Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A48: Superconductivity: Critical Current and Vortex Dynamics |
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Sponsoring Units: DCMP Chair: Morten Eskildsen, University of Notre Dame Room: Mile High Ballroom 1A |
Monday, March 2, 2020 8:00AM - 8:12AM |
A48.00001: Confined Vortex Matter with Anisotropic Interaction Lihao Yan, Boldizsar Janko, Xiaoyu Ma, Wenzhao Li In this project, we study confined superconducting vortex matter, when Abrikosov vortices are confined into a mesoscopic container. When the container is large, the vortices are arranged in a triangular Abrikosov vortex lattice. In contrast, in a mesoscopic container, vortices are arranged in other geometries strongly influenced by the container symmetry. In order to systematically study the system of confined vortices, we use the Molecular Dynamics (MD) simulation. In this work, we investigate the stability and melting of mesoscopic vortex matter as a function of vortex number, container size, and container geometry. We identify the so-called “magic number” states which correspond to very stable vortex configurations. We investigate the effect of container symmetry and size on these magic number states and differentiate between the so-called angular melting and general melting transitions. Finally, we explore the influence of anisotropic vortex interactions on the structure of mesoscopic vortex matter. |
Monday, March 2, 2020 8:12AM - 8:24AM |
A48.00002: Solid and liquid vortex phases in superconducting Nb/V hybrids Jose Vicent, Victor Rollano, Alicia Gomez, Alvaro Munoz-Noval, Javier del Valle, Mariela Menghini, Marina Calero de Ory, Jose Luis Prieto, Elena Navarro, Elvira Maria Gonzalez We have studied the vortex dynamics in a periodic potential created by local enhancement of superconductivity. This has been achieved by an array of Nb nanodots embedded in a V film of slightly lower critical temperature. A sample of Cu nanodots embedded in a V film is used as witness sample. In the former hybrid (Nb/V) the array of superconducting Nb dots induces: i) vanishing of commensurability effect between the vortex lattice and the Nb array; ii) hysteresis effects of the vortex dynamics in the solid phase; iii) softening of the vortex lattice, and iv) broadening of the liquid vortex phase. These outcomes can be controlled varying the temperature and/or the in-plane applied magnetic field. |
Monday, March 2, 2020 8:24AM - 8:36AM |
A48.00003: Nonlinear Dynamics and Dissipation of a Curvilinear Vortex Driven by a Strong Surface Current Manula Randhika Pathirana Walive Pathiranage, Alexander V Gurevich We report extensive numerical simulations of large-amplitude oscillations of a trapped vortex line subject to a strong ac magnetic field H(t) = H_{0}sin(ωt) parallel to the surface. The power dissipated by an oscillating vortex segment driven by the Meissner current was calculated by taking into account the nonlinear vortex line tension, vortex mass and a nonlinear Larkin-Ovchinnikov (LO) viscous drag force. It is shown that oscillations of trapped vortices perpendicular to the surface can radically change the field dependence of a residual surface resistance R_{i}(H_{0}) due to the LO decrease of the viscous drag coefficient with the vortex velocity. As the frequency increases,the conventional increase of R_{i}(H_{0}) with H_{0} at low ω evolves into a non-monotonic dependence of R_{i}(H_{0}) at larger ω, so that R_{i}(H_{0}) decreases with H_{0} at higher fields. As the electron mean free path on nonmagnetic impurities gets shorter, the field onset of the anomalous decrease of R_{i}(H_{0}) shifts to smaller field, and the drop of R_{i}(H_{0}) with H_{0} becomes more pronounced. |
Monday, March 2, 2020 8:36AM - 8:48AM |
A48.00004: Melting of the 2D vortex lattice in thin a-MoGe films Bal Pokharel, Jasminka Terzic, Dragana Popovic, Surajit Dutta, John Jesudasan, Pratap Raychaudhuri, Illaria MACCARI, Lara Benfatto The vortex lattice in a 2D type II superconductor is expected to melt via a BKT transition that involves an intermediate liquid phase (“hexatic fluid”), which preserves orientational order. Recent experiments combining transport and STM imaging made it possible to identify the hexatic state in a-MoGe thin films, showing that vortex diffusivity in the hexatic fluid state is strongly reduced with respect to the isotropic vortex liquid. To fully characterize the melting transition, we performed systematic transport measurements on weakly disordered, 22 nm-thick a-MoGe films down to 0.020 K and in magnetic fields up to 18 T. Our results unambiguously show that the persistence of orientational correlations in the hexatic phase significantly slows down the vortex motion, leading to vanishingly small flux-flow resistance. This effect is consistent with numerical simulations. |
Monday, March 2, 2020 8:48AM - 9:00AM |
A48.00005: Strong irradiation-induced vortex pinning in (Cu,C)Ba_{2}Ca_{3}Cu_{4}O_{11+}_{d} Wai-Kwong Kwok, Ulrich Welp, Alexei Koshelev, Asghar Kayani, Prashanta Niraula, Yue Zhang, Chengping He, Hai-Hu Wen We report on the strong enhancement of vortex pinning in the 115K-superconductor (Cu,C)Ba_{2}Ca_{3}Cu_{4}O_{11+}_{d }following irradiation with 6 MeV protons. While T_{c }decreases by only ~2K after doses of up to 10^{17} p/cm^{2}, the critical current density J_{c} is enhanced by a factor of 40 in fields above 2 Tesla and at temperatures above 60 K, matching well the range of intended applications. We estimate J_{c} from magnetization hysteresis measurements on polycrystalline samples using the Bean model. The whole-sample J_{c} values, deduced by using the sample size as dimension in Bean’s model, approach 5000 A/cm^{2} at 77 K and 1 T. These values are comparable to those of grain-aligned YBa_{2}Cu_{3}O_{x} intended for trapped-field magnets, underlining the application potential of this material if produced for instance in film form. The evolution of the field dependence of J_{c} with irradiation dose is consistent with a model of strong pinning by large-size inclusions. |
Monday, March 2, 2020 9:00AM - 9:12AM |
A48.00006: Lifting Degeneracy in the Vortex Lattice of MgB_{2} Allan Leishman, Morten Eskildsen, Nikolai D Zhigadlo, Markus Bleuel The superconducting vortex lattice (VL) of MgB_{2} hosts three structural phases associated with |
Monday, March 2, 2020 9:12AM - 9:24AM |
A48.00007: Superconducting vortex dynamics and critical current in mixed pinning landscapes Andreas Glatz, Roland Willa, Ivan Sadovskyy, Alexei Koshelev The transport properties of type-II superconductors are mostly determined by the dynamics of magnetic flux lines (vortices) threading the system and the material's capability to stop their motion. We provide clear evidence that antagonistic spherical and columnar defects can work hand in hand to provide an optimal pinning landscape for tilted magnetic fields. This establishes a new paradigm of mixed pinning optimization, where the search for optimal vortex pinning must span over a range of defect geometries, sizes, and orientations. |
Monday, March 2, 2020 9:24AM - 9:36AM |
A48.00008: Peak effects in NbSe_{2} with artificial pinning centers Tsuyoshi Tamegai, Wenjie Li, Sunseng Pyon, Satoru Okayasu, Ataru Ichinose Irradiations of swift particles into superconductors modify their vortex states and lead to the enhancement of critical current density, J_{c}, via creation of point defects (PDs) or columnar defects (CDs). Such an enhancement of J_{c} was demonstrated in cuprate superconductors [1] as well as in conventional [2] and iron-based superconductors [3]. However, how the J_{c} is enhanced depends strongly on the target material and ion species, energy, and density of swift particles. In some cases, J_{c} is monotonically enhanced, while in other cases, non-monotonic magnetic field dependence of J_{c}, peak effect, is observed after the irradiation. In the present study, we compare the effect of PDs and CDs on the J_{c} - H behavior of NbSe_{2}. With increasing the density of PDs by proton irradiation, the field range for the peak effect close to H_{c2} broadens significantly. In the case of CDs, the J_{c} - H behavior changes sensitively as a function of the direction of CDs and magnetic field with respect to the c-axis. |
Monday, March 2, 2020 9:36AM - 9:48AM |
A48.00009: Large and uniform critical currents in finite magnetic fields Ivan Sadovskyy, Andreas Glatz, Alexei Koshelev, Ulrich Welp, Wai-Kwong Kwok Loss-free superconducting transport is tremendously important for technological and energy applications. Heat dissipation from drifting magnetic vortices is one of the main limiting factors for superconductors usage. Here we discuss defect configurations that trap vortices almost independently of their direction. As a result, such pinning configurations produce near uniform critical current as a function of applied magnetic field angle. To cover the broadest class of pinning landscapes we employ a genetic algorithm varying the size, position, and orientation of each defect individually. We report several notable results and analyse them in details. The highest uniform critical current is 22% of the depairing current at the magnetic field 10% of the second critical field. |
Monday, March 2, 2020 9:48AM - 10:00AM |
A48.00010: New paradigm for a disordered superconductor in orbital magnetic field Amit Ghosal, Anushree Datta, Anurag Banerjee, Nandini Trivedi Orbital magnetic field, as well as disorder, weaken superconductivity when acting individually on a s-wave superconductor. The Abrikosov vortex lattice in a clean type-II superconductor, resulting from an orbital magnetic field, transforms into a metal beyond a critical magnetic field H_{c} once vortices start overlapping. Similarly, disorder drives a transition from a superconductor to a insulator past a critical disorder strength. Here we show that acting simultaneously on a two-dimensional superconductor, disorder and magnetic field lead to an intriguing evolution of the superconducting state. While for weak disorder, the critical field H_{c} for the suppression of superconducting energy gap matches the critical field at which the superfluid density collapses, the two diverge from each other with increasing disorder creating a pseudogap phase. Our results provide a natural explanation of the long standing puzzle of a strong magnetoresistance peak observed as a function of orbital magnetic field in thin disordered superconducting films. Our results also explain why the characteristic Caroli-de Gennes-Matricon zero bias peak in the local density of states in the vortex core of a clean superconductor might be absent in the presence of disorder, as observed in some recent experiments. |
Monday, March 2, 2020 10:00AM - 10:12AM |
A48.00011: Microwave flux-flow Hall effect of superconductors: (1) Method Ryo Ogawa, Fuyuki Nabeshima, Atsutaka Maeda The bound states in the vortex core of superconductors can be classified in terms of the cleanness of the vortex core. High T_{c} cuprate superconductors are expected to be in the clean limit because of the large superconducting gap. However, previous measurements of the effective viscous drag coefficient, η^{*}, for wide range of cleanness found that the vortex cores of high T_{c} cuprates were in the moderately clean limit, as a universal trend [1]. These results may suggest that there exists an extra dissipation mechanism due to the vortex motion which have not been considered so far. However, η^{*} is the mixture of the longitudinal viscous drag coefficient and the Hall coefficient [2]. Thus, η^{*} measurement may not reflect the true electronic states in the vortex core. Therefore, it is necessary to directly measure the flux flow Hall effect and to evaluate the true core states. In order to neglect pinning, we need high frequencies. Thus, we developed a new method for microwave Hall effect measurement using a cross shaped bimodal cavity. In this talk, we will show the detail of the measurement and the analysis. We applied our system to Bi, and obtained a good result. |
Monday, March 2, 2020 10:12AM - 10:24AM |
A48.00012: Microwave flux-flow Hall effect of superconductors: (2) Experiments in Bi_{2}Sr_{2}CaCu_{2}O_{y } Ryo Ogawa, Fuyuki Nabeshima, Atsutaka Maeda We have developed a new system to measure the microwave flux-flow Hall effect [1]. The new method enables us to measure microwave Hall effect of materials with high conductivity at low temperatures. Using the system, we investigate the flux flow Hall effect of high T_{c} cuprate superconductor, Bi_{2}Sr_{2}CaCu_{2}O_{y}. In the vortex state, with increasing magnetic field, Hall angle, tanq, approaches to a constant value at each temperature. We obtained ω_{0}τ ≈2-3 in the low temperature limit (ω_{0} and τ are the level spacing of quasiparticle in the vortex core and the quasiparticle scattering time in the vortex core, respectively), which is larger than the estimated value by the effective viscous drag coefficient η^{*}. In this talk, we will discuss the implication of the results. |
Monday, March 2, 2020 10:24AM - 10:36AM |
A48.00013: Dynamic Penetration Field of Vortices in a Superconductor in a Time-Dependent Magnetic Field Ahmad Sheikhzada, Alexander V Gurevich We address the nonlinear dynamics of penetration of vortices in a superconductor subject to a periodic magnetic field $H(t)=H_0\sin\omega t$ parallel to the surface. The time-dependent Ginzburg-Landau equations for a gapped superconductor were simulated numerically to calculate the frequency and temperature dependencies of the field onset $H_p(T,\omega)$ of vortex penetration at $T\approx T_c$. It is shown that $H_p(T,\omega)$ can exceed the dc superheating field $H_{s}$ at which the Meissner state becomes unstable. Here $H_p(T,\omega)$ increases with $\omega$ and approaches $\sqrt{2}H_{s}(T)$ at $\omega\tau\geq 1$, where $\tau(T)$ is the energy relaxation time of quasiparticles on phonons. We also investigated the effect of surface topographic defects on $H_p(T,\omega)$ and showed that they can substantially reduce $H_p(T,\omega)$ and cause additional power dissipation. |
Monday, March 2, 2020 10:36AM - 10:48AM |
A48.00014: Glassy and plastic vortex creep regimes in superconducting (Y,Gd)Ba_{2}Cu_{3}O_{y} films and coated conductors Leonardo Civale, Boris Maiorov, Ivan Nekrashevich, Masashi Miura Large thermal fluctuations in high T_{c} superconductors give rise to fast vortex dynamics that promotes the time relaxation of the metastable supercurrents, which is detrimental for applications. We are pursuing a general understanding of the lowest achievable flux creep rate (S) for a superconductor at any temperature (T) and magnetic field (H). Initially, we found that there is a universal lower limit for S in the Anderson-Kim (A-K) regime at T<<T_{c} (Eley et al., Nat. Mat. 2017). Later, we expanded our quest to higher T and H outside the A-K limit, where the universality is lost and different regimes occur. We will present results on (Y,Gd)Ba_{2}Cu_{3}O_{y} coated conductors with randomly distributed BaHfO_{3} nanoparticles. These samples exhibit extremely strong vortex pinning, with critical current densities among the highest in any known superconductor. We identify several glassy and plastic dynamics regimes, the boundaries among which are determined either by intrinsic vortex properties or by thickness effects. In particular, we find a thickness-controlled “second A-K regime” at high T, which sets the lowest S(T,H) limit in thin samples in technologically relevant T-H conditions. |
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