Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session B65: Topological Superconductivity in Josephson JunctionsFocus Recordings Available

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Sponsoring Units: DMP Chair: Jay Sau, UMD Room: Hyatt Regency Hotel Grant Park C 
Monday, March 14, 2022 11:30AM  12:06PM 
B65.00001: Topological superconductivity in semiconductorsuperconductor hybrid structures Invited Speaker: Karsten Flensberg Several proposals for inducing pwave superconductivity in semiconductors proximitized by swave superconductors have been put forward. This includes systems where timereversal symmetry is broken by either Zeeman energy or by phase differences in the proximitizing superconductors. In this talk, I discuss the detailed modeling of such structures and present results how for the comparison with experimental data for local and nonlocal conductance measurements. Finally, I discuss possible next steps in the search for nonabelian effects of Majorana zero modes. 
Monday, March 14, 2022 12:06PM  12:18PM 
B65.00002: Towards NonAbelian Statistics in Topological Planar Josephson Junctions Tong Zhou, Matthieu C Dartiailh, Kasra Sardashti, Jong E Han, Alex MatosAbiague, Javad Shabani, Igor Zutic Topological superconductivity supports exotic Majorana bound states (MBS) which are chargeless zeroenergy emergent quasiparticles. With their nonAbelian statistics, they are suitable for implementing faulttolerant topological quantum computing. While the main efforts to realize MBS have focused on 1D systems, the onset of topological superconductivity requires delicate parameter tuning and geometric constraints pose significant challenges for their control and demonstration of nonAbelian statistics. To overcome these challenges, building on recent advances in planar Josephson junctions (JJs) [1], we propose a novel platform of Xshaped JJs [2], where phase control can generate, exchange, and braid MBS. The underlying topological superconductivity exists over a large parameter space, consistent with our fabricated materials. With our experimental control of superconducting properties using five minigates in InAs/Albased JJs, we show how such minigate control demonstrates nonAbelian statistics through MBS fusion [3], paving a promising way to topological quantum computing. 
Monday, March 14, 2022 12:18PM  12:30PM 
B65.00003: Robust topological superconductivity in spatially modulated planar Josephson junctions Purna Paudel, Trey Cole, Benjamin D Woods, Tudor D Stanescu Semiconductorsuperconductor planar Josephson junctions are promising platforms for realizing topological superconductivity and Majorana zero modes. We purpose a spatially modulated structure consisting of a planar Josephson junction with periodically modulated width. Based on detailed numerical calculations for a lowenergy effective model of the proposed device, we demonstrate that the topological gap characterizing the modulated structure is enhanced significantly, which, in turn, enhances the robustness of the associated Majorana zero modes. The enhancement of the topological gap is the result of a stronger effective spinorbit coupling generated by the periodic potential induced by the spatial modulation of the junction. In addition, we find that the proposed structure supports a lowfield topological superconducting phase within a significant range of electrostatic control parameters, in the absence of a superconducting phase difference across the junction. The optimal regime for operating the device can be achieved by tuning the electrostatic potential in the junction region. 
Monday, March 14, 2022 12:30PM  12:42PM 
B65.00004: Visualizing Topological Phase Transitions in Planar Josephson Junctions Barış Pekerten, Alex MatosAbiague We investigate the signatures of topological superconductivity in the currentphase relation (CPR) of planar Josephson junctions (JJs). We show how the magneticfield dependence of the CPR in phasebiased JJs can be used to obtain a good description and visualization of topological phase transitions. In particular, we propose a new method for obtaining the topological phase diagram in the phase and magnetic field space from CPR measurements. This allows to identify the superconducting phase difference and magnetic field at which the junction transits into the topological superconducting state. Furthermore, the magneticfield dependence of the CPR in phasebiased JJs can also be used to predict the selftuning of phaseunbiased JJs into the topological state. We discuss the reliability of the proposed method by comparing to direct numerical calculations of the topological phase diagram and to signatures provided by other physical quantities, like conductance and spin susceptibility. 
Monday, March 14, 2022 12:42PM  12:54PM 
B65.00005: Enhancing topological superconductivity in phasebiased Josephson junctions William F Schiela, Bassel H Elfeky, Peng Yu, Neda Lotfizadeh, Seyed Mohammad Farzaneh, Joseph Yuan, Mehdi Hatefipour, Dylan Langone, William M Strickland, Javad Shabani The manifestation of spinorbit coupling in proximitized superconductivity is central to the formation of topological superconducting phases. The most exciting property of topological superconductivity is its capacity to harbor nonAbelian excitations that could be the key to developing faulttolerant quantum computers. Here we study planar Josephson junctions of highly transparent, epitaxial Al contacts on a nearsurface InGaAsInAsInGaAs quantum well confinining a 2DEG with strong spinorbit coupling and large gfactor. This system has been predicted to support topological superconductivity and previous experimental work has yielded promising results consistent with theoretically derived signatures of nontrivial topology. Our junction is embedded in a SQUID geometry with an integrated remote flux line, enabling phasesensitive measurements and phasecontrol via magnetic flux applied far from the junction so as to preserve the superconducting gap. We study the currentphase relation of the junction as a function of inplane Zeeman field and gate voltage. We further study the local density of states near the edge of the junction via tunneling spectroscopy with a quantum point contact while the junction is πphasebiased, where the Zeeman condition for topology is relaxed. 
Monday, March 14, 2022 12:54PM  1:06PM 
B65.00006: Tunneling spectroscopy in planar Josephson junctions Peng Yu, William F Schiela, Bassel Heiba Elfeky, Neda Lotfizadeh, Seyed Mohammad Farzaneh, Joseph Yuan, Mehdi Hatefipour, Dylan Langone, William M Strickland, Javad Shabani Planar Josephson junction based on twodimensional electron gas (2DEG) with strong spinorbit coupling, large gfactor and induced superconductivity is a promising platform to realize topological superconductivity and Majorana zero modes. Here we fabricate planar Josephson junctions on InAs 2DEG with epitaxial Al Contacts. Two tunneling probes are added to the ends of the junction to probe the density of states of edge modes. In a trivial regime, we show that we can screen devices and identify hard induced superconducting gaps in the junction. By application of an inplane magnetic field the system is expected to make a transition to topological regime. We will present the induced gap and lowenergy states measured at the two ends of the junction simultaneously as a function of the inplane magnetic field, the angle between the magnetic field and the junction. 
Monday, March 14, 2022 1:06PM  1:18PM 
B65.00007: Local Control of Supercurrent Density in Epitaxial AlInAs Josepshon Junctions Neda Lotfizadeh, Bassel H Elfeky, William F Schiela, William M Strickland, Matthieu C Dartiailh, Kasra Sardashti, Mehdi Hatefipour, Peng Yu, Natalia Pankratova, Hanho Lee, Vladimir Manucharyan, Javad Shabani Gate tunable Josephson junctions (JJs) made from two dimensional electron gas can host topological superconductivity, a unique property that can be used for fault tolerant topological quantum computation. In planar Josephson junctions the carrier density and, therefore the overall current distribution could be modified electrostatically via metallic gates. In this work, by applying an outofplane magnetic field to an epitaxial AlInAs JJ equipped with five minigates, we have extracted the spatial current distribution of our devices. We demonstrate that not only can the junction width be electrostatically defined and varied but also we can locally adjust the current profile to form superconducting quantum interference devices. Our studies show enhanced edge conduction in the junctions, which can be eliminated by minigates to create a uniform current distribution. The ability to locally tune the current distribution width of the junction could open a path for studying topological superconductivity in Josephson junctions. 
Monday, March 14, 2022 1:18PM  1:30PM 
B65.00008: Microwave Driven Epitaxial Josephson Junctions with an Inplane Magnetic Field Bassel Heiba Elfeky, Joseph J Cuozzo, William F Schiela, Neda Lotfizadeh, Peng Yu, Seyed Mohammad Farzaneh, Mehdi Hatefipour, Dylan Langone, William M Strickland, Joseph Yuan, Enrico Rossi, Javad Shabani The hunt for topological superconductivity has accelerated in recent years as it provides a suitable platform for faulttolerant quantum computing. Planar epitaxial AlInAs Josephson junctions (JJs) are a promising candidate to realize topological superconducting states due to their high transparency, strong spinorbit coupling, and large gfactor. When a microwave drive is applied to a JJ, quantizied integer and possibly fractional constant voltage steps, Shapiro steps, appear in the VoltageCurrent characteristic due to phase locking. The presence of the 4πperiodic Josephson effect, one of the signatures of topological superconductivity, results in missing odd Shapiro steps. However, highly transparent JJs in the topologically trivial regime, without a magnetic field, have also been shown to exhibit missing odd Shapiro. Theory predicts these JJs can be driven into a topological superconducting phase by applying an inplane magnetic field. We study the effect of the Zeeman field on trivial and nontrivial signatures with and without microwave drive. We further consider the field angle and junction geometry on the observed signatures. 
Monday, March 14, 2022 1:30PM  1:42PM 
B65.00009: Extracting strength of spinorbit coupling in InAsAl heterostructures: Implications for topological superconductivity Seyed Mohammad Farzaneh, Mehdi Hatefipour, Neda Lotfizadeh, Peng Yu, William F Schiela, Bassel H Elfeky, Joseph Yuan, Dylan Langone, William M Strickland, Javad Shabani Topological superconductivity is expected to emerge in materials with a strong spinorbit coupling. Beyond the realization of topological superconductivity it is important to control the topologicaltrivial phase transition as well. This can be achieved through a gate tunable spinorbit coupling in epitaxially grown twodimensional InAsAl heterostrcutres. Here, we discuss extracting the strength of linear Dresselhaus and Rashba spinorbit coupling terms from magnetoconductivity measurements which manifest as weak localization or antilocalization signatures. A fully quantum mechanical model is used to describe the electronic states and the interference effects resulting from scattering loops in the presence of an external outofplane magnetic field. The extracted experimental results are then compared with the solutions of the 8band Kane model for Zinc blende crystals with a reduced symmetry in the growth direction. We then discuss the symmetry and the interplay of Dresselhaus and Rashba spinorbit coupling terms as well as the effect of an inplane magnetic field in realizing topological superconductivity. 
Monday, March 14, 2022 1:42PM  1:54PM 
B65.00010: Topological superconductivity in the attractive RashbaHubbard model: A quantum Monte Carlo dynamical cluster approximation study Thomas A Maier, Peter Doak, Giovanni Balduzzi Fully gapped, spin singlet superconductors with antisymmetric spinorbit coupling in a Zeeman magnetic field provide a promising route to realize superconducting states with nonAbelian topological order and therefore faulttolerant quantum computation. Here we use a quantum Monte Carlo dynamical cluster approximation to study the superconducting properties of a doped twodimensional attractive Hubbard model with Rashba spin orbit coupling in a Zeeman magnetic field. In the absence of spin orbit coupling, we find that the Zeeman field completely suppresses superconductivity in this model as expected from the Pauli depairing effect. When the Rashba spin orbit coupling is turned on, however, superconductivity is restored at finite temperatures. By inspecting the Fermi surface of the interacting model, we can draw conclusions about the topological character of the superconducting state. 
Monday, March 14, 2022 1:54PM  2:06PM 
B65.00011: Observation of Phase Controllable Majoranalike Bound States in Metamaterialbased Kitaev Chain Analogues Kai Qian, David J Apigo, Karmela PadavicCallaghan, Keun Hyuk Ahn, Smitha VIshveshwara, Camelia Prodan Motivated by the recent theoretical studies on a quasionedimensional SuSchriefferHeeger ladder model Hamiltonian [1], we experimentally demonstrate that the Majoranalike Bound States (MBSs) can occur at the open edges of quasionedimensional mechanical metamaterials, analogous to the Majorana Zero Modes at the edges of the Kitaev chain. Specifically, using mechanical systems made of magnetically coupled spinners, we demonstrate (1) the topological phase transition signaled by the emergence of the midgap MBSs, (2) the localization of MBS characterized by decay and oscillation (3) the hybridization of MBSs and control of relative phases of wavefunction analogues by frequency tuning, and (4) the oscillation of the MBS hybridization splitting with varying the length of the ladder systems. We find strong agreement between experiments and theory. The implications to the Kitaev chain, qubits, and braidings, and the possibility of topologically protected mechanical memory are also discussed. 
Monday, March 14, 2022 2:06PM  2:18PM 
B65.00012: Fermion parity switches in MajoranaTransmon qubits Ramon Aguado, Elsa Prada, Pablo SanJose, Jesus Avila Recent efforts have focused on replacing the weak link in the Josephson Junction (JJ) of a superconducting qubit by electrostaticallygateable technologies compatible with high magnetic fields [1]. Such alternatives are crucial in order to reach a regime relevant for readout of topological qubits based on Majorana zero modes (MZMs) [2]. In this talk, I will focus on JJs based on semiconducting nanowires that can be driven to a topological superconductor phase with MZMs. A fully microscopic theoretical description of such hybrid semiconductorsuperconducting qubit allows to unveil new physics originated from the coherent interaction between the MZMs and the superconducting qubit degrees of freedom [3]. In the transmon regime, the microwave spectroscopy presents nontrivial features, including a full mapping of zero energy crossings and fermionic parity switches in the nanowire owing to Majorana oscillations [4]. 
Monday, March 14, 2022 2:18PM  2:30PM 
B65.00013: Local and nonlocal spectroscopy of phasebiased Josephson junctions in an InSbAs twodimensional electron gas Christian M Moehle, Chung Ting Ke, Prasanna Rout, Candice Thomas, Di Xiao, Geoffrey C Gardner, Srijit Goswami Planar Josephson junctions (JJs) have been proposed as a route to creating phasecontrollable Majorana zeromodes. We study such devices in a hybrid InSbAs/Al twodimensional electron gas, which has previously shown promising electrical properties for the realization of topological superconductivity (i.e., strong spinorbit coupling, a large gfactor and clean superconductorsemiconductor interfaces). Using simultaneous tunnelling spectroscopy at both ends of the junction, we observe Andreev bound states whose energies are modulated by the phase. The threeterminal nature of these devices also allows us to study the nonlocal conductance, giving additional insights into the bulk of the JJ. At finite inplane magnetic fields, we see the onset of a phasedependent inversion in the sign of the nonlocal conductance and the emergence of zerobias peaks in local spectroscopy. 
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