Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F61: Superconductivity: Josephson & Tunneling-IRecordings Available
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Sponsoring Units: DCMP Chair: Sergeri Urazhdin, Emory University Room: Hyatt Regency Hotel -Field |
Tuesday, March 15, 2022 8:00AM - 8:12AM |
F61.00001: Josephson Junctions with a CoGd Compensated Ferrimagnet Barrier Alexander E Madden, Josh C Willard, Reza Loloee, Norman O Birge We have studied the rare earth-transition metal ferrimagnet Co1-xGdx as the barrier in ferromagnetic Josephson junctions. At a particular thickness, the Co and Gd moments cancel leading to a zero-moment compensation point [1]. Due to differing participation of Co and Gd, the transport current is spin-polarized even at the compensation point [2]. This suggests that CoGd could be used for applications in spintronics and superconducting electronics requiring low magnetic moment but spin-polarized current. One potential application is in the development of a π Josephson junction with low moment and stray field. We present SQUID VSM data characterizing the compensation point in thin films at cryogenic temperatures. We also show transport data over a range of CoGd thicknesses that indicate the presence of a 0 - π transition at 1.2 nm. |
Tuesday, March 15, 2022 8:12AM - 8:24AM |
F61.00002: Josephson junctions containing Ni/Ru/Ni synthetic antiferromagnets Swapna Sindhu Mishra, Reza Loloee, Norman O Birge Ferromagnetic Josephson junctions with reliable control over the ground-state phase difference have applications in cryogenic memory. Phase control has been successfully demonstrated with devices containing a Ni fixed layer and a NiFe free layer [1,2]. However, the magnetic reliability and critical current magnitude of these junctions need to be improved. One idea is to replace the thin Ni fixed layer with a thicker unbalanced Ni/Ru/Ni synthetic antiferromagnet (SAF). In this work, we take the first step and measure supercurrent transmission through balanced Ni/Ru/Ni SAFs [3]. We observe that the supercurrent decays remarkably slowly, suggesting that the transport through the SAFs could be partially ballistic. |
Tuesday, March 15, 2022 8:24AM - 8:36AM |
F61.00003: Majorana bound states and parity transitions in S-TI-S lateral Josephson junctions Guang Yue, Xiong Yao, Deepti Jain, Jisoo Moon, Seongshik Oh, Dale J Van Harlingen It is predicted that lateral Josephson junctions fabricated by depositing electrodes of a conventional superconductor (S) on the surface of a 3-D topological insulator (TI), exhibit Majorana bound states (MBS) at the cores of the Josephson vortices. Here we present experimental results and compare to modeling of MBS features in Nb-Bi2Se3-Nb junctions. We show evidence of MBS including node-lifting and features in the critical current diffraction patterns at certain magnetic fields where the MBS enter the junction. Features of MBS may also be captured in asymmetric dc SQUID (Superconducting Quantum Interference Device) measurements designed to measure the current-phase relation of the junctions. We also report progress on experiments to probe the parity states of MBS pairs by studying the critical current distributions of the S-TI-S junctions. Data and comparison with simulations show that such measurements can provide information on MBS parity states and their lifetimes, which is crucial for applications of MBS based topologically protected quantum computing. |
Tuesday, March 15, 2022 8:36AM - 8:48AM |
F61.00004: Josephson effect in graphene-based multi-terminal Josephson junctions Fan Zhang, Asmaul Smitha Rashid, Wei Zhang, Krishnan Mekkanamkulam Ananthanarayanan, Run Xiao, Sang Yoo, Quyen T Tran, Tom Jackson, Nitin Samarth, Morteza Kayyalha A multi-terminal Josephson junction (MTJJ) consists of n (>2) independent superconductors coupled via a normal conductor. The supercurrent flowing in a MTJJ is a function of the (n-1) independent phase differences between the terminals. The phases act as quasimomenta and the resulting band structure may exhibit topological states and Weyl nodes in (n-1) dimension. A prior experiment (Phys. Rev. X 10, 031051) used InAs-based MTJJs involving multiple two-terminal Josephson junctions (2TJJs) coupled through a normal region. Here we studied a different geometry wherein Al terminals are edge-contacted to graphene without forming any 2TJJs. We measured the critical current contours (CCCs) and multiple Andreev reflections as functions of magnetic field and carrier concentration. In contrast to the prior results, we observed that the CCC expands (shrinks) with increasing carrier concentration (magnetic field) without deformation. We will discuss possible interpretations of our results. |
Tuesday, March 15, 2022 8:48AM - 9:00AM |
F61.00005: Triplet current Josephson junctions of ferromagnetic halfmetallic (La,Sr)MnO3 and low-Tc NbTi Jan Aarts, Junxiang Yao, Remko Fermin, Kaveh Lahabi Superconducting junctions with a ferromagnet as weak link, where the supercurrent is carried by triplet correlations, have been much studied in the last decade. Mostly, such junctions consist of simple metals or alloys such as Nb and Co. Generating a long range triplet current requires some form of inhomogeneous magnetization at, or close to, the ferromagnet / superconductor interface, which is brought about by using a second and different ferromagnet in the stack of layers. Junctions made with oxide ferromagnetic metals such as perovskite manganites have been much less studied, and those only in all-oxide combination with perovskite superconductors, in particular YBa2Cu3O7. Here we demonstrate clear supercurrents in lateral Josephson junctions consisting of ferromagnetic and fully spin-polarized La0.7Sr0.3MnO3 (LSMO), superconducting NbTi, and a trench made by Focused Ion Beam milling. The trench width, and therefore the junction length, was of the order 20 nm, larger than what was studied in all-oxide systems. When applying an out-of-plane magnetic field, bar shaped junctions show a Fraunhofer-type interference pattern, while disk-shaped junctions show stronger currents along the edge of the disk. We find Josephson couplings (IcRN products) around 50 μV, which is large compared to all-metal junctions. The half metal only allows a triplet current, and thereofore it is surprising that no extra ferromagnet is needed to generate the supercurrent. We surmise that sufficient magnetic inhomogeneity exists in the the LSMO layer close to the interface with NbTi to work as triplet generator. Combinations of oxide magnets with alloy superconductors appear a promising new route to realizing superconducting spintronics. |
Tuesday, March 15, 2022 9:00AM - 9:12AM |
F61.00006: Coexistance and tuning of spin-triplet superconductivity in spin-filter Josephson junctions Halima Giovanna Ahmad The Josephson effect in ferromagnetic Josephson junctions (SFS JJs) is a powerful tool to investigate intriguing processes occurring because of the interplay between the competition superconducting and ferromagnetic order parameters. Among these physical phenomena, the possibility to generate dissipationless spin-triplet supercurrents captured the attention of the spintronics and superconductivity research community, and a strong effort has been given to discuss the motivations and the conditions for which spin-triplet supercurrents are induced. We here propose a novel complementary study of unconventional spin-triplet mechanisms in high-quality tunnel-SFS JJs, made of an insulating ferromagnetic (FI) barrier (GdN) between two S-wave NbN electrodes, which involves the use of a microscopic theoretical approach. The goal is to give a detailed study of the unconventional thermal behavior of the critical current Ic in JJs with different FI barrier thicknesses, reported in previous works, and for the first time in presence of an external magnetic field. We report the demonstration of competition between spin-singlet and spin-triplet transport mechanisms, we discuss the physical mechanisms involved in the spin-triplet generation and we propose a technique to estimate the relative weight between spin-singlet and triplet correlation functions, which disentangles from the more standard study of the IcRN product of the JJ as a function of the interlayer thickness. Finally, we experimentally demonstrate how to tune the amount of spin-triplet supercurrents through weak external magnetic fields, thus giving a fundamental step to use such high-quality spin-filter JJs in superconducting spintronics devices. |
Tuesday, March 15, 2022 9:12AM - 9:24AM |
F61.00007: Half-integer Shapiro steps in strong ferromagnetic Josephson junctions. Liangliang Guo, YUNYAN YAO, Ranran Cai, See-Hun Yang, Wenyu Xing, Yang Ma, Michiyasu Mori, Yuan Ji, Sadamichi Maekawa, Xin-Cheng Xie, Wei Han Ferromagnetic Josephson junctions have been considered to be potential building blocks for quantum-flux qubits. In this talk, I will show the experimental results of half-integer Shapiro steps in the strong ferromagnetic Josephson junction (Nb-NiFe-Nb) by investigating the current-phase relation under radio-frequency microwave excitation. The half-integer Shapiro steps are robust in a wide temperature range from T = 4 to 7 K. The coexistence of 0 and π states in the Josephson junctions with the spatial variation of the NiFe thickness might contribute to the half-integer Shapiro steps and this scenario is supported by the high-resolution transmission electron microscopy characterization. Our results might help the applications of FM Josephson junction-based flux qubit, and for future investigation of the quantum superposition states. |
Tuesday, March 15, 2022 9:24AM - 9:36AM |
F61.00008: I-V characteristics of SNS Junctions Tony Liu, Boris Spivak, Anton Andreev We develop a theory of the I-V characteristics of superconductor-normal metal-superconductor (SNS) junctions. At sufficiently small voltages the conductivity can be expressed in terms of the superconducting phase dependence of the density of states in the normal region of the junction. In this regime, the conductivity isproportional to the inelastic relaxation time which, in typical situations, is much larger than the elastic one. At large voltages the conductivity of the junction is proportional to the elasticrelaxation time. In the presence of parallel to the film magnetic field and Rashba spin-orbit coupling the I-V characteristics of the junction has a part which is linear in the magnetic field and even in applied voltage. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F61.00009: Long range second harmonic Josephson coupling across a half metallic ferromagnet David Sanchez-Manzano, Salvatore Mesoraca, Fabian Cuellar, Mariona Cabero, Victor Rouco, Xavier Palermo, Adrian Balan, Lourdes Marcano, Sergio Valencia, Anke Sander, Mirko Rocci, Javier G Barriocanal, Fernando Gallego, Alberto Rivera, Federico Mompean, Mar Garcia-Hernandez, Jose Maria Gonzalez-Calbet, Carlos Leon, Cheryl Feuillet-Palma, Nicolas Bergeal, Alexander Buzdin, Jerome Lesueur, Javier E Villegas, Jacobo Santamaria The Josephson effect results from the coupling of two superconductors across a weak link or spacer to yield a phase coherent quantum state. In ferromagnets, singlet (opposite-spin) Cooper pairs decay over very short distances, and thus Josephson coupling requires a nanometric spacer. In systems with particular properties, however, equal spin triplet Cooper pairs can be generated, allowing the coupling of superconductors across magnetic barriers over much longer distances. We demonstrate extremely long range high-temperature Josephson coupling across the half-metallic manganite La0.7Sr0.3MnO3 combined with the superconducting cuprate YBa2Cu3O7. This is shown in planar junctions which display the hallmarks of Josephson physics: critical current oscillations (Fraunhofer pattern) and quantum phase locking under microwave excitation (Shapiro steps). Interestingly, both in the Fraunhofer pattern and in the Shapiro steps, a dominating second harmonic can be observed, which further confirms the exotic superconducting state that can be present in this system. [1-3] |
Tuesday, March 15, 2022 9:48AM - 10:00AM |
F61.00010: Curvature-induced long-range supercurrents in diffusive superconductor-ferromagnet-superconductor Josephson junctions with a dynamic 0−π transition Tancredi Salamone, Mathias B. M. Svendsen, Morten Amundsen, Sol H Jacobsen Including superconducting components in spintronic devices can improve spin-based data processing by providing dissipationless currents. For impurity-robust conventional s-wave superconductors, a long-ranged proximity effect is typically achieved by generating long-range triplet correlations via magnetic inhomogeneities or intrinsic spin-orbit coupling. We report that spin supercurrents can be induced in diffusive superconductor-ferromagnet-superconductor Josephson junctions without any magnetic misalignment or intrinsic SOC. Instead, the pathway to spin-triplet generation is provided via geometric curvature, and results in a long-range Josephson effect. In addition, the curvature can induce a dynamically tunable 0-π transition in the junction. This switching of current direction is therefore testable in a single sample. These results were first reported in Phys. Rev. B 104, L060505 (2021). |
Tuesday, March 15, 2022 10:00AM - 10:12AM |
F61.00011: Ferromagnetic Josephson memory elements with perpendicular magnetic anisotropy Nathan Satchell, Philippa Shepley, Gavin Burnell Traditionally considered competing phenomena, when artificially juxtaposed a wealth of new physics at the interface between superconductors and ferromagnets emerges. By combining ferromagnetism with the dissipationless property of superconductivity, superconducting spintronic memory and logic devices have potential to be both fast and highly energy efficient. |
Tuesday, March 15, 2022 10:12AM - 10:24AM |
F61.00012: Absence of supercurrent in edge-free Corbino graphene Josephson junction in the quantum Hall regime Seong Jang, Gil-Ho Lee, Sein Park, Jinho Park, Kenji Watanabe, Takashi Taniguchi The first observation of the supercurrent in graphene Josephson junction in the quantum Hall regime has attracted considerable attention, which demonstrated the hybridization of two seemingly incompatible phases: quantum Hall state and superconducting state. However, the mechanism of the quantum Hall supercurrent is still controversial due to not exactly quantized resistance at quantum hall plateau. This suggests that the supercurrent may flow through not perfectly insulating bulk state instead of quantum Hall edge states. In order to specify this issue, we fabricated and measured graphene Josephson junction both in conventional rectangular geometry with edges and Corbino geometry without any edge. Highly transparent molybdenum/rhenium superconducting contacts are realized to achieve Josephson coupling as high as at zero magnetic field in both rectangular and Corbino geometry. In the quantum Hall regime, supercurrent was observed in rectangular graphene Josephson junction with edges, however, supercurrent was absent in edge-free Corbino graphene Josephson junction. Our results strongly support that the supercurrent observed in quantum Hall regime flows along the edges of graphene. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F61.00013: Gate Tunable Supercurrent in Lateral Josephson Junctions Based on Epitaxial Topological Insulator Thin Films Ruoxi Zhang, Yifan Zhao, Lingjie Zhou, Run Xiao, Zijie Yan, Deyi Zhuo, Nitin Samarth, Moses H Chan, Cui-Zu Chang A topological insulator (TI) coupled to a superconductor is predicted to harbor an unusual form of superconductivity known as topological superconductivity (TSC). Over the past decade, the possibility of realizing the excitation of TSCs, i.e. Majorana zero modes, has generated much excitement, mainly due to its potential use in fault-tolerant topological quantum computations. The Majorana physics in TI/superconductor hybrid structures can be probed by fabricating a Josephson junction device in which the Dirac surface states of the TI layer serve as a weak connection. To date, most TI-based Josephson junctions are based on exfoliated TI flakes. However, for device fabrication and potential application in topological quantum computations, the molecular beam epitaxy (MBE)-grown wafer-scale TI films are much preferred. In this talk, we will introduce our recent progress on fabricating the lateral Josephson junction devices based on our MBE-grown (Bi,Sb)2Te3 films. By performing systematic measurements, we demonstrated that the induced superconductivity indeed arises in the TI’s Dirac surface states and found that the Josephson supercurrent can be altered by varying the bottom gates of the TI layer. Our findings advance the fundamental inquiries into Majorana physics in TI/superconductor hybrid devices. |
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