Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C35: Superconductivity: Vortices I |
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Sponsoring Units: DCMP Chair: Timir Datta, University of South Carolina Room: 343 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C35.00001: Strongly Enhanced Vortex Pinning by Conformal Crystal Arrays Dipanjan Ray, Cynthia Reichhardt, Boldizsar Janko, Charles Reichhardt Conformal crystals are non-uniform structures created by a conformal transformation of regular two-dimensional lattices. We show that gradient-driven vortices interacting with a conformal pinning array exhibit substantially stronger pinning effects over a much larger range of field than found for random or periodic pinning arrangements. The pinning enhancement is partially due to matching of the critical flux gradient with the pinning gradient, but the preservation of the sixfold ordering in the conformally transformed hexagonal lattice plays a crucial role. Our results can be generalized to a wide class of gradient-driven interacting particle systems such as colloids on optical trap arrays. [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C35.00002: Vortex Pinning in Superconducting MoGe Films Containing Conformal Arrays of Nanoscale Holes and Magnetic Dots Y.L. Wang, M.L. Latimer, Z.L. Xiao, L.E. Ocola, R. Divan, U. Welp, G.W. Crabtree, W.K. Kwok Recent numerical simulations by Ray et al. predict that a conformal pinning array can produce stronger vortex pinning effect than other pinning structures with an equivalent density of pinning sites [1]. Here we present experimental investigations on conformal pinning structures. Direct and conformal pinning arrays of triangular and square lattices were introduced into MoGe superconducting films using focused-ion-beam milling or electron-beam lithography. Transport measurements on critical currents and magnetoresistances were carried out on these samples to reveal the advantages of conformal pinnings. Effects of random pinnings with the same average density were also studied for comparison. Details on sample fabrications and effects of pinning types (holes versus magnetic dots) will be presented. Reference: [1] D. Ray, C.J. Olson Reichhardt, B. Janko, C. Reichhardt, arXiv:1210.1229 (2012) Work supported by the US DoE-BES funded Energy Frontier Research Center (YLW), and by Department of Energy, Office of Science, Office of Basic Energy Sciences (MLL, ZLX, LEO, RD, UW, WKK), under Contract No. DE-AC02-06CH11357 [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C35.00003: Matching of the Flux Lattice to Geometrically Frustrated Pinning Arrays J. Trastoy, R. Bernard, J. Briatico, J.E. Villegas, J. Lesueur, C. Ulysse, G. Faini We use vortex dynamics on artificial nanoscale energy landscapes as a model to experimentally investigate a problem inspired by ``spin ice'' systems. In particular, we study the matching of the flux lattice to pinning arrays in which the geometrical frustration is expected to impede a unique stable vortex configuration and to promote metastability. This is done with YBCO films in which the nanoscale vortex energy landscape is fabricated via masked ion irradiation. Surprisingly, we found that minimal changes in the distance between pinning sites lead to the suppression of some of the magneto-resistance matching effects, that is, for certain well-defined vortex densities. This effect strongly depends on the temperature. We argue that this behavior can be explained considering the arrays' geometrical frustration and the thermally activated reconfiguration of the vortex lattice between isoenergetic states. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C35.00004: Critical currents, magnetic relaxation and pinning in Nd$Ba_2$$Cu_3$$O_{7-\delta}$ films with BaZr$O_3$ generated columnar defects A.O. Ijaduola, S.H. Wee, A. Goyal, P.M. Martin, J. Li, J.R. Thompson, D.K. Christen The critical current density $J_c$ and the magnetic relaxation (creep) properties have been studied for a set of $NdBa_2Cu_3O_{7-\delta}$(NdBCO) films doped with $BaZrO_3$ (BZO) nanoparticles to form columnar defects. The dependence of $J_c$ on the magnitude and orientation of the applied magnetic field $H_{app}$ (0-6.5 T) and temperature T (5 K-$T_c$) was investigated. The normalized flux-creep rate $S = -d ln(J)/d ln(t)$ was determined as a function of T. The current dependence of the effective activation energy $U_{eff}(J)$ was derived using the formalism developed by Maley. The results are well described by an inverse power law type barrier of the form $U_{eff}(J)\sim U_0(J_0/J)^{\mu}$ with fitted values for the pinning energy scale $U_0$ and the glassy exponent $\mu$. When comparing values for these parameters in the BZO-doped samples with those for their undoped control counterparts, the most striking difference is the larger scale of current density $J_0$ in the doped samples (a factor of 2.4 higher), while the other pinning parameters do not differ strongly. In the BZO-doped materials, the pinning energy scale $U_0$ increases with vortex density and $J_0$ decreases, with both following simple power law dependences on the field. [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C35.00005: Polaronic pinning of vortex in magnetic superconductors and magnetic-superconducting multilayers Shi-Zeng Lin, Lev Bulaevskii We present a new type of vortex pinning by enhancing the viscosity of vortex in magnetic superconductors with long relaxation time of magnetization and large magnetic susceptibility. In the absence of current, vortices are dressed by nonuniform magnetic polarization and form vortex-polarons. Under a small current and consequently low Lorentz force, the magnetic polarization follows the vortex motion. However, at long magnetic relaxation time of magnetization, there is additional dragging force by the magnetization besides the Bardeen-Stephen one, thus the effective viscosity of vortex is significantly enhanced resulting in suppression of dissipation. For a large current, the magnetic polarization cannot follow the vortex motion and the vortex-polaron dissociates, i.e. the magnetization and vortex become decoupled. In the IV characteristic, the decoupling transition shows as a voltage jump and can be identified as a depinning transition. The polaronic pinning mechanism successfully explains the observed enhancement of critical current in the ErNiBC superconductor at low temperatures. The polaronic pinning can be optimized in magnetic-superconducting multilayers. We show also that vortex-polaron creep is suppressed at low temperatures. [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C35.00006: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 3:42PM - 3:54PM |
C35.00007: Role of twin boundaries on the vortex dynamics in CSD YBCO nanocomposites V. Rouco, A. Palau, M. Coll, R. Guzman, J. Gazquez, S. Ye, A. Llordes, J. Arbiol, S. Ricart, X. Obradors, T. Puig Vortex pinning landscape engineering is foreseen as the route to high performance YBCO coated conductors at high fields. Solution-derived nanocomposites with randomly oriented nanoparticles were shown to be an excellent low cost option with huge isotropic pinning forces associated to a highly dense defect structure. We find that the local strain ensuing from the partial dislocation associated to intergrowths breaks the vertical coherence of twin boundaries (TB) and reduce the TB spacing. The lack of TB coherence will affect their role as pinning centers or channels for easy vortex flow. Transport measurements at different temperatures and magnetic fields realized in tracks with different crystallographic orientations has enabled to determine the effect of TB in a quantitative manner and establish their contribution (pinning and/or channeling) in a H-T diagram. We show that the anisotropic pinning coming from TBs has a minor role compared with the huge enhancement of isotropic pinning in nanocomposites. On the contrary, we demonstrate that the reduction of the TB vertical coherence has a relevant effect on precluding vortex channelling at low temperatures avoiding a J$_{\mathrm{c}}$ suppression for field parallel to the c-axis. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C35.00008: Vortex dynamics in YBCO films with engineered antidots and ferromagnetic Nanostructures A. Palau, V. Rouco, J.C. Gonz\'alez, C. Monton, T. Puig, X. Obradors, R. C\'ordoba, J.M. de Teresa Understanding vortex pinning mechanisms and the interaction between vortices and defects is still one of the major goals to enhance properties of nanostructured superconductors. We have used high resolution lithography techniques (FIB, EBL and C-AFM) to create artificial pinning sites in YBCO films. Model systems with antidots and blind antidots with different geometries, distribution and density have been generated. Moreover, with the aim to study interactions in hybrid superconductor-ferromagnetic systems we have filled the antidots with cobalt rods by focused electron beam induced deposition. In-field critical current measurements have been performed in a wide temperature (T) and magnetic field (H) range in order to study vortex dynamics in these novel systems. As far as YBCO films with blind antidots, collective guided vortex motion is observed when we generate spatial asymmetric (ratchet) pinning potentials. By tuning H and T, the vortices undertake the single vortex pinning to vortex-vortex interaction transition which determines the region where the ratchet effect is activated. As far as YBCO films with ferromagnetic rods, we demonstrate a clear interaction between the magnetic field generated by the cobalt nano-rods and the superconducting matrix. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C35.00009: Non-equlibrium relaxation of vortex lines in disordered type-II superconductors Ulrich Dobramysl, Hiba Assi, Michel Pleimling, Uwe C. T\"{a}uber Vortex matter in disordered type-II superconductors display a remarkable wealth of behavior, ranging from hexagonally arranged crystals and a vortex liquid to glassy phases. The type and strength of the disorder has a profound influence on the structural properties of the vortex matter: Randomly distributed weak point pinning sites lead to the destruction of long range order and a Bragg glass phase; correlated, columnar disorder can yield a Bose glass phase with infinite tilt modulus. We employ a three-dimensional elastic line model and apply a Langevin molecular dynamics algorithm to simulate the dynamics of vortex lines in a dissipative medium. We investigate the relaxation of a system of lines that were initially prepared in an out-of-equilibrium state and characterize the transient behavior via two-time quantities. We vary the disorder type and strength and compare our results for random and columnar disorder. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C35.00010: Large Thermal Fluctuation effects on Vortex Matter in Iron Arsenide Superconductors Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ and Ca$_{10}$(Pt$_{4}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ Oscar Ayala-Valenzuela, Jeehoon Kim, Laonardo Civale At finite temperatures, thermal fluctuations (TF) may allow vortices in a superconductor to jump from one pinning center to another in response to the driving force of the current, even if the current density is lower than the critical current density ($J_{c})$. This effect, known as flux creep, is orders of magnitude larger in cuprate HTS than in conventional low critical temperature ($T_{c})$ materials, mainly due to the much smaller coherence length and large anisotropy. TF in cuprates also produce melting of the vortex lattice and the appearance of vortex liquid phases (VLP), characterized by $J_{c}=$0, near $T_{c}$. In general iron-based superconductors (FeSC) also exhibit large vortex fluctuation effects, with creep rates as large as or even larger than cuprates, in spite of the lower $T_{c}$. VLP are observed in many of FeSC; their extension and characteristics are topics of extensive current research. We have explored vortex fluctuation effects in single-crystals of the novel FeSC Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ and Ca$_{10}$(Pt$_{4}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ by measuring the magnetization and its time decay in a SQUID magnetometer. We found creep rates even higher than in YBCO. We also observed extensive VLP (characterized by a reversible magnetization) that cover most of the mixed state region in the H-T phase diagram, as well as superconducting fluctuations above $T_{c}$. These unusually strong fluctuations are a consequence of the very large penetration depth $\lambda $, which results in Ginzburg numbers ($G_{i})$ higher than in cuprates. [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C35.00011: Flux-quantum-discretized dynamics of intermediate state flux structures in current-driven type-I superconductors G.R. Berdiyorov, A.D. Hernandez-Nieves, M.V. Milosevic, F.M. Peeters, D. Dominguez Nonlinear flux dynamics in a current-carrying type-I superconductor is studied using Ginzburg-Landau theory. The current induces the intermediate state, where nucleation of flux domains is \textit{discretized to a single fluxoid at a time}, while their final shape (tubular or laminar) and size depend on applied current. The current induces opposite flux domains on opposite edges, and subsequently drives them to annihilation---which is \textit{also discretized}, as a sequence of vortex-antivortex pairs. In the presence of pinning centers, both pinning and depinning processes occur in a single flux-discretized form regardless of the shape and size of the flux configurations. Repulsive centers (i.e., obstacles) can results in splitting of tubular domains and branching of laminar structures or transformation of them into tubular patterns. The discretization of nucleation and annihilation, as well as pinning/depinning processes leaves measurable traces in the voltage across the sample and in locally probed magnetization. The reported dynamic phenomena thus provide an unambiguous proof of a flux quantum being the smallest building block of the intermediate state in type-I superconductors. [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C35.00012: Comparison of local electrodynamic responses of superconducting materials--from bulk Nb to MgB$_2$ and Nb thin films Tamin Tai, Behnood Ghamsari, Teng Tan, Xiaoxing Xi, Steven Anlage A near-field magnetic field microwave microscope that enables mapping of the local electrodynamic response in the GHz frequency regime at liquid helium cryogenic temperatures was successful built using the combination of a magnetic writer and a near field-microwave microscope [1]. Many superconducting materials, especially the candidate materials for superconducting RF cavities, were tested at a fixed location to analyze the local electromagnetic response, including both the intrinsic and extrinsic nonlinearities. The bulk Nb materials only show extrinsic nonlinearity, consistent with vortex generation and annihilation in the material. The measurements on Nb and MgB$_{2}$ thin film materials shows not only the extrinsic nonlinearity due to the vortex mechanism, but also intrinsic nonlinearity. The intrinsic nonlinearity comes from the modulation of the superconducting order parameter near T$_{\mathrm{c}}$, but behaves differently for single band gap (Nb) and two-gap (MgB$_{2})$ superconductors. Quantitatively analyzing the nonlinear mechanisms will enable the microscope to extract many material parameters and image the superconducting properties by raster scanning. \\[4pt] [1] Tamin Tai, \textit{et al}., IEEE Trans. Appl. Supercond. \underline {21}, 2615 (2011). [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C35.00013: Lower critical magnetic field for a 2-D superconductor in a non-uniform field John Draskovic, Thomas Lemberger Our exploration of nonlinear effects in two-coil measurements of superfluid density in thin superconducting films led us to calculate the lower critical field of infinite-radius, thin superconducting films in the non-uniform field of a nearby coil powered by an external current supply. We obtain an expression for the Helmholtz free-energy of vortices and antivortices added to the vortex-free Meissner state, and for the work done by the current supply when vortices appear. From these quantities we construct the appropriate ``Gibbs'' free energy $\Delta G$ to minimize. We find that $\Delta G$ of a vortex-bearing state dips below that of the Meissner state when the applied magnetic field exceeds: $B_0^* \approx \frac{8\sqrt{2}\Lambda}{R}B_{c1}^{2D}$ where the intrinsic 2D lower critical field is defined as: $B_{c1}^{2D} \equiv \frac{\Phi_0}{4 \pi \Lambda^2}ln \left (\frac{\Lambda}{\xi}\right )$. Here, $\Lambda$ is the 2-D penetration depth, $2\lambda^2/t$, and $t$ is film thickness. We show that in amorphous MoGe films, this theoretical field is much smaller than the experimental field where vortices become evident in the data. Experimentally, the onset of vortex physics is close to the point where the Meissner screening supercurrent density approaches its theoretical maximum value. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C35.00014: Nature of the Cuprate Pseudogap State in the Presence of High Magnetic Fields Peter Scherpelz, Dan Wulin, Kathryn Levin, Attipat Rajagopal We address the important question of how to characterize the pseudogap state of superconductors under the influence of magnetic fields strong enough to lead to vortex lattices in the condensate. Here we adopt a preformed pair interpretation of the pseudogap in which non-condensed pairs (arising from a stronger-than-BCS attraction) are present above and below $T_c$. We use a simple extension of Gor'kov theory to arrive at a formalism for treating BCS-to-BEC crossover physics. We thereby demonstrate how these pairs organize above the transition $T_c$ into precursors of a vortex configuration via small distortions of the superconducting vortex state. We believe this provides a possible scenario for a normal state ``pseudo''-vortex state which has been the object of discussion in the literature. Because their dispersion is no longer effectively one-dimensional, this precursor vortex configuration appears to enable otherwise problematic ``Bose condensation'' in a field. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C35.00015: ABSTRACT WITHDRAWN |
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