Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T1: Invited Session: Superfluids under Nanoscale Confinement |
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Sponsoring Units: DCMP Chair: Jeevak Parpia, Cornell University Room: Ballroom I |
Thursday, March 21, 2013 8:00AM - 8:36AM |
T1.00001: Topological superfluids confined in a nanoscale slab geometry Invited Speaker: John Saunders Nanofluidic samples of superfluid $^3$He provide a route to explore odd-parity topological superfluids and their surface, edge and defect-bound excitations under well controlled conditions. We have cooled superfluid $^3$He confined in a precisely defined nano-fabricated cavity to well below 1 mK for the first time. We fingerprint the order parameter by nuclear magnetic resonance, exploiting a SQUID NMR spectrometer of exquisite sensitivity. We demonstrate that dimensional confinement, at length scales comparable to the superfluid Cooper-pair diameter, has a profound influence on the superfluid order of $^3$He. The chiral A-phase is stabilized at low pressures, in a cavity of height 650 nm. At higher pressures we observe $^3$He-B with a surface induced planar distortion. $^3$He-B is a time-reversal invariant topological superfluid, supporting gapless Majorana surface states. In the presence of the small symmetry breaking NMR static magnetic field we observe two possible B-phase states of the order parameter manifold, which can coexist as domains. Non-linear NMR on these states enables a measurement of the surface induced planar distortion, which determines the spectral weight of the surface excitations. The expected structure of the domain walls is such that, at the cavity surface, the line separating the two domains is predicted to host fermion zero modes, protected by symmetry and topology. Increasing confinement should stabilize new p-wave superfluid states of matter, such as the quasi-2D gapped A phase, which breaks time reversal symmetry, has a protected chiral edge mode, and may host half-quantum vortices with a Majorana zero-mode at the core. We discuss experimental progress toward this phase, through measurements on a 100 nm cavity. On the other hand, a cavity height of 1000 nm may stabilize a novel ``striped'' superfluid with spatially modulated order parameter.\\[4pt] In collaboration with L.V. Levitin, R.G. Bennett, A.J. Casey, B. Cowan, J. Parpia, E.V. Surovtsev [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 9:12AM |
T1.00002: Probing Chirality in Superfluid $^3$He-A: Free surface as an ideal boundary condition Invited Speaker: Kimitoshi Kono Superfluid $^3$He is known as a typical topological superfluid. A recent theoretical investigation suggests Majorana surface states at the free surface of superfluid $^3$He-B phase [1]. On the other hand, superfluid $^3$He-A is known as a chiral superfluid. The scattering of quasiparticle from small object is predicted to be skew with respect to an anisotropy axis [2]. We have developed an experimental technique to study transport properties of ions under the free surface of superfluid $^3$He [3]. By using this technique, we can investigate interaction between elementary excitations in superfluid $^3$He and small objects under well-controlled conditions. For example, in $^3$He-B interaction with Majorana surface states, although no interaction is expected, will be investigated, whereas in $^3$He-A skew scattering of quasiparticle from electron bubbles will be probed. In this paper, we present the recent results of transport properties of electron bubbles trapped below the free surface of superfluid $^3$He. In particular, experimental evidences of the skew scattering and chirality of superfluid $^3$He-A will be presented. The skew scattering of quasiparticle in $^3$He-A from electron bubble results in a bubble transport analogous to the Hall effect, where the anisotropy vector of $^3$He-A behaves as if it was a magnetic field in the Hall effect. Under experimental conditions, the effect is observed as an analogue of edge magnetoplasmon effect. After the analysis of data, we obtained a reasonable qualitative agreement with the theory [2].\\[4pt] [1] S. B. Chung and S.-C. Zhang: Phys. Rev. Lett. 103, 235301 (2009).\\[0pt] [2] R. H. Salmelin, M. M. Salomaa, and V. P. Mineev: Phys. Rev. Lett. 63, 868-871 (1989).\\[0pt] [3] T. Shiino, H. Mukuda, K. Kono, W. F. Vinen: J. Low Temp. Phys. 126, 493-498 (2002). [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:48AM |
T1.00003: Surface Majorana cone of the topological superfluid $^3$He B phase Invited Speaker: Ryuji Nomura Topological superfluids and superconductors are characterized by a non-trivial topological number in the gapped bulk state and gapless edge states on their surfaces. The surface states are proposed to be Majorana fermions as they satisfy the Majorana condition, i.e., a particle and its antiparticle are equivalent, and their linear dispersion is called Majorana cone. It is an urgent issue in condensed matter physics to confirm the realization of the topological matters in nature and their bulk-edge correspondence. Superfluid $^3$He is a suitable system to reach a definite conclusion since the spin-triplet p-wave symmetry is well established in the bulk state. We measured transverse acoustic impedance of the superfluid $^3$He B phase changing the boundary condition of a wall from a diffusive scattering up to practically specular limit by coating the wall with thin layers of superfluid $^4$He. A growth of low-energy peak in the transverse acoustic impedance was observed at higher specularities, which is the clear evidence of low-lying quasiparticle states in the vicinity of the wall. A self-consistent theoretical calculation reproduces the experimental results well and shows that the observed growth of the peak is the reflection of the linear dispersion of the surface Andreev bound states. Thus, we experimentally confirmed Majorana fermions on the surface of the superfluid $^3$He B phase and showed that the superfluid $^3$He B phase is truly a topological superfluid with the bulk-edge correspondence. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:24AM |
T1.00004: Symmetry Protected Topological Order in Superfluid $^3$He-B Invited Speaker: Takeshi Mizushima The superfluid $^{3}$He-B has been recognized as a concrete example of topological superconductors, where the time-reversal symmetry ensures a nontrivial topological number and the existence of helical Majorana fermions. This may indicate that any time-reversal breaking disturbance wipe out the topological nature. In this talk, I will demonstrate that the B phase under a magnetic field in a particular direction stays topological due to a discrete symmetry, that is, in a symmetry protected topological order [1]. Due to the symmetry protected topological order, helical surface Majorana fermions in the B phase remain gapless and their Ising spin character persists. I unveil that the competition between the Zeeman magnetic field and dipole interaction involves an anomalous quantum phase transition where a topological phase transition takes place together with spontaneous breaking of symmetry. Based on the quasiclassical theory, I illustrate that the phase transition is accompanied by anisotropic quantum criticality of spin susceptibilities on the surface, which is detectable in NMR experiments~[1,2].\\[4pt] [1] T. Mizushima, M. Sato, and K. Machida, Phys. Rev. Lett. \textbf{109}, 165301 (2012).\\[0pt] [2] T. Mizushima, Phys. Rev. B \textbf{86}, 094518 (2012). [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 11:00AM |
T1.00005: Critical Point Coupling and Proximity Effects in He-4 at the Superfluid Transition Invited Speaker: Francis Gasparini We report measurements of specific heat and superfluid density for $^{4}$He confined in an array of (2$\mu$m)$^{3}$ boxes at 2$\mu$m separation and linked through a 33 nm film [1]. We find a strong enhancement of the specific heat and the superfluid density relative to control measurements where the boxes are placed farther apart [2]; and, measurements of the film itself in the absence of the boxes. We demonstrate that this coupling is due to the finite-size correlation length associated with the helium in the boxes. The surprising result, however, is that this coupling extends over distances 30-50 times the correlation length. This cannot be understood on the basis of the meaning of the correlation length as the distance over which order propagates in a critical system. These observations have implications in the understanding of experiments with helium confined in heterogeneous media, and, more generally, to other coupled critical systems where competing order is present.\\[4pt] [1] J. K. Perron, and F. M. Gasparini, Phys. Rev. Lett. \textbf{109}, 035302 (2012)\\[0pt] [2] J. K. Perron, M. O. Kimball, K. P. Mooney, and F. M. Gasparini, Nature Phys. \textbf{6}, 499 (2010) [Preview Abstract] |
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