Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V05: Topological Superconductivity: General I |
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Sponsoring Units: DMP Chair: Liang Fu, Massachusetts Institute of Technology Room: BCEC 108 |
Thursday, March 7, 2019 2:30PM - 2:42PM |
V05.00001: Gauge-Invariant Variables Reveal the Quantum Geometry of Fractional Quantum Hall States Rudro Biswas, YingKang Chen We introduce the framework of gauge invariant variables to describe fractional quantum Hall (FQH) states, and prove that the wavefunction can always be represented by a unique holomorphic multi-variable complex function. We use this representation to combine the quantum geometry of charged particles in a magnetic field with the theory of coherent states and provide an analytical route, hitherto elusive, for deriving the properties of fractional quantum Hall phases from experimentally relevant microscopic Hamiltonians. |
Thursday, March 7, 2019 2:42PM - 2:54PM |
V05.00002: Connecting the responses of quantum Hall states to gravitational and electric fields YingKang Chen, Guodong Jiang, Rudro Biswas We introduce the framework of gauge invariant variables to describe quantum Hall states This representation exploits novel aspects of the quantum geometry of charged particles in a magnetic field. We use this representation to reveal the geometric response of cyclotron orbits to non-uniform electric fields. We visualize the known connection between the gravitational response of quantum Hall states and their current response to nonuniform electric fields. We also conjecture that gravitational response is connected to the charge response of quantum Hall states to non-uniform electric fields. |
Thursday, March 7, 2019 2:54PM - 3:06PM |
V05.00003: Dephasing dynamics of noisy Majorana-based qubits: Topological versus Andreev Ryan Mishmash, Bela Bauer, Felix von Oppen, Jason Alicea Topological quantum computation schemes encode quantum information nonlocally through non-Abelian anyons separated by macroscopic distances $L$, typically spanning the length of the constituent qubit device. This nonlocality renders topological qubits exponentially immune to dephasing from \emph{all} sources of classical noise with operator support local on the scale of $L$. We explore detailed theoretical and numerical analyses of a time-domain Ramsey-type protocol for noisy Majorana-based qubits which is designed to validate this coveted topological protection in near-term nanowire devices. By assessing dependence on wire length $L$, our proposed protocol can sharply distinguish a bona fide Majorana qubit from one constructed from Andreev bound states, which can otherwise closely mimic the true Majorana scenario in local probes; for a fixed wire length, the protocol can also inform which scenario is likely realized. This proposed experiment requires no pulsing and only (relatively slow) measurement of two nearby Majorana modes for both initialization and readout---achievable, for example, by tunnel coupling to a nearby quantum dot---and thus serves as an enticing pre-braiding experiment aimed at quantifying the utility of Majorana-based qubits. |
Thursday, March 7, 2019 3:06PM - 3:18PM |
V05.00004: Vortices in a Monopole Superconducting Weyl Semimetal Canon Sun, Shu-Ping Lee, Yi Li A monopole superconductor is a novel topological phase of matter with topologically protected gap nodes as a result of Cooper pairs acquiring a non-zero Berry phase. We study the vortex structure of a Weyl semimetal with broken time-reversal symmetry in the monopole superconducting phase. Within each single isolated vortex, there exists a zero-energy Majorana bound state. The spin up and down components of the Majorana zero mode exhibit a non-trivial winding as a consequence of spin-orbit coupling. |
Thursday, March 7, 2019 3:18PM - 3:30PM |
V05.00005: Phases and phase transitions in Kitaev ladders: symmetries and finite size scaling Ke Wang, Tigran Sedrakyan We discuss phases and phase transitions in Kitaev ladders with and without interactions. In the noninteracting system, the phase diagram of a two-leg Kitaev ladder with time-reversal (TR) symmetry contains three distinct phases possessing 0, 2, and 4 Majorana edge modes and one tricritical line separating these phases. We discuss the finite-size corrections to energy along the tricritical line and finite-size scaling away from it which exhibits a non-trivial universal shape. Upon breaking the protecting TR symmetry, we find that the phase described by four Majorana edge modes in TR protected state crosses over to a trivial phase with no edge modes via symmetry broken path G in parameter space. The energy gap remains finite along G indicating that Z-classification (class BDI )reduces to Z_2 (class D) classification upon breaking of TR. Remarkably, the finite size scaling function in the vicinity of a critical line separating trivial phase from the nontrivial one in TR broken phase, exhibits a new nontrivial behavior. We also show that there is a phase transition of Ising universality in an interacting Kitaev chain and discuss its implications in ladder models. |
Thursday, March 7, 2019 3:30PM - 3:42PM |
V05.00006: Spin-Orbit Coupling Effects on the Current-Phase Relation of a DC SQUID Alex Matos Abiague, Narayan Mohanta, William Andrew Mayer, SiChao Yu, Kaushini Wickramasinghe, Joseph Yuan, Javad Shabani, Igor Zutic We consider a dc superconducting quantum interference device (SQUID) composed of two Josephson junctions (JJs) in a loop threaded by a magnetic flux. The JJs are built on a heterostructure where a semiconducting two-dimensional electron gas (2DEG) is partially covered by a conventional superconductor. Due to proximity effect, superconductivity is induced in the covered 2DEG, while the uncovered region remains in the normal state. Top gates allow for tuning both the carrier density and Rashba spin-orbit coupling (RSOC) strength in the normal region of each JJ, individually. We theoretically investigate the effects of self-inductance, charge density, and RSOC as well as their distinctive signatures on the corrent-phase relation (CPR) of the device. The sizable effects of RSOC tuning on the CPR make the considered SQUID a promising device not only for the detection of RSOC fields in proximitized materials but also for studying tunable topological transitions and the potential formation of Majorana bound states upon the application of a Zeeman field. The theoretical results are in good agreement with recent experimental measurements of the CPR in SQUIDs composed of InAs/Al JJs. |
Thursday, March 7, 2019 3:42PM - 3:54PM |
V05.00007: Spontaneous Edge Current in Higher Chirality Superconductors Xin Wang, Zhiqiang Wang, Catherine Kallin The effects of finite temperature, Meissner screening and surface roughness on the spontaneous edge current for higher chirality quasi-two dimensional superconductors are studied in the continuum limit using the quasiclassical Eilenberger equations. We find that the total spontaneous current is non-zero at finite temperature T and maximized near T=T_{c}/2, where T_{c} is the transition temperature, although it vanishes at T=0. In the presence of surface roughness, we observe a surface current inversion in the chiral d-wave case that can be understood in terms of a disorder induced s-wave pairing component in the rough surface regime. This conclusion is supported by a Ginzburg-Landau analysis. However, this current inversion is non-universal beyond the continuum limit as demonstrated by self-consistent lattice Bogoliubov-de Gennes calculations. |
Thursday, March 7, 2019 3:54PM - 4:06PM |
V05.00008: Unconventional Josephson Effect in a topological Kondo insulator Xuecheng Ye, Jacob Cook, Erik Huemiller, Aaron D Finck, Pouyan Ghaemi Mohammadi, Thomas Vojta, Shanta Saha, Johnpierre Paglione, Cihan Kurter Proximity-induced superconductivity in three dimensional (3D) topological insulators forms a new quantum phase of matter and harbors exotic quasiparticles such as Majorana bound states. One of the biggest drawbacks of the commonly studied 3D topological insulators is having conducting bulk that obscures the role of surface states. Introducing superconductivity in topological Kondo insulators such as SmB_{6} is particularly promising due to their robust insulating bulk at low temperatures. In this work, we develop an unconventional Josephson junction by coupling superconducting Nb leads to the surface states of a SmB_{6} crystal. We observe a distinct critical current at low temperatures that exhibits a Fraunhofer diffraction pattern with the application of an out-of-plane magnetic field. The appearance of Shaphiro steps under microwave irradiation gives further evidence of a Josephson Effect. The Fraunhofer patterns show an anomalous hysteresis with magnetic field sweep direction suggesting coexistence of ferromagnetism with superconductivity in the topological surface states of SmB_{6}. The experimental work will advance the current understanding of topologically nontrivial superconductors and emergent states associated with such unconventional superconducting phases. |
Thursday, March 7, 2019 4:06PM - 4:18PM |
V05.00009: Simulating the spectral response of a Majorana-transmon device Anna Keselman, Bernard Van Heck, Bela Bauer We perform a numerical study of a toy-model for the Majorana-transmon device, obtaining its spectral response across the topological phase transition. To this end, we employ the DMRG and TEBD techniques. We model the superconducting wires on either end of the Josephson junction as spinful fermions hopping on a 1D lattice with Rashba spin-orbit coupling in a Zeeman field perpendicular to the spin-orbit direction. As the Zeeman field is increased and the superconducting wires are driven across the topological phase transition, we observe the splitting of the plasma mode due to the zero-energy Majorana modes forming at the ends of the wires. Interestingly, the spectral function is largely unaffected by the gap closing in the bulk of the superconducting wires. When the length of the junction is finite, additional Andreev bound states can form. We show that the appearance of these states gives rise to additional spectral lines, obscuring the signatures of the topological phase. |
Thursday, March 7, 2019 4:18PM - 4:30PM |
V05.00010: Prediction of Hall effects from geometrical properties of the Fermi surface Elena Derunova, Mazhar Ali, Yan Sun, Stuart S Parkin Recently, the intrinsic Hall effects (e. g. spin, anomalous, and planar Hall effects) have been investigated as topological properties stemming from a material's electronic band structure. In our work we show the connection of these properties to the local geometry of the Fermi surface. Specifically we introduce the concept of geodesic flow of the Fermi surface and using this concept we link the Berry curvature term in the semiclassical equation of motion of Bloch electrons with the evolution of the Riemannian metric in the tangent bundle of the Fermi surface. As an example, we consider the comparison of Kubo formalism for the spin Hall effect with the geometrical analysis of the Fermi surface for Pt and Beta-W. Such geometrisation of topological properties can be applied to an algorithmic material search in crystallographic databases, paving the way for high throughput analysis of materials and topologically driven properties. |
Thursday, March 7, 2019 4:30PM - 4:42PM |
V05.00011: Parafermions in the Fractional Quantum Hall Spin Transitions Jingcheng Liang, George E Simion, Leonid Rokhinson, Yuli Lyanda-Geller Parafermion zero modes are promising for universal topological quantum computation because of their richer non-Abelian braiding properties. However, physical systems that are predicted to host these exotic excitations are rare and difficult to realize in experiments. In this work, we show that parafermion zero modes can emerge in the spin transitions in the fractional quantum Hall regime. Exact diagonalization of the Hamiltonian in a disk and torus geometries demonstrates formation of counter-propagating edge states with different spin polarizations at a boundary between polarized and unpolarized ν=2/3 phases. By analytical and numerical methods we find conditions for parafermion zero modes to emerge when these edge states are coupled to an s-wave superconductor. The phase diagram shows that the parafermionic phase, which is represented by the six-fold ground state degeneracy, is separated from gapped phases by a topological phase transition. Parafermion modes in fractional quantum Hall systems coupled to s-wave superconductors are experimentally feasible. |
Thursday, March 7, 2019 4:42PM - 4:54PM |
V05.00012: Blurring the boundaries between topological and non-topological phenomena in dots Denis Candido, Michael Flatté, Carlos Egues In this work we investigate the electronic and transport properties of topological and non-topological InAs_{0.85}Bi_{0.15} quantum dots (QDs) described by a Bernevig-Hughes-Zhang (BHZ) model with cylindrical confinement, i.e., "BHZ dots''. We analytically show that {\it non-topological} dots have discrete helical edge states, i.e., Kramers pairs with spin-angular-momentum locking similar to topological dots. These unusual and unexpectedly non-topological edge states are geometrically protected due to confinement in a wide range of parameters and are not guaranteed to exist by the bulk-edge correspondence. In addition, for a conduction window with four edge states, we find that the two-terminal conductance G vs. the QD radius R and the gate V_{g} controlling its levels shows a double peak at 2e^{2}/h for both topological and trivial BHZ QDs. Our results blur the boundaries between topological and non-topological phenomena for conductance measurements in small systems such as QDs thus showing an equivalence between the BHZ QDs in different topological phases. |
Thursday, March 7, 2019 4:54PM - 5:06PM |
V05.00013: Topological Superconductivity in a Phase-Controlled Josephson Junction Hechen Ren, Falko Pientka, Sean J Hart, Andrew T Pierce, Michael Kosowsky, Lukas Lunczer, Raimund Schlereth, Benedikt Scharf, Ewelina Hankiewicz, Laurens W Molenkamp, Bertrand I. Halperin, Amir Yacoby While signatures of Majorana bound states have been observed in one-dimensional systems, there is an ongoing effort to find alternative platforms that do not require fine-tuning and can be easily scalable. Using a Josephson junction made of HgTe quantum well coupled to thin-film aluminum, we can tune between a trivial and a topological superconducting state by controlling the phase difference φ and an applied in-plane magnetic field, as we measure the tunneling conductance at the edge of the junction. At low magnetic fields, we observe a minimum in the tunneling spectra near zero bias, consistent with a trivial superconductor. As the field increases, the tunneling conductance develops a zero-bias peak which persists over a range of φ that expands systematically with increasing magnetic fields. Consistent with theoretical predictions for this system, our observation establishes this system as a promising platform for realizing topological superconductivity and for creating and manipulating Majorana modes in two-dimensional systems. |
Thursday, March 7, 2019 5:06PM - 5:18PM |
V05.00014: Proximity-Induced Superconductivity in a Topological Crystalline Insulator Bryan Rachmilowitz, He Zhao, John Schneeloch, Ruidan Zhong, Genda Gu, Ilija Zeljkovic Inducing superconductivity at the surface of topological crystalline insulators (TCIs) can give rise to new topologically protected phases. These phases are theorized to be different than the ones observed at the surface of superconducting Z_{2} topological insulators. Here we report molecular beam epitaxy synthesis of a heterostructure involving a prototypical TCI SnTe and a high temperature superconductor Fe(Te,Se). Using low-temperature scanning tunneling microscopy and spectroscopy, we provide a strong evidence for proximity-induced superconductivity at the surface of a few bi-layer thick SnTe films. Our work provides a platform for investigating the emergent phenomena in superconducting TCIs. |
Thursday, March 7, 2019 5:18PM - 5:30PM |
V05.00015: Spectrocopic visualization of aluminum islands in the Coulomb blocakade regime deposited on InAs nanowires Haim Beidenkopf, Jonathan Reiner, Abhay K Nayak, amit tulchinsky, Yuval Oreg, hadas shtrikman, Nurit Avraham Hybrid InAs/Al nanowires are studied as a platform for topological superconductivity. Here we visualize spectroscopically the Coulomb blockade regime of aluminum islands deposited in situ on InAs nanowires. We find no changes in the chemical potential before and after aluminum deposition signifying absence of charge doping and of local band bending at the immediate vicinity of the islands. On the aluminum islands we find Coulomb blockade features typical to double barrier tunneling junction. From these we characterize the resistive and capacitive properties of the nanowire-aluminum interface. We identify a finite barrier for tunneling of electrons across that interface. Our observations strongly reflect on the ability to induce robust topological superconductivity by deposition of aluminum on InAs nanowires. |
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