Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V08: Superconducting Heterostructures and Quantum Dots 
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Sponsoring Units: DCMP Chair: James Eckstein, Univ of Illinois  Urbana Room: BCEC 150 
Thursday, March 7, 2019 2:30PM  2:42PM 
V08.00001: Towards the Kitaev model with quantum dot chains in semiconductor nanowires Hao Wu, Po Zhang, Jun Chen, Sasa Gazibegovic, Roy Op het Veld, Ghada Badawy, Erik P. A. M. Bakkers, Sergey M Frolov We experimentally explore whether chains of quantum dots in semiconductor nanowires can be used to emulate important onedimensional Hamiltonians such as Kitaev model that describes a topological pwave superconductor. In our devices, quantum dots are electrostatically confined in an InSb nanowire by an array of closely spaced narrow gates. Quantum dots are coupled to NbTiN superconducting leads. We study transport properties and magnetospectroscopy of Andreev bound states in these quantum dots with magnetic fields parallel to the nanowire axis. Up to three quantum dots and three superconducting contacts are defined in a single nanowire. We compare results from quantum dot chain experiments to those of experiments on closely related Majorana nanowires. 
Thursday, March 7, 2019 2:42PM  2:54PM 
V08.00002: Analytical Noise Spectra Calculations for HighTemperature Superconducting Nanowires Corey Ostrove, Linda E Reichl In this talk we present analytical results for the noise spectrum of a superconducting heterostructure formed by a high temperature superconductor (HTS) nanowire. The scattering matrix is calculated from first principles using the Bogoliubovde Gennes equations with appropriate boundary conditions and without the Andreev approximation. The currentcurrent correlation function is calculated directly using the scattering coefficients and from this the noise spectrum (and in particular the shot noise contribution) is generated. The effects of the anisotropy of the order parameter in the scattering properties of the structures are investigated. We also consider some of the consequences of the quasi1d geometry in terms of the restrictions on allowed propagation modes through the nanowire. Recent advances in materials synthesis have brought the production of high quality HTS nanowires closer widespread use in the design and construction of superconducting devices, which makes theoretical characterization of these structures of particular importance. 
Thursday, March 7, 2019 2:54PM  3:06PM 
V08.00003: Competition between the YuShibaRusinov and Kondo screening in double quantum dot based Cooper pair splitters Ireneusz Weymann, Kacper Wrzesniewski The Andreev transport properties of double quantum dot based Cooper pair splitters with one superconducting and two normal leads are studied theoretically in the Kondo regime. The influence of the YuShibaRusinov screening resulting from superconducting pairing correlations on the local density of states, Andreev transmission coefficient, and Cooper pair splitting efficiency is thoroughly analyzed. It is shown that finite superconducting pairing potential quickly suppresses the SU(2) Kondo effect, which can however reemerge for relatively large values of coupling to superconductor. In the SU(4) Kondo regime, a crossover from the SU(4) to the SU(2) Kondo state is found as the coupling to superconductor is enhanced. 
Thursday, March 7, 2019 3:06PM  3:18PM 
V08.00004: Andreev Blockade in a Double Quantum Dot with a Superconducting Lead David Pekker, Sergey M Frolov A normal metal source reservoir can load two electrons onto a double quantum dot in the spintriplet configuration. We show that if the drain lead of the dot is a spinsinglet superconductor, these electrons cannot form a Cooper pair and are blockaded on the double dot. We call this phenomenon Andreev blockade because it arises due to suppressed Andreev reflections. We identify transport characteristics unique to Andreev blockade. Most significantly, it occurs for any occupation of the dot adjacent to the superconductor, in contrast with the wellstudied Pauli blockade which requires odd occupations. Andreev blockade is lifted if quasiparticles are allowed to enter the superconducting lead, but it should be observable in the hard gap superconductorsemiconductor devices. Andreev blockade should be considered in the design of topological quantum circuits, hybrid quantum bits and quantum emulators. 
Thursday, March 7, 2019 3:18PM  3:30PM 
V08.00005: Transport in superconductor/ferromagnet multilayers with Néeltype domain walls Xavier Palermo, Anke Sander, Sophie D'Ambrosio, Salvatore Mesoraca, K Bouzehouane, Sophie Collin, Nicolas Reyren, Vincent Cros, Javier Villegas We experimentally investigate the coupling between magnetization reversal and magnetotransport in the metallic superconductor/ferromagnet multilayers. The ferromagnet consists itself of a multilayer in which the materials’ stack (Co, Pt, Ir and Ru) is engineered in order to tailor i) the magnetic domains morphology and ii) their size. These variables have a striking effect on the magnetotransport properties below the superconducting critical temperature. In particular, in the presence of magnetic domains the mixedstate resistance is radically diminished to an extent that depends on the domains structure morphology. Contrarily, when the ferromagnet is saturated, an excess resistance is observed. Furthermore, and interestingly, those changes in the longitudinal resistance are accompanied by an anomalous transverse resistance which strongly depends on the applied magnetic field direction and magnetic history. We will discuss the mechanisms contributing to that behavior, which include Pearl vortex pinning and generation, as well as inhomogeneous superconductivity induced by the magnetic template. 
Thursday, March 7, 2019 3:30PM  3:42PM 
V08.00006: Anisotropic purity of entangled photons from Cooper pairs in heterostructures HaeYoung Kee, Jacob S. Gordon

Thursday, March 7, 2019 3:42PM  3:54PM 
V08.00007: Subgap states and Dynes parameter in hardgap tunnel junctions Christian Scheller, Mario Palma, Lucas Casparis, Taras Patlatiuk, Luca Chirolli, Anna V Feshchenko, Dario Maradan, Matthias Meschke, Jukka P Pekola, Dominik Zumbuhl Normal metal – insulator  superconductor (NIS) tunnel junctions are the basic building blocks for Josephson junctions, superconducting qubits and many other applications. Ideally, the single particle current is zero within the superconducting gap; while photon assisted tunneling (PAT) can lead to subgap currents (Dynes parameter). Here, we examine hard gap NIS tunnel junctions in a well filtered and shielded environment where PAT is negligible. We observe discrete, sharp current steps in the subgap regime which can be used as a primary thermometer, in excellent agreement with regular NIS thermometry [1], but cooling as low as 4 mK on a nuclear refrigerator [2]. In an inplane magnetic field, the steps exhibit Zeeman splitting with g = 2 and display diamagnetic shifts. We model the steps as geometric resonances within the weakly disordered normal metal giving enhanced Andreev reflection due to multiple reflections. The model shows that disorder is a possible microscopic origin for Dynestype linear leakage current. 
Thursday, March 7, 2019 3:54PM  4:06PM 
V08.00008: Induced superconducting gap vs. barrier thickness in hybrid Al/Al_{0.15}In_{0.85}As/InAs heterostructures Saeed Fallahi, Ray Kallaher, Sergei Gronin, Candice Thomas, Tian Wang, Geoffrey C. Gardner, Georg W. Winkler, Jan Gukelberger, John Gamble, Andrey Antipov, Roman Lutchyn, Antonio Fornieri, Alexander Whiticar, Asbjorn C. C. Drachmann, Eoin C O'Farrell, Fabrizio Nichele, Charles M Marcus, Michael Manfra The two dimensional electron gas (2DEG) in InAs proximitized by aluminum (Al) is a promising platform for scaling topological qubits based on Majorana zero modes. The 2DEG lies in an InAs quantum well and is separated from the epitaxial Al layer by a barrier of Al_{0.15}In_{0.85}As with thickness d. Due to hybridization between the wave functions of 2DEG and superconductor, the strength of induced gap in the 2DEG will largely depend on the barrier thickness. We present a systematic study of the strength of the induced gap in hybrid Al/Al_{0.15}In_{0.85}As/InAs superconductor/semiconductor heterostructures as a function of barrier thickness. We estimate the induced gap by analyzing multiple Andreev reflections in SNS junctions and differential conductance measurement in SQPCN junctions in the tunneling regime. We compare our results with selfconsistent calculations of induced gap for our particular geometry. 
Thursday, March 7, 2019 4:06PM  4:18PM 
V08.00009: Interaction of Skyrmions and Pearl Vortices in SuperconductorChiral Ferromagnet Heterostructures Samme Dahir, Anatoly F. Volkov, Ilya Eremin We investigate a hybrid heterostructure with magnetic skyrmions (Sk) inside a chiral ferromagnet 
Thursday, March 7, 2019 4:18PM  4:30PM 
V08.00010: Tunneling Anomalous Hall Effect (TAHE) in Ferromagnet/Superconductor Junctions Andreas Costa, Alex Matos Abiague, Jaroslav Fabian The competition of two antagonistic interactions, spinsinglet superconducting pairing and ferromagnetic exchange, in one heterojunction leads to extraordinary phenomena. Owing to the additionally broken inversion symmetry, not only the interplay of superconductivtity and ferromagnetism, but also the induced strong interfacial spinorbit fields (SOFs) offer interesting subjects for experimental and theoretical investigations; several studies unraveled an intriguing impact of interfacial SOFs on transport already in normalconducting systems, e.g., TAMR^{1} and TAHE^{2} effects. Our theoretical work focuses on ferromagnet/superconductor junctions, demonstrating the existence of a superconducting TAHE effect. While the effect's fundamental characteristics are comparable to what has been found in the normalconducting analog^{2}, our numerical simulations predict a much larger tunability of the TAHE conductance in the superconducting scenario, mostly due to the presence of Andreev reflection. Together with a simultaneously generated transverse supercurrent response in the superconductor, these findings might offer an interesting future perspective for experimentalists. 
Thursday, March 7, 2019 4:30PM  4:42PM 
V08.00011: Optimizing spintriplet supercurrent though a ferromagnetic Josephson junction Victor Aguilar, Joseph A Glick, Reza Loloee, William P Pratt, Norman Owen Birge Ferromagnetic Josephson junctions show promise for application to energy efficient cryogenic memory [1]. Both spinsinglet and spintriplet supercurrents are being studied by our group for this purpose. Engineering adjacent F layers in a threelayer system to have perpendicular magnetizations allows singlet pairs to convert to spinaligned triplet pairs. Recent work in our group exploited a synthetic antiferromagnet (SAF) with perpendicular magnetic anisotropy (PMA) for the middle F layer [2]. These junctions exhibit phase control [2], but have a low critical current when compared to singlet junctions. To make the triplet junction a viable option for memory we show that removing the SAF while maintaining the PMA increases the critical current. To estimate the fraction of the supercurrent carried by spintriplet pairs in our junctions, we also fabricate comparison junctions with the order of the F layers shuffled to suppress generation of spintriplet pairs. 
Thursday, March 7, 2019 4:42PM  4:54PM 
V08.00012: Geometry and pairingsymmetry effects in superconductor/halfmetallic ferromagnet vertical junctions Salvatore Mesoraca, Sophie D'Ambrosio, Xavier Palermo, David SánchezManzano, Fabian Cuellar, Sophie Collin, Anke Sander, Jacobo Santamaria, Javier Villegas The interactions between superconductors (S) and ferromagnets (F) are widely studied nowadays due to the possibility of obtaining equalspin triplet pairing, which is immune to the exchange field and therefore can propagate over long distances into the F. This effect has fundamental interest as well as a potential for spintronic applications. In this context, we study vertical S/F/S junctions made of a halfmetallic F (La_{0.7}Sr_{0.3}MnO_{3} or La_{0.7}Ca_{0.3}MnO_{3}) with either a dwave S (YBa_{2}Cu_{3}O_{7}) or a swave one (Mo_{80}Si_{20}), aiming to understand to role of the pairing symmetry on the proximity effect and, particularly, on the generation of equalspin triplet correlations. We will compare lowtemperature magnetotransport measurements made in vertical microjunctions having different geometries, namely threeprobe vs. a novel fourprobe junction scheme, which is designed to separate conductance features linked to the top electrode’s contact resistance from effects due to transport across the entire S/F/S junction. 
Thursday, March 7, 2019 4:54PM  5:06PM 
V08.00013: Transport in ferromagnetic/superconducting heterostructures Evan Moen, Oriol T Valls We consider the transport theory in nanoscale ferromagnetic/superconducting heterostructures with multiple ferromagnetic layers. We do so using fully self consistent, numerical methods in the clean limit. Many properties unique to these systems arise due to the proximity effects between ferromagnets and superconductors. We determine the conductance in each spin channel and analyze its dependence on the magnetic misalignment angle between the ferromagnets, as well as on other physical parameters pertinent to multilayered systems. We quantitatively describe some of these properties and we discuss our results in connection to proposed experiments. 
Thursday, March 7, 2019 5:06PM  5:18PM 
V08.00014: Thermoelectric effects in superconductorferromagnetic hybrids Kirsten Blagg, Zachary Parrott, Michael P Lilly, Meenakshi Singh The conflicting spin order in superconductorferromagnetic systems has lead to a host of facinating phenomena. However, most experimental studies have focused on electrical and spin transport measurements. While there have been numerous theortical predictions of exciting thermal phenomena, thermal properties of these systems remains largley unexplored. In particular, thermoelectric effects which are usually negligible in superconductors have been predicted to increase dramatically in superconductor hybrid structures when in the presence of a spin splitting exchange field. This presentation will focus of the experimental design and measurement of the Seebeck coeffient in superconductorferromagnetic hybrids. 
Thursday, March 7, 2019 5:18PM  5:30PM 
V08.00015: Highcriticalfield superconducting heterostructures using anodic oxidation Asbjørn Drachmann, Henri J Suominen, Alex R Hamilton, Sergei Gronin, Tian Wang, Geoffrey Gardner, Candice Thomas, Alexander Whiticar, Antonio Fornieri, Michael Manfra, Charles M Marcus, Fabrizio Nichele Insitu growth of Al on top of shallow InAs 2DEG heterostructures gives close to perfect proximity effect [12]. The transparent super/semi interface combined with customizable 2DEG lithography hold promise to many interesting applications, e.g. topological quantum computation [34]. 
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