Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V23: Novel Transport Properties of Electrons and Ions Near the Surface of the Helium LiquidsInvited
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Sponsoring Units: DCMP Chair: James Sauls, Northwestern University Room: New Orleans Theater B |
Thursday, March 16, 2017 2:30PM - 3:06PM |
V23.00001: Electron Bubbles in Chiral Superfluid $^3$He-A: Weyl Fermions and Anomalous Hall Effect Invited Speaker: Oleksii Shevtsov The A phase of $^3$He is a chiral topological superfluid that spontaneously breaks parity and time-reversal symmetries. These broken symmetries were recently revealed by the RIKEN group in transport measurements showing electrons exhibit an anomalous Hall effect in superfluid $^3$He-A. We report results based on our theory\footnote{O. Shevtsov and J. A. Sauls, Phys. Rev. B \textbf{94}, 064511 (2016).} of the anomalous Hall effect for electron transport in $^3$He-A. The theory is based on a quantum mechanical treatment of multiple scattering of Bogoliubov quasiparticles by negative ions embedded in the chiral superfluid. The key ingredient to the scattering theory is the role of broken mirror and time-reversal symmetries on the quasiparticle scattering rate. Quantum interference of quasi-particles and quasi-holes leads to the formation of a branch of sub-gap Weyl Fermions bound to the surface of the ion, a mesoscopic realization of the bulk-edge correspondence. We show that these states are responsible for the ion's angular momentum which is inherited from the chiral ground state, and that the Weyl spectrum plays a central role in the skew scattering of thermally excited Bogoliubov quasiparticles off the negative ion in $^3$He-A. Our theoretical results are shown to be in quantitative agreement with the RIKEN experiments for both the longitudinal and transverse forces acting on the ion, and our theory provides both a qualitative and quantitative explanation of the processes responsible for the anomalous Hall effect in $^3$He-A. [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:42PM |
V23.00002: Topological aspects of superfluid $^3$He investigated by ions trapped at the surface Invited Speaker: Hiroki Ikegami Topology is a key concept for understanding fundamental aspects of quantum states of matter. The $p$-wave superfluid $^3$He offers a textbook example of topological superfluids, showing a number of exotic phenomena associated with its topology and complex broken symmetry. Here we present investigations of topological aspects of the A and B phases of the superfluid $^3$He by transport of electrons (electron bubbles) trapped below the free surface. The A phase ($^3$He-A) is a chiral $p$-wave superfluid with broken time-reversal and parity symmetries. This phase is recently recognized as a Weyl superfluid with Weyl points in momentum space. In this phase, we found that electron bubbles trapped below the free surface show the anomalous Hall effect\footnote{H. Ikegami {\it et al.}, Science {\bf 341}, 59 (2013).}. The observation of the anomalous Hall effect provides a direct demonstration for broken time-reversal and parity symmetries of $^3$He-A. Furthermore, this observation could be a strong evidence for the spectrum of Weyl fermions in $^3$He-A\footnote{O. Shevtsov and J. A. Sauls, Phys. Rev. B {\bf 94}, 064511 (2016).}. The B phase ($^3$He-B) is a time-reversal invariant topological superfluid, which hosts Majorana bound states at a surface. To show the presence of the bound states, we measured mobility of electron bubbles trapped just below the free surface. The observed mobility is suppressed from that in bulk $^3$He at low temperatures\footnote{H. Ikegami {\it et al.}, Phys. Soc. Jpn. {\bf 82}, 124607 (2013).}. The recent theoretical calculation\footnote{Y. Tsutsumi, arXiv:1609.02720} shows that the suppression of mobility is caused by the scattering of surface bound states by the electron bubble, and the theoretical mobility perfectly reproduces our experimental data. This agreement provides a direct evidence of the presence of the surface bound states at the free surface of $^3$He-B. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 4:18PM |
V23.00003: Stick-Slip Motion of the Wigner Solid on the Surface of Liquid Helium Invited Speaker: David Rees We present time-resolved transport measurements of a Wigner solid (WS) trapped above the surface of superfluid $^4$He, which reveal new insights into the dynamical coupling between the electron system and excitations at the liquid surface[1]. The WS forms at high electron densities and low temperatures, as electrons trapped in surface states above the helium surface self-organize to form a triangular lattice. The static WS is ‘dressed’ by quantized capillary waves (ripplons), resulting in the formation of a shallow depression (or `dimple’) in the helium substrate beneath each electron[2]. Because the combined electron-dimple mass is much greater than the bare electron mass, the SE conductance drops dramatically when the system enters the WS phase. However, the WS can be decoupled from the dimple lattice (DL) by the application of a strong electric field parallel to the helium surface. After decoupling, the WS `slides’ freely across the helium with high velocity. Here we present time-resolved measurements of the WS-DL decoupling process. In our experiment the helium is confined in a microchannel several microns in width, and the electron density at the helium surface is controlled using an array of gate electrodes[3]. On applying a smoothly ramped driving potential, the WS initially remains coupled to the DL, and its velocity is limited to the ripplon phase velocity[4]. As the driving force builds, the WS eventually decouples from the DL and electron velocity increases dramatically. The driving force is then released, allowing the WS to re-couple with the DL, and the cycle is repeated. The consequent ‘stick-slip’ motion of the electron system results in spontaneous current oscillations, the frequency of which depends on the strength of coupling between the WS and the helium substrate. We discuss the influence of lattice defects on the WS-DL coupling, and examine the transport of the WS when it is decoupled from the DL, a regime that until now has remained largely unexplored. \\ \text{[1]} D. G. Rees, N. R. Beysengulov, J.-J. Lin and K. Kono, Phys. Rev. Lett. \textbf{116}, 206801 (2016).\\ \text{[2]} K. Shirahama and K. Kono, Phys. Rev. Lett \textbf{74}, 5 (1995).\\ \text{[3]} D. G. Rees \textit{et al}, Phys. Rev. B \textbf{94}, 045139 (2016).\\ \text{[4]} M. I. Dykman and Yuri G. Rubo, Phys. Rev. Lett \textbf{75}, 25 (1997). [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:54PM |
V23.00004: Hybrid Circuit QED with Electrons on Helium Invited Speaker: Ge Yang Electrons on helium (eHe) is a 2-dimensional system that forms naturally at the interface between superfluid helium and vacuum. It has the highest measured electron mobility [1], and long predicted spin coherence time [2]. In this talk, we will first review various quantum computer architecture proposals that take advantage of these exceptional properties [3]. In particular, we describe how electrons on helium can be combined with superconducting microwave circuits to take advantage of the recent progress in the field of circuit quantum electrodynamics (cQED). We will then demonstrate how to reliably trap electrons on these devices hours at a time, at millikelvin temperatures inside a dilution refrigerator. The coupling between the electrons and the microwave resonator exceeds 1 MHz, and can be reproduced from the design geometry using our numerical simulation [4]. Finally, we will present our progress on isolating individual electrons in such circuits, to build single-electron quantum dots with electrons on helium. [1] K. Shirahama et al., Low Temp. Phys. 101, 439 (1995). [2] S. A. Lyon, Phys. Rev. A 74, 052338 (2006). [3] D. I. Schuster et al., Phys. Rev. Lett. 105, 040503 (2010). [4] Ge Yang et al., Phys. Rev. X 6, 011031 (2016). [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:30PM |
V23.00005: Strong coupling of an electron ensemble on the surface of liquid helium to a microwave cavity Invited Speaker: Denis Konstantinov Recently there has been a significant interest in the strong coupling of an ensemble of quantum particles to the electro-magnetic modes of a resonator. Besides traditional systems used in cavity QED experiments such as Rydberg atoms, strong coupling regime has been recently studied in various electron and nuclear spin ensembles, as well as two-dimensional electron systems (2DES) in semiconductors [1,2]. The hallmark of the strong coupling regime is the splitting in the resonator spectrum revealed in the signal reflected from or transmitted through the resonator. In case of a collection of $N$ quantum particles this splitting scales as $\sqrt{N}$. Besides general interest in the fundamental problem of light-matter interaction, the particular interest in the strong coupling regime comes from the quantum information processing as strong coupling to a high-Q resonator enables coherent information transfer between, for example, a qubit and quantum system excitations. Therefore, most of the recent observations of strong coupling have been interpreted as pure quantum phenomena. However, it is rarely mentioned that the strong coupling between large $N$-particle ensemble and the coherent state of electromagnetic mode in a resonator can be described completely classically in many cases. We present experimental observation of the strong coupling between cyclotron mode of 2DES on the surface of liquid helium and a microwave cavity resonator. The splitting in the eigen spectrum of coupled motion is observed in the cavity reflection signal, as well as in the ac current of electrons detected by measuring their bolometric photoresponse [3]. A simple model based on classical mechanics and electrodynamics accounts for all experimental features including the observed splitting. The $\sqrt{N}$-scaling of the splitting follows naturally from our model. Thus, our work reproduces all main features of the strong coupling regime for a large N-particle 2DES, including those reported in Refs. [1,2], but puts them on a completely classical ground. [1] G. Scalari, Science 335, 1323 (2012). [2] Q. Zhang et al., Nature Physics 12, 1005 (2016). [3] L. V. Abdurakhimov et al., Phys. Rev. Lett. 117, 056803 (2016). [Preview Abstract] |
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