Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H45: Topological Josephson JunctionFocus

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Sponsoring Units: DMP Chair: James Williams, University of Maryland Room: 392 
Tuesday, March 14, 2017 2:30PM  3:06PM 
H45.00001: Magnetic field induced 4$\pi$ periodic Josephson effect in InAs nanowires Invited Speaker: Dominique Laroche, Dani\"{e}l Bouman, David van Woerkom, Alex Proutski, Ruben van Gulik, Micha\l{} Nowak, Dmitry Pikulin, Jesper Nyg\r{a}rd, Peter Krogstrup, Charles Marcus, Leo Kouwenhoven, Attila Geresdi 
Tuesday, March 14, 2017 3:06PM  3:18PM 
H45.00002: Abstract Withdrawn We theoretically study a one dimensional pwave superconducting mesoscopic ring interrupted by a weak link and coupled inductively to a microwave cavity. We establish an inputoutput description for the cavity field in the presence of the ring, and identify the electronic contributions to the cavity response and their dependence on various parameters, such as the magnetic flux, chemical potential, and cavity frequency. We show that the cavity response is 4$\pi$ periodic as a function of the magnetic flux in the topological region, stemming from the so called fractional Josephson current carried by the Majorana fermions, while it is 2$\pi$ periodic in the nontopological phase, consistent with the normal Josephson effect. We find a strong dependence of the signal on the cavity frequency, as well as on the parity of the ground state. Our model takes into account fully the interplay between the lowenergy Majorana modes and the gaped bulks states, which we show is crucial for visualizing the evolution of the Josephson effect during the transition from the topological to the trivial phase. 
Tuesday, March 14, 2017 3:18PM  3:30PM 
H45.00003: Signatures of Majorana assisted tunneling in 1D Josephson junction rings. Rosa Rodriguez Mota, Smitha Vishveshwara, Tami PeregBarnea Majorana zero modes in condensed matter settings have drawn immense attention recently and the search for unequivocal signatures of Majorana modes is underway. In this respect the presence of 4$\pi$ periodic tunneling between topological superconductors, which is assisted by the Majorana modes is a promising candidate. In this work, we study how the inclusion of Majorana assisted tunneling modifies physics in a ring of Josephson junction coupled islands. In addition, we study parity breaking processes in a controlled manner by coupling the ring to a twolevel electronic system, such as found in quantum dots. For the ring alone, we find that the effects of the 2$\pi$ periodic terms are suppressed by increasing the number of islands in the ring. This allows a clearer distinction of the 4$\pi$ periodicity. In the combined system, we find that the periodicity can be tuned by controlling the occupation energy of the two level system. We also discuss the signatures of Majoranas that remain after parity breaking effects restore the $2\pi$ periodicity. [Preview Abstract] 
Tuesday, March 14, 2017 3:30PM  3:42PM 
H45.00004: Detecting topological superconductivity using doubled Shapiro steps Jay Sau, F setiawan The fractional Josephson effect has been observed in many instances as a signature of a topological superconducting state containing zeroenergy Majorana modes. However, it has been suggested that such a fractional Josephson effect might occur in nontopological systems. Here, we apply a scattering matrixbased noise formalism to calculate the fractional ac Josephson effect in one such nontopological scenario that may occur generically in semiconductorbased Josephson junctions. We find that the presence of a weakly coupled resonant impurity bound state in a semiconductor in the vicinity of a highly transparent channel produces a nearly topological fractional Josephson effect similar to Majorana modes. The Josephson effect signature for Majorana modes become distinct from this nontopological scenario only at low frequency. We argue that a variant of the fractional Josephson effect, namely the doubled Shapiro step, which can be performed at lower frequency and will generically appear in topological superconductors, can provide a more reliable signature of the topological superconducting state. [Preview Abstract] 
Tuesday, March 14, 2017 3:42PM  3:54PM 
H45.00005: Conductance spectroscopy of nontopologicaltopological superconductor junctions F. Setiawan, William Cole We calculate the zerotemperature differential conductance $dI/dV$ of a voltagebiased onedimensional junction between a nontopological and a topological superconductor for arbitrary junction transparency using the scattering matrix formalism. We consider two models for the topological superconductors: (i) spinful $p$wave and (ii) $s$wave with spinorbit coupling and spin splitting. In the tunneling limit (small junction transparencies) where only single Andreev reflections contribute to the current, the conductance for voltages below the nontopological superconductor gap $\Delta_s$ is zero and there are two conductance peaks at $eV = \pm \Delta_s$ with the quantized value $(4\pi)2e^2/h$ due to resonant Andreev reflection from the Majorana zero mode. However, when the junction transparency is not small, there is a finite conductance for $eV < \Delta_s$ arising from multiple Andreev reflections. The conductance at $eV = \pm \Delta_s$ in this case is no longer quantized. In general, the conductance is particlehole asymmetric except for sufficiently small transparencies. We further show that, for certain values of parameters, the tunneling conductance of a zeroenergy conventional Andreev bound state can resemble that of Majorana. Ref: Setiawan et. al. arXiv:1609.09086 [Preview Abstract] 
Tuesday, March 14, 2017 3:54PM  4:06PM 
H45.00006: Magnetic field and gate tunable supercurrent fluctuations in multimode semiconductor nanowires Vincent Mourik, Kun Zuo, Daniel Szombati, Dmitry Pikulin, Bas Nijholt, Viacheslav Ostroukh, Anton Akhmerov, David van Woerkom, Attila Geresdi, Diana Car, Sebastien Plissard, Erik Bakkers, Leo Kouwenhoven, Sergey Frolov Semiconducting nanowires coupled to superconductors are crucial in proposals for inducing, detecting, and controlling topological superconductivity and Majorana fermion bound states. Using NbTiNInSbNbTiN Josephson junctions, we study supercurrents flowing in quasiballistic nanowires with strong spinorbit interaction and in high magnetic fields, thus combining the essential ingredients required for Majorana fermions. Operating in a multimode regime, we observe aperiodic supercurrent fluctuations with several nodes at a finite magnetic field. The node fields are gate tunable. By numerically calculating the supercurrent in a realistic, three dimensional device geometry, we analyze the possible causes of these fluctuations: Zeeman splitting, spinorbit interaction, and interference between modes. We find that interference dominates over the other effects in the investigated magnetic field range of 02 T. Demonstrating and controlling supercurrent interference effects is crucial to understanding the signatures of Majorana fermions in Josephson junctions, and it will benefit the future construction of topological quantum circuits. [Preview Abstract] 
Tuesday, March 14, 2017 4:06PM  4:18PM 
H45.00007: Josephson currents induced by the Witten effect Jeroen van den Brink, Zohar Nussinov, Flavio Nogueira We reveal the existence of a new type of topological Josephson effect involving type II superconductors and threedimensional topological insulators as tunnel junctions. We predict that vortex lines induce a variant of the Witten effect that is the consequence of the axion electromagnetic response of the topological insulator: at the interface of the junction each flux quantum attains a fractional electrical charge of e/4. As a consequence, if an external magnetic field is applied perpendicular to the junction, the Witten effect induces an AC Josephson effect in absence of any external voltage. We derive a number of further experimental consequences and propose potential setups where these quantized, flux induced, Witten effects may be observed. \\F. S. Nogueira, Z. Nussinov, and J. van den Brink, Phys. Rev. Lett. {\bf 117}, 167002 (2016). \newline F. S. Nogueira, Z. Nussinov, and J. van den Brink, Phys. Rev. D {\bf 94}, 085003 (2016). [Preview Abstract] 
Tuesday, March 14, 2017 4:18PM  4:30PM 
H45.00008: Electrical Transport in Bi2Te3 Thin Film Devices: Effective Band Structure, Charge Carrier Distribution and Josephson Coupling Martin Stehno, Prosper Ngabonziza, Hiroaki Myoren, Alexander Brinkman Engineering of topological states in Josephson junctions requires understanding of the effective band structure of the topological insulator (TI) material that forms the weak link. Previous work has focused on optimizing materials aspects to boost the effective Josephson coupling while providing little insight into underlying mechanisms. We present transport measurements on backgated Bi$_2$Te$_3$ thin film devices with low levels of bulk doping. Carrier densities are obtained from a twocarrier model fit to the Hall effect data. To validate the fits, we calculate the band structure of a thin film using a tightbinding model and find excellent agreement in the number of transport carriers. We conclude that the bottom region of the film changes from a deep quantum well to a depletion zone as the backgate voltage is lowered, which explains a change in the observed weakantilocalization signal with gate bias. For our Nb/Bi$_2$Te$_3$/Nb Josephson junctions, we observe that the Josephson coupling energy scales according to the theory of long diffusive SNS contacts with a smaller, effective value for the Thouless energy of the weak link that takes into account the longer dwell time in the junction area due to interface barriers and the conductivity mismatch between the materials. [Preview Abstract] 
Tuesday, March 14, 2017 4:30PM  4:42PM 
H45.00009: Experimental studies on hybrid superconductortopological insulator nanoribbon Josephson devices Morteza Kayyalha, Luis Jauregui, Aleksander Kazakov, Ireneusz Miotkowski, leonid Rokhinson, Yong Chen The spinhelical topological surface states (TSS) of topological insulators in proximity with an swave superconductor are predicted to demonstrate signatures of topological superconductivity and host Majorana fermions. Here, we report on the observation of gatetunable proximityinduced superconductivity in an intrinsic BiSbTeSe$_{\mathrm{2}}$ topological insulator nanoribbon (TINR) based Josephson junction (JJ) with Nb contacts. We observe a gatetunable critical current (I$_{\mathrm{C}})$ with an anomalous behavior in the temperature (T) dependence of I$_{\mathrm{C}}$. We discuss various possible scenarios that could be relevant to this anomalous behavior, such as (i) the different temperature dependence of supercurrent generated by ingap, where phase slip plays an important role, and outofgap Andreev bound states or (ii) the different critical temperatures associated with the top and bottom topological surface states. Our modeling of I$_{\mathrm{C}}$ vs. T suggests the possible existence of one pair of ingap Andreev bound states in our TINR. We have also studied the effects of magnetic fields on the critical current in our TINR Josephson junctions. [Preview Abstract] 
Tuesday, March 14, 2017 4:42PM  4:54PM 
H45.00010: Josephson Tunneling between Superconducting Topological Surface States Lin Li, Hui Zhang, XiaoDong Ma, George Nichols, Sangil Kwon, David Cory, ChangGan Zeng, GuoXing Miao Proximity at the interface of a 3D topological insulator (TI) and a conventional swave superconductor can induce a 2D topological superconducting state. We grew swave superconductor (Nb)/TI (BiSbTe)/swave superconductor(Nb) heterostructures using molecular beam epitaxy (MBE) and fabricated sandwich typed Josephson junctions. Through the superconducting proximity effect, we realized topological superconducting surface states on both the top and bottom surfaces of the topological insulator film. Transport measurements showed an anomalous zerobias conductance peak (ZBCP) at 135mK, in addition to the conventional Josephson effect between the two superconductor leads. This suggests Josephson tunneling occurs between the two proximitydriven topological surface states. [Preview Abstract] 
Tuesday, March 14, 2017 4:54PM  5:06PM 
H45.00011: Detecting Topological Superconductivity with $\varphi_{0}$ Josephson Junctions Constantin Schrade, Silas Hoffman, Daniel Loss Topological superconductivity can emerge in conventional superconductors in the presence of spinorbit interaction and magnetic fields. Remarkably, the recent experimental discovery of $\varphi_{0}$ Josephson junctions by Szombati et al., characterized by a finite phase offset in the supercurrent, require the same ingredients as topological superconductors, which suggests a profound connection between these two distinct phenomena. Here, we theoretically show that a quantum dot $\varphi_{0}$ Josephson junction can serve as a new qualitative indicator for topological superconductivity: Microscopically, we find that the phase shift in a junction of $s$wave superconductors is due to the spinorbit induced mixing of singly occupied states on the qantum dot, while for a topological superconductor junction it is due to singlettriplet mixing. Because of this important difference, when the spinorbit vector of the quantum dot and the external Zeeman field are orthogonal, the $s$wave superconductors form a $\pi$ Josephson junction while the topological superconductors have a finite offset $\varphi_{0}$ by which topological superconductivity can be distinguished from conventional superconductivity. Our prediction can be immediately tested in nanowire [Preview Abstract] 
Tuesday, March 14, 2017 5:06PM  5:18PM 
H45.00012: Controlling Majorana bound states in a Josephson junction on a topological insulator SangJun Choi, H.S. Sim We theoretically study a Josephson junction on a topological insulator under a magnetic field such that the number of flux quanta inside the Josephson junction is larger than one. Majorana bound states emerge along the junction where the superconducting phase difference of the junction equals to $\pi $. We find that the fermionic state composed of the Majorana bound states can be manipulated and controlled by a bias voltage between the two superconductors. The features of Majorana bound states such as exchange can be detected by measuring the Josephson current. [Preview Abstract] 
Tuesday, March 14, 2017 5:18PM  5:30PM 
H45.00013: Tenfold way of topological Josephson effects in superconductors YuanMing Lu, Fan Zhang We obtain the tenfold way classification of topological Josephson effects in superconductors using the Ktheory. We show how the ten AltlandZirnbauer symmetry classes can be realized in singlet or triplet superconductors with or without spinorbit couplings and/or coexisting magnetic orders. In particular, we show that in three spatial dimensions there always exists a topological Josephson effect in each symmetry class, and that in one spatial dimension there are novel topological pumps mediated by paired or unpaired Majorana bound states. [Preview Abstract] 
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