Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P31: Superconducting/Magnetic Structures |
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Sponsoring Units: DCMP GMAG Chair: James Eckstein, University of Illinois Room: LACC 407 |
Wednesday, March 7, 2018 2:30PM - 2:42PM |
P31.00001: Microwave Studies of Josephson Junctions Based on InAs Quantum Wells with an Epitaxial Superconductor Gino Graziano, Joon Sue Lee, Mihir Pendharkar, Chris Palmstrom, Vlad Pribiag Two-dimensional electron gases (2DEGs) with strong spin-orbit coupling are expected to host topological states in the presence of superconductivity and broken time-reversal symmetry. Josephson junctions based on such 2DEGs are predicted to exhibit an unconventional current-phase relation, characterized by a large component at twice the Josephson frequency due to the presence of gapless topological Andreev bound states. In order to study the current-phase relation of these bound states, here we characterize the microwave response of Josephson junctions based on InAs quantum wells proximitized with epitaxial Al. |
Wednesday, March 7, 2018 2:42PM - 2:54PM |
P31.00002: Odd-frequency superconducting pairing in SNS junctions with spin-orbit coupling Jorge Cayao, Annica Black-Schaffer We study the emergence of disorder-robust s-wave odd-frequency superconducting pairing in superconductor-normal metal-superconductor (SNS) junctions with spin-orbit coupling (SOC), including topological insulator surface states. We perform a fully analytical investigation based on Green’s functions and analyze the consequences of such exotic pairing on the local density of states and supercurrents. Furthermore, we discuss potential scenarios for the enhancement of odd-frequency superconducting pairing and its experimental realization. |
Wednesday, March 7, 2018 2:54PM - 3:06PM |
P31.00003: Revealing the role of magnetic domains in superconducting tunneling spectroscopy of EuS/Al bilayers. Elia Strambini, Vitaly Golovach, Giorgio De Simoni, Jagadeesh Moodera, Fernando Bergeret, Francesco Giazotto A renewed interest in studying Ferromagnetic/Superconductor structures came with the development of superconducting spintronics [1]. Ferromagnetic insulators (FIs) attached to a superconductor are known to induce triplet superconducting pairing and an exchange energy splitting in the Bardeen-Cooper-Schrieffer (BCS) density of states [2]. |
Wednesday, March 7, 2018 3:06PM - 3:18PM |
P31.00004: Supercurrent in ferromagnetic Josephson junctions with heavy metal interlayers Nathan Satchell, Gavin Burnell, Norman Birge For exploring superconductor-ferromagnet proximity effects, Josephson junctions typically take the form: S/N/F/N/S, where S denotes a superconductor, F a ferromagnet and N a normal metal. The role of the N spacer layer is usually to promote the growth of the F layer – which may not be well lattice matched with the S layer. The critical current density of these junctions diminishes over very small thicknesses of F due to the exchange field dephasing the Cooper pairs. By including two additional F’, F” layers in a S/N/F’/N/F/N/F”/N/S junction, singlet Cooper pairs can be converted to spin aligned triplet Cooper pairs, which are not dephased by the exchange field and penetrate deep into the middle F layer. It has been theoretically predicted that replacing the N/F’/N and N/F”/N layers with a single layer NSOC (where NSOC is a heavy metal with Rashba spin orbit coupling) can also generate triplet Cooper pairs. In this work we describe our experimental progress to realize such a junction. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P31.00005: Long range proximity effect in Ni nanowires Jue Jiang, Weiwei Zhao, Renzhong Du, Ludi Miao, Qi Li, Moses Chan We report measurements on the proximity effect of superconducting Nb electrodes on Ni nanowires (500nm to 1μm(length)× 400nm(width)× 40nm(thickness)). The fabrication of the devices was assisted by e-beam lithography. The spatial extent of the proximity effect is found to be on the order of 2 nm. Such a short range is expected due to the competing spin order of ferromagnetism and superconductivity. Interestingly, when a thin (3nm) Cu buffer layer is sandwiched between Nb and Ni, the spatial extent is dramatically increased to between 35 to 130 nm. Subsequent SQUID measurements of a sandwich film of Nb(40nm)/Cu(3nm) on SiO2 substrate show evidence of weak ferromagnetism at 50K. The thermally evaporated Cu film on SiO2, as in same the case for the fabrication of the nanowire devices, was exposed to air before the Nb film was sputtered. One way to understand the proximity effect result is that the buffer CuO layer with weak ferromagnetism facilities the conversion of s-wave superconductivity in Nb into triplet supercurrent along the Ni nanowires1. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P31.00006: Ferromagnetic Josephson junctions incorporating both in and out-of-plane magnetic anisotropy. Victor Aguilar, Joseph Glick, Reza Loloee, William Pratt, Norman Birge Josephson junctions containing multiple ferromagnetic layers are a topic of intense research. In a junction where the magnetizations of all the layers are either aligned parallel or antiparallel, only the spin-singlet electron pairs carry supercurrent. Engineering adjacent F layers to have perpendicular magnetizations allows singlet pairs to convert to spin-aligned triplet pairs. Recent work in our group has shown the conversion to the spin-aligned triplet by the addition of a synthetic antiferromagnet (SAF) with perpendicular magnetic anisotropy (PMA) [1]. In this work, we show that by changing the order in which these layers are stacked inside the junction, one can either optimize or suppress generation of spin-triplet pairs. Such an approach should provide an unambiguous estimate of the fraction of the supercurrent carried by spin-singlet pairs in an otherwise dominant spin-triplet junction. |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P31.00007: Phase-controllable spin-triplet Josephson junctions containing a synthetic antiferromagnet Joseph Glick, Adel Gougam, Victor Aguilar, Reza Loloee, William Pratt, Norman Birge We present the first experimental demonstration of phase-controllable Josephson junctions (JJs) that carry long-range spin-triplet supercurrent. Phase-sensitive detection was achieved by measuring the interference between two such JJs in a Superconducting QUantum Interference Device (SQUID) loop. Spin-triplet supercurrent in the JJs is generated through the use of three ferromagnetic layers with noncolinear magnetizations [1]. The central layer is a synthetic antiferromagnet with magnetization perpendicular to the plane, while the other two ferromagnetic layers have in-plane magnetization, as recently demonstrated [2]. We show that the phase-state of the JJs can be switched between 0 and π over a thousand times without error, opening possibilities for their use in superconducting memory. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P31.00008: Interplay of Ferromagnetism and Superconductivity in Ni nanowires with Nb Contacts Haowen Ren, Sohini Manna, Sheena Patel, Yuxuan Xiao, Eric Fullerton Combining superconductivity and spintronics effects has become a promising area for the next generation electronic devices. Owing to the mismatch of spin-spin coupling states between superconductivity and ferromagnetic materials, Cooper pairs can typically survive only a few nanometers in ferromagnetic materials. However, recent studies showed a long-range proximity effect superconductivity in a crystalline Co nanowire when capped with Pb. Meanwhile, studies showed a strong evidence for the existence of triplet superconductivity in Co-based Josephson junctions. Inspired by their works, we fabricated epitaxial Ni nanowires by CVD and magnetron sputtering deposition followed by ebeam lithography and contacted by Nb leads. RF bias cleaning was applied before Nb contacts deposition. AMR results confirm the single crystallization of Ni nanowires. The results showed that at a certain level of RF bias cleaning before Nb deposition, nanowires with gap around 500 nm between two Nb contacts exhibit zero resistance, which indicates possible supercurrent path. Furthermore, we fabricated weak-link SQUID devices by two Nb wires. The transport measurements revealed an interesting oscillatory behavior with changing applied magnetic field. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P31.00009: Superconducting-Magnetic Proximity Systems in the Clean Limit: Inhomogeneous Magnetization and Itinerant Heterostructures Alberto Garcia, Christian Martens, Luis Leal, Goetz Seibold, Andreas Bill We analyze pair correlations and the magnetic structure in superconducting-magnetic heterostructures in the clean limit. Two models are considered. In the first, we study the effect of doping and impurities on pair correlations in a magnetic trilayer embedded between two superconductors. We focus on the singlet-triplet mixing for different inhomogeneities of the magnetic multilayer and compare the results with the diffusive regime. The second model is a heterostructure which is made from itinerant ferromagnets and superconductors. Contrary to the majority of previous models we do not fix the magnetic configuration in the material but study Hubbard-Heisenberg type single-band models which display itinerant ferromagnetism as a mean-field ground state. This allows us to investigate the influence of adjacent superconducting layers on the properties of the ferromagnetic. |
Wednesday, March 7, 2018 4:18PM - 4:30PM |
P31.00010: The role of canting in superconducting-spin valve structures Thomas Baker, Andreas Bill Superconducting spintronic devices have become a popular conceptual alternative to classical computing, although the development of these technologies is in the early stages. Common designs revolve around various magnetic configurations that compose the interlayer in a Josephson junction. We study the dependence of canting on the Josephson current in a magnetic structure with symmetric and anti-symmetric configurations of trilayers and pentalayers. Fully numerical calculations are justified by a toy model. The two structures are chosen because they display opposite behavior under canting. We analyze the components of the Gor'kov function and how they affect the 0-π transition. A recent classification according to the symmetry of the paired states is used to analyze the results. We also propose a novel feature that is experimentally observable and arises from the interplay of singlet and triplet correlations. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P31.00011: The superconducting absolute spin valve Giorgio De Simoni, Elia Strambini, Jagadeesh Moodera, Fernando Bergeret, Francesco Giazotto A ferromagnetic insulator, such as EuS, in contact with a superconductor drives an exchange field resulting in a splitting of the Bardeen-Cooper-Schrieffer density of states by a magnitude proportional to its magnetization [1]. It was suggested that the combination of two such spin-split superconductors could implement an ideal spin valve [2]: a hybrid ferromagnetic-superconducting tunnel junction where the transport of one of the spin species is open while it is completely blocked for the other. Here we report on a switchable superconducting spin-valve based on two EuS/Al bilayers coupled through a tunnel barrier. Our devices exhibit a relative differential resistance variation -between the parallel and antiparallel configuration of the EuS layers- up to almost 1000%. Our devices implement an efficient spin polarized current source and can be exploited as switching logical elements in a memory cell to be used in cryogenics superconductor-based computers. |
Wednesday, March 7, 2018 4:42PM - 4:54PM |
P31.00012: Bias Current Dependence of Superconducting Transition Temperature in Superconductor/Spin-Valve Nanowires Alejandro Jara, Ilya Krivorotov Superconductor/ferromagnetic (S/F) heterostructures have been a focus of intensive research over the past two decades. Apart from their importance for understanding of fundamental physics of the superconducting proximity effect, S/F nanostructures may find applications in low-power cryogenic computing. Competition between superconducting and ferromagnetic ordering in S/F heterostructures can lead to unusual types of magneto-transport effects emerging from the proximity effect at the S/F interfaces. One example is the spin switch effect observed in S/F/F spin valves near the superconducting transition temperature Tc. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P31.00013: Controllable 0 – π Switching in Ferromagnetic Josephson Junctions for Cryogenic Memory Alexander Madden, Joshua Willard, Joseph Glick, Bethany Niedzielski, Victor Aguilar, Reza Loloee, William Pratt, Norman Birge Josephson junctions containing ferromagnetic layers have generated recent interest for application in cryogenic memory. In a junction containing a magnetically hard fixed layer and soft free layer with specific thicknesses the phase state can be switched between zero and π by changing the relative magnetization of the two layers from antiparallel to parallel [1]. This phase switching has been observed in junctions with Ni fixed layers and NiFe free layers [2]. We present phase-sensitive measurements of such junctions in low-inductance symmetric SQUID loops, which simplify analysis relative to our previous work. We observe controllable switching in multiple junctions with Ni fixed layers of thickness 1.6nm and 2.0nm to begin mapping a phase diagram of zero and π states vs. Ni and NiFe thicknesses. |
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