Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A31: Superconductivity: Vortex Phases and Dynamics |
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Sponsoring Units: DCMP DMP Chair: Steven Anlage, University of Maryland Room: LACC 407 |
Monday, March 5, 2018 8:00AM - 8:12AM |
A31.00001: Vortex matter in clean single crystals of the intermediate-anisotropy superconductor Hg1201 Leonardo Civale, Serena Eley, Mun Chan, Eric Bauer The rich vortex physics in oxide high Tc superconductors (HTS) arises from the strong influence of thermal fluctuations, which produce vortex liquid phases and fast dynamics (large creep rates, S) in the solid phases. The fluctuations effects increase with increasing anisotropy (γ). In moderately anisotropic 3D materials, the strength of the thermal fluctuations is measured by a combination of parameters, the Ginzburg number (Gi). In very anisotropic systems, 2D pancake vortices are formed and fluctuation effects are determined by a different number, Gi2D. Vortex physics in the paradigmatic 3D and 2D HTS— YBCO (γ~5-7) and Bi-based compounds (γ>100), respectively— has been extensively explored. Here we investigate vortex matter in clean Hg1201 single crystals, an ideal intermediate-γ HTS to learn about the interplay of 3D and 2D effects. From the equilibrium magnetization in the liquid phase we calculate Gi and Gi2D. In the solid phase the magnetization loops show a fishtail shape. Pinning at low fields (H) is very low, allowing us to study the single vortex regime in the dilute disorder limit. S is large at low H and decreases as vortex interactions strengthen with increasing H. |
Monday, March 5, 2018 8:12AM - 8:24AM |
A31.00002: Thermal vortex creep and depinning in strong pinning theory Martin Buchacek, Roland Willa, Vadim Geshkenbein, Johann Blatter We study creep-type motion (due to temperature $T>0$) of vortices pinned in type-II superconductors by a low density $n_p$ of strong defects. Kramer's rate theory is used to calculate the thermal transitions between the pinned and free vortex branches characteristic for the strong pinning. In the critical drive region $j\lesssim j_c$, the steady-state branch occupation is derived as a modification of the critical (Bean) state. Transitions over the activation barrier $U(j)=\alpha e_p(1-j/j_c)^{3/2}$ ($e_p$ is the pinning potential depth and $\alpha$ a known numerical) shift the critical current, $1-j_c(T)/j_c(0)\sim (T/e_p)^{2/3}$ and smooth the current-velocity characteristic. Beyond $j_c$, $U(j)$ varies logarithmically and creep results in the shift of the $T = 0$ excess-current characteristic for drives $j<(1+\alpha') j_c(0),\, (\alpha'\propto T/n_p)$ covering a large region if $n_p$ is small. At small drives $j\sim (T/e_p)j_c$, we derive the thermally assisted flux flow (TAFF) characteristic $v = (j/j_c)v_0e^{-U_0/T}$ with activation barrier $U_0=\alpha''e_p$ governing transitions close to the thermal equilibrium. Results are supported by various experiments (Xiao et al., Phys. Rev. B 65, 094511 (2002), Palstra et al., Phys. Rev. Lett. 61, 1662 (1988)). |
Monday, March 5, 2018 8:24AM - 8:36AM |
A31.00003: Dramatic Increase in Vortex Creep Rate with Decreasing Film Thickness in Disordered Superconductors Serena Eley, Roland Willa, Masashi Miura, Michio Sato, Michael Henry, Leonardo Civale In type-II superconductors, the interplay between vortex elasticity, vortex motion, and material disorder is a primary determinant of electronic and magnetic properties, such as creep (thermally activated vortex motion) and the critical current. Here, we present a systematic study of the dependence of the creep rate S on film thickness d in Nb and (Y,Gd)Ba2Cu3O7-x (YGdBCO). We observe a progression from a thickness-independent to a thickness-dependent S with decreasing d, suggestive of a magnetic 3D-to-2D transition. For the Nb film, the critical thickness dc at which this crossover occurs coincides with the collective pinning length Lc, i.e., the size of vortex segments that bend to adjust to the energy landscape provided by point defects. However, for YGdBCO, we find that dc»Lc because the dynamics are strongly influenced by sparse large precipitates within its microstructure. We proceed by leveraging the sensitivity of S to d to determine the effective pinning length in the YGdBCO film and find surprisingly high values, e.g., ~800 nm at T/Tc ~ 0.5 and a field of 1 T. Finally, we show evidence that the dramatic thickness-dependence of creep is due to a change from elastic to rigid vortex dynamics. [S. Eley et al., submitted, arxiv.org/abs/1709.02776] |
Monday, March 5, 2018 8:36AM - 8:48AM |
A31.00004: Dynamics of Driven Vortex Lattice Probed by Microwave Technique Hodaka Kurokawa, Fuyuki Nabeshima, Atsutaka Maeda H. Kurokawa, F. Nabeshima and A. Maeda |
Monday, March 5, 2018 8:48AM - 9:00AM |
A31.00005: Vortex Variable Range Hopping in a Conventional Superconducting Film Ilana Percher, Irina Volotsenko, Aviad Frydman, Boris Shklovskii, Allen Goldman We will present resistance versus temperature data from a highly disordered thin film of superconducting amorphous indium oxide that is tuned through the superconductor-insulator transition by way of a perpendicular magnetic field. In the regime of non-zero field below the critical field, the transition broadens in temperature, which is understood to result from vortex motion across the film. The temperature dependence of the resistance suggests that vortices move by 2D Mott variable range hopping over a range of fields, and several decades of resistance. |
Monday, March 5, 2018 9:00AM - 9:12AM |
A31.00006: Spontaneous disordering of the Vortex Lattice in the chiral phase of UPt3 with H parallel to c. Keenan Avers, Stephen Kuhn, Morten Eskildsen, William Halperin, William Gannon, Jorge Gavilano, Urs Gasser, Gergely Nagy, Charles Dewhurst, Lisa DeBeer-Schmitt The unconventional, topological superconductor UPt3 is suspected to have one chiral superconducting phase. We have performed small angle neutron scattering (SANS) from the vortex lattice (VL) in UPt3 at the Paul Scherrer Institute, ILL, and Oak Ridge National Lab with the magnetic field parallel to the hexagonal c-axis. The VL Bragg peaks in the chiral phase show a time-dependent behavior exhibiting a clear decrease of intensity with time. This spontaneous disordering occurs when the applied magnetic field is left constant, but order can be restored by small oscillations of the applied field. The spontaneous disordering occurs faster in higher quality samples which rules out defect pinning as a cause. The process also occurs faster at higher vortex densities suggesting that vortex-vortex interactions are driving the disorder. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A31.00007: Visualizing the subsurface bending of vortex lattices under tilted magnetic fields in the isotropic superconductor β-Bi2Pd. Edwin Herrera-Vasco, Isabel Guillamon, Jose Galvis, Alexandre Correa, Anton Fente, Sebastian Vieira, Hermann Suderow, Vladimir Kogan In type-II superconductors vortices are quantized magnetic flux lines are usually oriented parallel to the magnetic field and tend to be straight to minimize their elastic energy. For many years, the orientation of vortices when the field is approximately parallel to a surface has been commonly overlooked, particularly for high values of the magnetic field. Here, we use scanning tunneling microscopy to study vortex lattices in the weak-pinning and nearly isotropic superconductor β-Bi2Pd in tilted magnetic fields. We show that vortices exit the sample perpendicular to the surface, even when the magnetic field is parallel to the surface. Thus, vortices are bent beneath the surface. We analyze how the structure and orientation of the tilted vortex lattices in the bulk are strongly affected by Coulomb-type intervortex repulsion at the surface due to the stray (Pearl-like) magnetic fields [1]. |
Monday, March 5, 2018 9:24AM - 9:36AM |
A31.00008: Vortex chains in superconducting nanowires Nina Markovic, Ahmed Ibrahim, Robert Lynn, Leo Sementilli We have studied the superconducting properties of narrow aluminum nanowires in a magnetic field. As the magnetic field is increased, vortices enter the nanowire and self-organize in a periodic one-dimensional lattice. By tuning the width of the nanowire and the magnetic field, we can tune the vortex density, starting from a sparse array to a merging line of vortex cores in the center of the wire. We will discuss the phase diagram and the transport regimes in the current-voltage characteristics as a function of temperature and magnetic field. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A31.00009: Molecular Dynamics Simulations on Melting Transition of Vortex Lattices in Mesoscopic Superconductors Masaru Kato, Osamu Sato Because of weak pinning and strong thermal and quantum fluctuations in cuprate high-Tc superconductors, there is a melting line of a vortex lattice in the H-T phase diagram. Recently, Ooi et al. showed the melting temperature in a mesoscopic superconducting cuprate square plate oscillates with increasing magnetic field. They explained this oscillation comes from the stability of configurations of n2 vortices (n=1, 2, 3, ...) [1]. In order to confirm this explanation, we study the vortex lattice melting in a mesoscopic superconductor, using the molecular dynamics method [2,3]. Temperature dependence is included in the thermal fluctuation force and the penetration depth in the vortex-vortex interaction. In order to determine the melting point, we use the standard deviation of time development of vortex positions. We have found that melting temperature oscillates with increasing vortex number [4]. Also, we will show how vortex lattices melt. |
Monday, March 5, 2018 9:48AM - 10:00AM |
A31.00010: Stability and Magic Number Configurations of Confined Vortex Matter via Network Science Xiaoyu Ma, Zoltan Toroczkai, Boldizsar Janko The ground state and dynamics of vortex matter has been widely studied by molecular dynamics (MD) simulation. However, this method only probes a small region of the whole configuration space. For a systematic study of the configuration stability under different conditions, we propose a novel 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 with graph theoretical properties that correlate with energetic properties, the stability of metastable n-body configurations and the transitions between them. This network mapping provides a reduced degree complexity description of the n-body system, yet detailed enough to capture novel behavior as function of the confinement geometry and the pinning potential configuration. We implement this method to study the stability of vortex matter as well as identify “magic number” configurations, i.e., ground states with extraordinary high stability at specific vortex number. |
Monday, March 5, 2018 10:00AM - 10:12AM |
A31.00011: Structural Transitions in Vortex Systems with Anisotropic Interactions Maciej Olszewski, Morten Eskildsen, Charles Reichhardt, Cynthia Reichhardt We introduce a model of vortices in type-II superconductors with an anisotropic vortex-vortex interaction potential. Using numerical simulations we show that the vortex lattice (VL) undergoes structural transitions as the anisotropy is increased. For a four-fold anisotropy we reproduce the well-known VL evolution from a triangular lattice at low anisotropy, to a rhombic intermediate state, and finally a square lattice for high anisotropy. In some cases a multi-q state is observed, consisting of an Archimedean tiling that combines square and triangular local ordering. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A31.00012: Tuning the Vortex Lattice Phase Diagram of MgB2 through Doping Allan Leishman, Elizabeth De Waard, Soham Manni, P.C. Canfield, John Barker, Lisa DeBeer-Schmitt, Charles Dewhurst, Morten Eskildsen
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Monday, March 5, 2018 10:24AM - 10:36AM |
A31.00013: Post-Synthesis Enhancement of Critical Current in Commercial Grade YBCO Coated Conductors Wai-Kwong Kwok, Matthew Smylie, Ulrich Welp, Shaofei Zhu, Asghar Kayani, Ado Umezawa, Venkat Selvamanickam, Yifei Zhang, Marty Rupich We present critical current measurements on a series of commercial grade YBCO coated conductors. These coated conductors contain a variety of inherent defects such as nanoparticle inclusions, twin boundaries, dislocations and self-aligned nanorods. In addition to these inherent defects, we induce new defects via low and high-energy particle irradiation in the form of point, cluster and columnar defects to probe the additive/non-additive nature of vortex pinning in these coated conductors. We show that commercial grade coated conductors can be further improved by a combination of post-synthesis induced defect landscapes and annealing schedules. |
Monday, March 5, 2018 10:36AM - 10:48AM |
A31.00014: Genetic apporach to targeted evolution of pinning landscapes in type-II supercondctors using large-scale simulations Andreas Glatz, Ivan Sadovskyy, Wai-Kwong Kwok, Alexei Koshelev The critical current in type-II superconductors largely depends on the pinning effectiveness of magnetic vortices. In practice, pinning defects can be grown chemically, e.g., self-assembled nanoparticles and nanorods, or introduced artificially by, e.g., ion irradiation. Here, we present a novel numerical technique which make it possible to determine the pinning landscape having the maximum possible critical current among all arbitrary combinations of defect types. Our approach is based on a genetic algorithm, which can evolve the number of the defects as well as position, size, and shape of each individual defect, in combination with the time-dependent Ginzburg-Landau equation, which is used to calculate the critical current for a given configuration. As an application, we determined the best possible configuration of metallic inclusions in a sample in fixed magnetic field perpendicular to the current. |
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