantum Vortices in the polar phase of He-3 in nematic aerogel Invited Speaker: Vladimir Mineev Unlike to superfluid He-4 the superfluid He-3A support the existence of vortices with half quantum of circulation as well as single quantum vortices. The singular single quanta vortices as well as nonsingular vortices with 2 quanta of circulation have been revealed in rotating He-3A. However, the half quantum vortices in open geometry always possess an extra energy due to spin-orbit coupling leading to formation of domain wall at distances larger than dipole length from the vortex axis. Fortunately the same magnetic dipole-dipole interaction does not prevent the existence of half-quantum vortices in the polar phase of superfluid He-3 that can be realized in peculiar porous media ``nematically ordered'' aerogel. Here we discuss this exotic possibility. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A39.00003: Engineering the glass phase of superfluid $^3$He-A with disorder Invited Speaker: J.I.A. Li It is established theoretically that an ordered state with continuous symmetry is inherently unstable to arbitrarily small amounts of disorder [1, 2]. Based on this idea it was predicted [3] that $^3$He-A in high porosity aerogel would become a superfluid glass, provided the aerogel has no global anisotropy that breaks the continuous symmetry of the system. We report here our nuclear magnetic resonance (NMR) measurements on $^3$He in an aerogel sample, characterized to be extremely homogeneous and isotropic. When the superfluid state is generated by cooling from the normal state, the long range orientational order of the intrinsic superfluid orbital angular momentum is destroyed, confirming the existence of a superfluid glass of $^3$He-A[3]. In this disordered glass state, the NMR response of the superfluid state vanishes and the order parameter structure of the superfluid is completely hidden, a behavior of potential significance for understanding exotic superconductors such as URu$_2$Si$_2$. In contrast, $^3$He-A generated by warming from superfluid $^3$He-B has perfect long-range orientational order, providing a mechanism for switching off this effect. Furthermore, by uniaxial compression of $\approx 20\%$ on the same sample we introduce uniform global anisotropy into this aerogel which breaks the 3-D continuous symmetry and restores perfect orientational order along the compression axis. However, in the plane perpendicular to the compression axis, the remaining 2-dimensional continuous symmetry gives rise to an in-plane glass state. \\[4pt] [1] Larkin, A. I. JETP Lett. 31, 784-786 (1970).\\[0pt] [2] Imry, Y. and Ma, S. Phys. Rev. Lett. 35, 1399-1401 (1975).\\[0pt] [3] Volovik, G. E. JETP Lett. 63, 301-304 (1996). [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A39.00004: Signatures of Superfluid Phases in Torsion Pendulum Experiments on He-3 Confined in Uniaxially Compressed Silica Aerogel Invited Speaker: Nikolay Zhelev Embedding superfluid He-3 into the random matrix of porous aerogel has proven to be a practical way to introduce disorder in the otherwise absolutely pure system. ``Dirty'' He-3 confined in aerogel exhibits markedly different properties compared to the bulk fluid. I present data from an experiment in which a deliberate anisotropy has been induced in the aerogel sample. A 98\% open silica aerogel, grown in a cell within the head of a torsion pendulum, is compressed by 10\% along the pendulum's axis. Through observing the pendulum's period shift and dissipation ($Q^{-1}$), we map out the modification of the superfluid phase diagram by anisotropic disorder [1]. Data for $Q^{-1}$ of the pendulum in both the superfluid phases cannot be fully explained by the existing theoretical framework, and as such should motivate new models for the interaction of the superfluid and the aerogel network [2]. \\[4pt]This experiment was done in collaboration with R.G. Bennett, E.N. Smith, J. Pollanen, W.P. Halperin and J.M. Parpia. \\[4pt][1] R. G. Bennett et al., Phys. Rev. Lett. \textbf{107}, 235504 (2011). \\[0pt][2] N. Zhelev et al., arXiv:1308.4724 [cond-mat.supr-con]. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 11:00AM |
A39.00005: Ultrasonic measurements of normal and superfluid He-3 in high porosity aerogel Invited Speaker: Yoonseok Lee Ultrasound spectroscopy and nuclear magnetic resonance have been proven to be the most valuable spectroscopic tools in the study of superfluid $^{3}$He. These experimental methods provide complementary information on the orbital and spin structure of the Cooper pairs. In particular, the rich spectrum of the order parameter collective modes, a direct consequence of the exotic broken symmetry in the superfluid phases, have been mapped out by ultrasonic spectroscopic techniques. Aerogel possesses a unique structure, whose topology is at the antipode of conventional porous media such as Vycor glass and metallic sinters. High porosity aerogel presents additional scattering channel that substantially changes the ultrasonic behavior in both normal and superfluid phase of $^{3}$He. For example, in the normal fluid the classic first to zero sound crossover is effectively prohibited due to the residual elastic scattering from aerogel. However, the hydrodynamic-Knudsen crossover arises owing to the unique structure and the widely varying inelastic mean free path in $^{3}$He. In superfluid, no signatures of the order parameter collective modes were observed but the gapless superfluidity has been clearly verified through ultrasound measurements. In this paper, we will present the experimental results obtained in the past decade using ultrasonic techniques. [Preview Abstract] |
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