Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W65: Topological Superconductivity IFocus Recordings Available

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Sponsoring Units: DMP Chair: William Strickland, New York University Room: Hyatt Regency Hotel Grant Park C 
Thursday, March 17, 2022 3:00PM  3:36PM 
W65.00001: From Majorana to parafermion corner states in secondorder topological superconductors Invited Speaker: Katharina Laubscher Recently, the generalization of conventional topological insulators (TIs) and topological superconductors (TSCs) to socalled higherorder TIs and TSCs has raised significant interest. While conventional ddimensional TIs and TSCs host gapless edge states at their (d1)dimensional boundaries, nthorder ddimensional TIs and TSCs exhibit gapless edge states at their (dn)dimensional boundaries. In particular, a twodimensional secondorder TSC (SOTSC) hosts Majorana bound states at the corners of a rectangular sample. Here, we demonstrate how the presence of strong electronelectron interactions can ‘fractionalize’ such Majorana corner states [1,2] and promote them to even more exotic parafermion corner states. [3,4] For this, we construct an SOTSC from an array of weakly coupled onedimensional wires, which allows us to treat the strong interactions analytically using bosonization techniques. As a possible platform to realize such a system, we discuss coupled helical channels in gated bilayer graphene. 
Thursday, March 17, 2022 3:36PM  3:48PM 
W65.00002: Crossed Andreev reflection in the fractional quantum Hall state of graphene Jonathan Zauberman, Onder Gul, Yuval Ronen, Si Young Lee, Hassan Shapourian, Jonathan Zauberman, YoungHee Lee, Kenji Watanabe, Takashi Taniguchi, Ashvin Vishwanath, Amir Yacoby, Philip Kim We construct highquality graphenebased van der Waals devices with narrow superconducting niobium nitride (NbN) electrodes, in which superconductivity and robust fqH can coexist. We find crossed Andreev reflection (CAR) across the superconductor separating two qH edges. In integer fillings, we find a fillingindependent CAR probability robust to changes in magnetic field and temperature, which we attribute to spinorbit coupling in NbN allowing for Andreev reflection between spinpolarized edges. Interestingly, while CAR in the holeconjugate fractions has similar behavior, CAR probabilities in the particlelike fractional fillings are markedly higher and depend strongly on temperature and magnetic field. We investigate this behavior by tuning the doping of the NbN interface while keeping bulk filling constant. These results provide a route to realize novel topological superconducting phases in fqH–superconductor hybrid devices based on graphene and NbN. 
Thursday, March 17, 2022 3:48PM  4:00PM 
W65.00003: Skyrmion control of Majorana states in planar Josephson junctions Narayan Mohanta, Satoshi Okamoto, Elbio R Dagotto Planar Josephson junctions provide a versatile platform, alternative to the nanowirebased geometry, for the generation of the Majorana bound states, due to the additional phase tunability of the topological superconductivity. The proximity induction of chiral magnetism and superconductivity in a twodimensional electron gas showed remarkable promises to manipulate topological superconductivity. In this talk, we shall present our proposal for a new geometry, involving a Josephson junction and a skyrmion crystal, that hosts the Majorana bound states. The chiral magnetism of the skyrmions can create and control the Majorana bound states without the requirement of an intrinsic Rashba spinorbit coupling. Interestingly, the Majorana bound states in our geometry are realized robustly at zero phase difference at the junction. The skyrmion radius, being externally tunable by a magnetic field or a magnetic anisotropy, brings a unique control feature for the Majorana bound states that can be useful for topological quantum computing. 
Thursday, March 17, 2022 4:00PM  4:12PM 
W65.00004: Edelstein effect and induced topological phase transition in superconductor hybrids Mostafa Tanhayi Ahari, Yaroslav Tserkovnyak we study topological phase transitions induced by the Edelstein effect in a onedimensional spinorbitcoupled wire in proximity to a noncentrosymmetric metallic superconductor. We showed that the Cooper pair flow in the substrate metallic superconductor, depending on the crystal symmetry of the metal, can be utilized to manipulate the Majorana fermions localized at the boundary of the topological phase in the wire. In a selfconsistent calculation, we extended our results to finite temperatures and showed that our simple hybrid structure may be relevant to the coherent braiding of Majorana bound states and to further developing the fabrication of quantum gates in a web of semiconducting nanowires. 
Thursday, March 17, 2022 4:12PM  4:24PM Withdrawn 
W65.00005: Supercurrentinduced topological phase transitions Kazuaki Takasan, Shuntaro Sumita, Youichi Yanase We will present our recent results showing that finite current in superconductors can induce topological phase transitions, as a result of the deformation of the quasiparticle spectrum by a finite centerofmass (COM) momentum of the Cooper pairs [1]. To show the wide applicability of this mechanism, we examine the topological properties of three prototypical systems, the Kitaev chain, swave superconductors, and dwave superconductors. We introduce a finite COM momentum as an external field corresponding to supercurrent and show that all the models exhibit currentinduced topological phase transitions. In this talk, we will also address the possibility of observing the phase transitions in experiments and the relation to the other finite COM momentum pairing states. 
Thursday, March 17, 2022 4:24PM  4:36PM 
W65.00006: Signatures of enhanced spintriplet superconductivity induced by spinorbit coupling Chenghao Shen, Jong E Han, Mohammad Alidoust, Thomas Vezin, Igor Zutic Through structural inversion asymmetry in superconducting junctions, the resulting interfacial spinorbit coupling provides a versatile platform to realize proximityinduced spintriplet superconductivity sought in superconductor spintronics and topological quantum computing. However, the signatures of such spintriplet conductivity and the conditions for its enhancement remain debated. By calculating superconducting correlations in ferromagnet/swave superconductor junctions and zerobias conductance contributions from equalspin Andreev reflection, we reveal how very different signatures give a consistent picture for an enhanced spintriplet superconductivity. Unlike the common expectation that strong spinorbit coupling promotes the spintriplet pairing or a small interfacial barrier is necessary for robust proximity effects, we reveal a more complex picture. An enhanced spintriplet superconductivity, realized for intermediate values of spinorbit coupling and interfacial barrier strength, is consistent with the experimentally observed huge increase in the conductance magnetoanisotropy. 
Thursday, March 17, 2022 4:36PM  4:48PM 
W65.00007: Effects of quantum dotcontinuum coupling in topological superconductorquantum dottopological superconductor system: implications for quantum dotbased measurements of Majorana qubits and the 0π transition Aleksei Khindanov, Torsten Karzig, Dmitry I Pikulin Quantum dot coupled to Majorana Zero Modes (MZMs) in onedimensional topological superconductorquantum dottopological superconductor junctions can potentially serve two purposes for the needs of topological quantum computing. First, it is believed that absence of the 0π transition in the junction can indicate presence of the MZMs and the topological phase. Second, quantum dot can be used as a tool to measure the state encoded by the MZMs. Considering proximitized Rashba nanowire with Zeeman energy as a model for a topological superconductor, here we investigate effect of continuum nanowire modes coupled to a quantum dot in topological superconductorquantum dottopological superconductor junctions. Using perturbation theory in the strength of the coupling between the quantum dot and the continuum modes, we find that for realistic parameters coupling to continuum modes does not considerably alter visibility of the quantum dotbased measurements of Majorana qubits. However, we also find that presence of this coupling can lead to a πjunction behavior even in the topological phase which would limit capabilities of the 0π transition as a diagnostic tool for the presence/absence of MZMs. 
Thursday, March 17, 2022 4:48PM  5:00PM 
W65.00008: Scanning Microscopy Probes of Majorana States Michael Gottschalk, Eric W Goodwin, Elinore McLain, Reza Loloee, Kaedon ClelandHost, Stuart H Tessmer Majorana bound states are difficult to distinguish from trivial nearzero energy states. Singleelectron transistors (SETs) have been proposed as onchip probes to solve this problem. Combining these devices with scanning probe methods allows us to extend the reach of these probes to test many of the systems that theoretically support Majorana bound states (vortices in a 2D topologogical superconductor, for example.) In this talk, we present progress on (i) scanning tunneling microscopy (STM) probes of Majorana states in Josephson junctions and (ii) development and testing of the Scanning Majorana Microscope (SMM). We present a platform that implements a capacitancebased device constructed from a highelectronmobility transistor that can measure the counting statistics of electrons tunneling into a quantum dot. Theoretically, the cumulants of the counting statistics change as the quantum dot hybridizes with a Majorana bound state. This device is integrated into a probe tip which fabricated by thermally evaporating two Al leads (one tunneling lead and one capacitance lead) onto a sharplypointed optical fiber. Using a focused ion beam, we create a flat end of the pointed optical fiber where we deposit our quantum dot. For this talk, we will focus on key milestones that have been accomplished. The first, is a demonstration of the probes counting statistics in vacuum. Second, is confirmation that the probe can adequately perform the basic functions of an STM tip: approaching safely, and measuring the topography of the sample's surface. Additionally, we present progress on developing a device featuring planar Josephson junctions on the surface of a topological insulator. This device is fabricated with conventional electronbeam lithography and reactive ion etching. Such a device should theoretically support Majorana bound states in the presence of an external magnetic field. Our nearterm goal is to probe this device with STM and the SMM. 
Thursday, March 17, 2022 5:00PM  5:12PM 
W65.00009: Transport of Majorana Zero Modes in 1D Topological Superconductors Bill P Truong, Tami PeregBarnea, Kartiek Agarwal We consider the transport of Majorana zero modes across a 1D topological superconductor by applying local gate voltages across sections of the superconductor. This “piano key” method allows for sections of the superconductor to switch between the trivial and topological phases, thereby facilitating the motion of a Majorana zero mode. As a single section, or piano key, undergoes a phase transition, it is possible for the ground state to experience excitations, especially near criticality. The excitation probability has been studied for a large piano key in Ref. [1] which casts the problem in terms of a simple LandauZener transition. In our work, we consider the excitation probability when a Majorana zero mode is transported using a series of smaller piano keys. We calculate the excitation probability numerically by simulating a sequence of piano keys. Furthermore, we demonstrate an analytical calculation of the excitation probability and compare this to the numerical results. 
Thursday, March 17, 2022 5:12PM  5:24PM 
W65.00010: Direct Measurement of the Andreev Bound State Spin and Charge During the SingletDoublet Transition David van Driel, Guanzhong Wang, Alberto Bordin, Nick van Loo, Grzegorz P Mazur, Sasa Gazibegovic, Ghada Badawy, Erik P. A. M. Bakkers, Gijs De Lange, Tom Dvir The past decade has seen many proposals to measure Majorana quasiparticles definitively. One such measurement involves probing the bulk spin polarization of a system undergoing a topological phase transition. An Andreev bound state (ABS) is expected to similarly show both charge and spin reversal at the singletdoublet quantum phase transition. 
Thursday, March 17, 2022 5:24PM  5:36PM 
W65.00011: Tuning singlet vs triplet Cooperpair splitting in InSb nanowires Guanzhong Wang, Nick van Loo, Chunxiao Liu, Grzegorz P Mazur, Sasa Gazibegovic, Ghada Badawy, Erik P. A. M. Bakkers, Michael Wimmer, Gijs De Lange, Tom Dvir Cooper pairs are a natural source of entangled electrons provided by superconductors. Semiconductingsuperconducting hybrid systems allow the splitting of Cooper pairs into separate single electrons and their subsequent manipulation. Using two fewelectron InSb nanowire quantum dots separated by a narrow strip of proximitized superconducting Al, we achieve a high degree of control in Cooperpair splitting via selecting for the single electron's desired charge as well as spin. The resulting Cooperpair splitting signal is free of contributions from the competing process, elastic cotunneling. Under an applied magnetic field parallel to the Rashba spinorbit field of InSb, we observe complete blockade of the equalspin splitting process due to spinsinglet superconducting pairing. Rotating the applied field to be perpendicular to the Rashba field introduces a triplet component in the effective superconducting pairing between electrons and lifts this blockade. The spin blockade results open new pathways to the precise manipulation and entanglement testing of the split electrons. 
Thursday, March 17, 2022 5:36PM  5:48PM 
W65.00012: Transport signatures of Majorana zero modes in magnetic insulator nanowire hybrid systems Roshni Singh, Bhaskaran Muralidharan Semiconducting nanowires with Rashba spinorbit coupling and proximityinduced superconductivity are the forerunners as platforms to host Majorana Zero Modes (MZMs). Recent evidence of a topological transition in hybrid Rashba nanowires epitaxially coated by superconductormagnetic insulator bilayers points to the formation of MZMs at significantly lower external magnetic fields than in conventional systems, making it experimentally viable for scaling networks of nanowires hosting MZMs. 
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