Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K09: Superconducting Critical Current and Vortex Dynamics 
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Sponsoring Units: DCMP Chair: Timir Datta, University of South Carolina Room: BCEC 151A 
Wednesday, March 6, 2019 8:00AM  8:12AM 
K09.00001: WITHDRAWN ABSTRACT

Wednesday, March 6, 2019 8:12AM  8:24AM 
K09.00002: Tuning vortex fluctuations and the resistive transition in superconducting films with a thin overlayer Alexander Gurevich It is shown that the temperature of the resistive transition T_{r} of a superconducting film can be increased by a thin superconducting or normal overlayer due to an "antiproximity effect" which manifests itself in an initial increase of T_{r}(d_{2}) with the overlayer thickness d_{2} followed by a decrease of T_{r}(d_{2}) at larger d_{2}. The nonmonotonic dependence of T_{r}(d_{2}) resulting from the interplay of vortex fluctuations and the conventional proximity effect, was obtained by solving the Usadel equations to calculate the BKT transition temperature and the temperature of the resistive transition due to thermallyactivated hopping of vortices. The model may explain the nonmonotonic dependence of T_{r}(d_{2}) observed on (Ag, Au, Mg, Zn)coated Bi films, Agcoated Ga and Pb films or NbN and NbTiN films on AlN buffer layers. The transition temperature can be optimized by tuning the overlayer parameters, which can significantly weaken vortex fluctuations and nearly restore the meanfield T_{c}.These results suggest that bilayers can be used as model systems for systematic investigations of optimization of fluctuations in superconductors. 
Wednesday, March 6, 2019 8:24AM  8:36AM 
K09.00003: Superconductivity in a disordered vortex lattice Amit Ghosal, Anushree Datta, Anurag Banerjee, Nandini Trivedi Orbital magnetic field, as well as disorder, weaken superconductivity of a typeII superconducting film when acting individually. The Abrikosov vortex lattice, resulting from magnetic field, melts with the increasing field strength turning the superconductor into a metal. On the other hand, disorder causes a twodimensional superconducting film to undergo a transition to an insulating state beyond a critical strength of disorder. Here we show that the simultaneous presence of these perturbations leads to an interesting evolution of superconductivity. We demonstrate that the local superconductivity strengthen due to a selfconsistent spatial reorganization of order parameters. At weak disorder strengths, the same critical field that collapses the superconducting energy gap which is also responsible for the vanishing of the superfluid density. However, these two critical fields diverge from one another at large disorder strengths. In addition, disorder is found to distort the Carolide GennesMatricon bound state, which in the clean system features a strong zerobias peak in the local density of states (LDOS) at the vortexcore. This peak disappears featuring a dip in LDOS at core, even for weak disorders. We comprehend such behavior and discuss possible experimental signatures. 
Wednesday, March 6, 2019 8:36AM  8:48AM 
K09.00004: Flux flow solution as a boundary value problem of the Timedependent GinzburgLandau equation Shunki Sugai, Noriyuki Kurosawa, Yusuke Kato We study force on vortices in type II superconductors (SC) with finite GinzburgLandau parameter κ within the generalized timedependentGinzburgLandau (TDGL) equations developed by WattsTobin, Krähenbühl Kramer (1981) for dirty swave SC.While earlier studies addressed the flow conductivity only via the solvability condition for the flux flow problem, we calculate the full solution for boundary value problem with the Meissner current state as the asymptotic state in a distant region from the vortex core. We find transport current j_{t} which appears in the forcebalance equation is not the local (actual) current through the vortex core, but the extrapolated value at the vortex center defined from the boundary condition. We also calculate the hydrodynamic force, magnetic Lorentz force, environmental force and the driving force, which is the sum of the hydrodynamic force and magnetic Lorentz force, on the basis of the full solution of TDGL equation. We find that only the sum of the hydrodynamic and magnetic Lorentz force is eligible as a welldefined driving force on vortex. We also identify the radius of dissipation region around the vortex core as “the radius of vortex” from the viewpoint of defining the force on vortex. 
(Author Not Attending)

K09.00005: Attractive interaction between superconducting vortices in tilted magnetic fields Hermann Suderow Practical applications of superconductors need layered very anisotropic or twodimensional materials and magnetic fields applied on an arbitrary direction with respect to the layers. Then, Cooper pair currents can only circulate either fully within or perpendicular to the layers. In tilted magnetic fields this creates mutually perpendicular circular currents, leading to Josephson and pancake vortices, with quantized circulation across and within layers. Vortices form intertwined lattices that have been visualized mostly using noninvasive magnetic imaging techniques, but no direct manipulation has been achieved. Here we present magnetic force microscopy experiments in tilted magnetic fields in $Bi_{2}Sr_{2}CaCu_{2}O_{8}$. We trigger pancake vortex motion in between Josephson vortices, and find that the Josephson vortex lattice is highly manipulable, such that Josephson vortices in different layers can be crossed by the combined action of the tip's force and the rotating magnetic field. This unprecedented malleabilty originates in an attractive component of the interaction between Josephson vortices in tilted magnetic fields caused by an out of equilibrium arrangement of pancake vortices and might be characteristic of strongly anisotropic superconductors in tilted magnetic fields. 
Wednesday, March 6, 2019 9:00AM  9:12AM 
K09.00006: Imaging individual superconducting vortices in amorphous Mo_{0.8}Si_{0.2} by scanning SQUIDontip Lorenzo Ceccarelli, Denis Vasyukov, Marcus Wyss, Giulio Romagnoli, Martino Poggio Understanding vortex pinning and dynamics in superconductors is crucial for the design of devices carrying nondissipative currents. Here, we use a scanning nanometerscale superconducting quantum interference device (nanoSQUID) [1,2] to image individual vortices in a Mo0.8Si0.2 amorphous thin film. MoxSi1x has recently gained prominence in sensitive superconducting singlephoton detectors due to its ease of growth, homogeneity, and high critical temperature (~7 K) [3]. The high flux sensitivity and spatial resolution of our scanning nanoSQUIDontip allow us to study the behaviour of individual vortices in the presence of applied field and current. In particular, we investigate flux pinning by driving vortices through Lorentz forces exerted by applied currents [4,5], hopping between pinning sites, and the expulsion of magnetic flux upon lowering the applied magnetic field. 
Wednesday, March 6, 2019 9:12AM  9:24AM 
K09.00007: Network Science Approach to Confined Vortex Matter Xiaoyu Ma, Wenzhao Li, Zoltan Toroczkai, Boldizsar Janko Previous studies of confined vortex matter typically focus on ground state or a single dynamical process. Although metastable states are less studied, they are important to the dynamics and stability of the system. In order to perform a systematic study of these properties, we propose a method based on a network science approach. As first shown by Stillinger and Weber, the metastable states (vertices) and the transition states between them (edges) form a complex network, serving as a concise representation of the potential energy landscape. By applying this method to confined vortex matter, we study the network properties as a function of vortex number and confinement geometry. We show that the ground state is always at the “core” of this network, which justifies the use of simulated annealing via molecular dynamics simulations, in finding the ground state. Furthermore, from transition properties restricted by network topology, we study the stability of vortex matter as well as identify “magic number” configurations, i.e., ground states with high stability at specific vortex numbers. 
Wednesday, March 6, 2019 9:24AM  9:36AM 
K09.00008: Computer Simulation Study on Confined Vortex Matter Wenzhao Li, Xiaoyu Ma, Boldizsar Janko, Zoltan Toroczkai The socalled magic number configurations have been studied for several decades in a wide variety of systems where confinement is important. Recent progress in nanoscale patterning of superconducting films and single crystals provides unprecedented control of the size and shape of nanoscale superconducting samples and allows for the experimental study of confined superconducting vortex matter. In order to explore theoretically the properties of confined vortex matter, we are using computational methods developed for LennardJones clusters, and we are able to explore the energy landscapes of vortex systems as well. Furthermore, we propose an analysis based on a network representation of the energy landscape, with metastable states being represented by nodes and the transition states between adjacent local minima as edges. We predict the emergence of magic number configuration by studying the metastable states and transition states of a finite number of vortices confined by containers of different geometry. Our results agree well with recent experiments. 
Wednesday, March 6, 2019 9:36AM  9:48AM 
K09.00009: Time analysis of the voltage response created by a critical current pulse of superconducting NbTi nanowires Khalil Harrabi, Abdelkarim Mekki, Jean Paul Maneval We have investigated in time domain the voltage response of superconducting NbTi filaments with different thickness to a supercritical (I>Ic) steppulse of electrical current. The resistive state is created in the filament for exceeding the critical current, where a hot spot is found far below to the critical temperature and a permanent phaseslip center close to Tc. In both cases, the resistive response appears after a certain delay time t_{d }depending upon the temperature and the ratio I/I_{c}(T) which can been analyzed through a TimeDependent GinzburgLandau equation. We found that the experimental data can be fitted by using a gap relaxation time, independently of the sample widths. However, it depends strongly on the filament thickness. Assuming proportionality to sample thickness, this indicates a phonon escape time of about 80 ps/nm for a NbTi film sputtered on polished crystalline Al_{2}O_{3}. 
Wednesday, March 6, 2019 9:48AM  10:00AM 
K09.00010: Critical current and persistent supercurrent in asymmetric superconducting rings Jacob Hudis, Irinel Chiorescu, Peng Xiong Magnetic flux quantization through superconducting rings dictate that they contain a persistent supercurrent which switches direction at halfinteger external flux quanta. The oscillatory persistent supercurrent manifests in critical current and resistance (Little–Parks) oscillations. Recently, it was shown theoretically and experimentally [1] that in an asymmetrically connected superconducting ring, the critical current may exhibit anomalous periodic behavior with external magnetic flux, including discontinuous jumps. Significant inconsistencies exist between experimental and simulation results [1]. We have performed modeling of the critical current in a superconducting ring as a function of geometrical asymmetries. Our simulations indicate that persistent current might not switch direction at the most energetically favorable locations; and the geometric asymmetry could be used to control the switching. Experimentally, asymmetric superconducting rings have been fabricated by electronbeam lithography, and the critical current will be measured and compared with the simulations. The results may find implications in novel superconducting electronics relying on flux quantization such as superconducting qubits and nano SQUIDs. 
Wednesday, March 6, 2019 10:00AM  10:12AM 
K09.00011: Rotational transition, dislocations and domain formation in vortex systems with combined six and 12fold anisotropic interactions Maciej Olszewski, Morten Eskildsen, Charles Reichhardt, Cynthia Reichhardt We introduce a model of vortices in typeII superconductors with a combined 6 and 12fold anisotropy in the interaction potential, motivated by the vortex lattice (VL) phase diagram in MgB_{2} and UPt_{3}. Using numerical simulations we show that the VL undergoes a continuous rotational transition as the ratio of the 6 and 12fold anisotropy is changed, causing the VL to fracture into domains. We explore the structure of domain boundaries, and isolated dislocations present with single VL domains. Furthermore, we calculate the stress field associated with both dislocations and domain boundaries. The simulations provide a real space complement to results obtained from smallangle neutron scattering studies. We discuss how our simulations may be extended to model metastable VL phases observed experimentally, and the kinetics of the metastabletoequilibrium transition. Finally we compare our numerical results to similar work on graphene lattices, skyrmions and colloids. 
Wednesday, March 6, 2019 10:12AM  10:24AM 
K09.00012: Structural transition kinetics and activated behavior in the superconducting vortex lattice Morten Eskildsen, Elizabeth R Louden, Catherine Rastovski, Stephen J Kuhn, Allan Leishman, Lisa DeBeerSchmitt, Charles D Dewhurst, Nikolai D Zhigadlo Structural phase transformations are ubiquitous in solids, exhibiting common features independent of the microscopic properties of a particular material. We show that this commonality extends to phase transition kinetics associated with the superconducting vortex lattice. Using smallangle neutron scattering, we investigated the behavior of a metastable vortex lattice (VL) state in MgB_{2} as it is driven towards equilibrium by an AC magnetic field. This shows an activated behavior, where the AC field amplitude and cycle count are equivalent to, respectively, an effective “temperature” and “time”. The activation barrier increases as the metastable state is suppressed, corresponding to an aging of the VL. Furthermore, we find a crossover from a partial to a complete suppression of metastable domains depending on the AC field amplitude, which may empirically be described by a single free parameter. This represents a novel kind of collective vortex behavior, most likely governed by the nucleation and growth of equilibrium VL domains. 
(Author Not Attending)

K09.00013: Numerical approach for vortex dynamics in the presence of mesoscopic pining centers Ivan Sadovskyy, Roman Lutchyn The efficient and accurate description of superconducting vortex dynamics is required for a number of technological applications extending from microelectronics to dissipationless power transmission lines and powerful magnets. 
Wednesday, March 6, 2019 10:36AM  10:48AM 
K09.00014: Superconducting Nanostructures Grown by Ga^{+} and He^{+} Focused Ion Beam Induced Deposition (FIBID) Rosa Córdoba, Pablo Orús, Alfonso Ibarra, Dominique Mailly, JOSE MARIA DE TERESA NOGUERAS Since the discovery of superconductivity in WC deposits grown by Ga^{+ } Focused Ion Beam Induced Deposition (FIBID) [1], this material has been used as a model system to investigate the interplay of the vortex lattice with geometrical restrictions [2] or with periodic thickness modulations [3, 4]. 
Wednesday, March 6, 2019 10:48AM  11:00AM 
K09.00015: Suppression of dissipation in superconducting nanostrips by parallel magnetic field Andreas Glatz, YongLei Wang, Igor S Aronson, Zhili Xiao, WaiKwong Kwok Common wisdom dictates that an increase in the magnetic field escalates the loss of energy since the number of vortices increases. Here we show that this is no longer true if the magnetic field and the current are applied parallel to each other. Experimental studies on superconducting MoGe nanostrips reveal that the initial dissipative state with increasing magnetic field is followed by a pronounced resistance drop, signifying a reentrance of the superconducting state. Largescale simulations of the 3D timedependent Ginzburg–Landau model indicate that the intermediate resistive state is due to an unwinding of twisted vortices. When the magnetic field increases, this instability is suppressed due to a better accommodation of the vortex lattice to the pinning configuration. Our findings show that magnetic field and geometrical confinement can suppress the dissipation induced by vortex motion. [PNAS 114, E10274 (2017)] 
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