Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L11: Engineering Vortex Matter in Superconductors II |
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Sponsoring Units: DMP Chair: Maxime Leroux, Argonne National Laboratory Room: 007B |
Wednesday, March 4, 2015 8:00AM - 8:12AM |
L11.00001: Effects of Heat Transport and Quasi-particle Relaxation on Dynamics of Vortices in Superconductors Masaru Kato, Osamu Sato We study dynamics of vortices in superconductors using the molecular dynamics simulation. Motion of vortices causes a heat generation and also decreases amplitude of the superconducting order parameter on their trail because of quasi-particle relaxation time. We incorporate these effects into the Molecular dynamics methods. In a superconducting corbino-disk, where vortices move along concentric circles, vortex motion is more rapid around the center of the disk than that around the edge. Therefore the heat generation is not uniform and there appears heat transport from the center to the edge. In such case, our simulation shows that vortices move toward the edge of the disk. Also vortices show some dynamical spiral structures depend on the pinning impurity sites and heat resistance between the superconductor and a substrate. We show details of these motions and analyze the effects of the heat transport to the vortex dynamics. [Preview Abstract] |
Wednesday, March 4, 2015 8:12AM - 8:24AM |
L11.00002: Thermoelectric Measurements of Electronic Diffusivity in Bad Metals Jiecheng Zhang, Eli Levenson-Falk, Aharon Kapitulnik Many interesting materials, including cuprate superconductors and heavy-fermion systems, exhibit ``bad metal'' behavior at high temperatures, where the electronic mean free path is shorter than the de Broglie wavelength. Recent theory [1] postulates that conduction in such systems is best described by collective incoherent transport, instead the standard quasiparticle model. This has implications for the temperature dependence of electronic diffusivity in these systems. We present a setup for measuring electronic diffusivity: a laser beam is focused onto a material surface and chopped, creating a periodic, concentrated heat source. The resulting thermoelectric signal is measured at various positions on the same surface with sharp voltage probes. By sweeping temperature in the range 10-450 K, we are able to measure the temperature dependence and anisotropy of the electronic diffusivity of the material. We discuss experimental improvements and measurements of cuprate superconductors. \\[4pt] [1] arXiv:1405.3651 [Preview Abstract] |
Wednesday, March 4, 2015 8:24AM - 8:36AM |
L11.00003: Heat Transport in non-uniform superconductors Caroline Richard, Anton B. Vorontsov Thermal transport in superconductors is used as an experimental probe of both quasiparticles states and order parameter structure. Although thermal properties of uniform superconductors have been studied a lot, much less is known about details of the transport in non-uniform phases such as vortex states or the speculative Fulde-Ferrell-Larkin-Ovchinnikov state. Using the quasi-classical Keldysh technique, we theoretically investigate the heat transport in a inhomogeneous superconducting state where the order parameter contains a domain wall. We study the effect of Andreev states, bound to the domain wall, on thermal conductivity, and find the response to be strongly non-local. In linear response, we determine the density of states, impurity self-energy and the effective local temperature gradient required to sustain a stationary heat current through the sample. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 8:48AM |
L11.00004: Heat transport along domain walls and surfaces of superconductors Anton Vorontsov, Caroline Richard We calculate thermal transport in non-uniform states of unconventional superconductors, that appear near pairbreaking surfaces, or due to formation of domain walls in the order parameter. The spectrum of the quasiparticles states in these regions is dominated by the Andreev bound states, including topologically protected modes. We investigate how these states contribute to the heat transport, using non-equilibrium quasiclassical theory in linear response. We report self-consistent calculation of the order parameter, impurity self-energies, density of states and vertex corrections. Particular attention is paid to the non-local nature of the response. We show differences and similarities between domain walls in d-wave materials, and surfaces of multi-component chiral superconducting states. We describe results for Born and unitary impurity scattering limits, and effects of the Zeeman magnetic field on thermal transport. [Preview Abstract] |
Wednesday, March 4, 2015 8:48AM - 9:00AM |
L11.00005: Vortex relaxation in type-II superconductors following current quenches Harsh Chaturvedi, Hiba Assi, Ulrich Dobramysl, Michel Pleimling, Uwe T\"auber The mixed phase in type-II superconductors is characterized by the presence of mutually repulsive magnetic flux lines that are driven by external currents and pinned by point-like or extended material defects. We represent the disordered vortex system in the London limit by an elastic directed line model, whose relaxational dynamics we investigate numerically by means of Langevin Molecular Dynamics. We specifically study the effects of sudden changes of the driving current on the time evolution of the mean flux line gyration radius and the associated transverse displacement correlation functions. Upon quenching from the moving into the pinned glassy phase, we observe algebraically slow relaxation. The associated two-time height-autocorrelations display broken time translation invariance and can be described by a simple aging scaling form, albeit with non-universal scaling exponents. [Preview Abstract] |
Wednesday, March 4, 2015 9:00AM - 9:12AM |
L11.00006: Ordered Pinning Arrays with Tunable Geometry via Thermal Effects Juan Trastoy, Rozenn Bernard, Javier Briatico, Javier E. Villegas, Maxime Malnou, Nicolas Bergeal, Jerome Lesueur, Christian Ulysse, Giancarlo Faini We have used geometrically frustrated pinning arrays to create artificial vortex-ice [1]. The pinning arrays are fabricated via ion irradiation of high-Tc superconducting films. These arrays present a very unique characteristic: the frustration can be reversibly switched on/off using temperature as a control knob, which allows stabilizing either a vortex-ice or a square vortex lattice. We have further investigated the thermal switching mechanism by studying the matching of the flux lattice to arrays that are incrementally deformed upon fabrication by introducing minute variations of the distance between pins. The array deformation exacerbates the thermal effects, leading to dramatic variations of the vortex distribution as a function of temperature. These results illustrate the strength of the temperature-induced reconfiguration effects, which may constitute a novel knob in fluxtronic devices based on vortex manipulation.\\[4pt] [1] Trastoy et al. Nature Nanotechnology 9, 710-715 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:24AM |
L11.00007: Controlling Superconductivity in Thin Film with an External Array of Magnetic Nanorods Wonbae Bang, K.D.D. Rathnayaka, I.F. Lyuksyutov, W. Teizer, D.G. Naugle We have fabricated a novel type of magnet-superconductor hybrid: an ordered array of magnetic nanorods on top of a superconducting film which is electrically insulated from the array. We have studied the transport properties of this magnet-superconductor hybrid including the R(T) and R(H) dependences for temperatures 0.96T$_C$$<$T$<$1.06T$_C$ and magnetic fields H$<$700 Oe (T$_C$ denotes the critical temperature of the hybrid system). We compare these results with those for a superconducting film on top of alumina template with an array of magnetic nanowires. [Preview Abstract] |
Wednesday, March 4, 2015 9:24AM - 9:36AM |
L11.00008: Effects of heavy ion irradiation on the thermodynamic and transport properties of YBCO Xu Luo, Maxime Leroux, Vivek Mishra, Xinsheng Ling, Ulrich Welp, Wai-Kwong Kwok The effects of Au heavy ion irradiation (HII) on the transport and thermodynamic properties of untwined YBCO crystals irradiated to dose matching fields of B$_{\mathrm{\Phi }} \quad =$ 6 Tesla and 1 Tesla along the crystallographic c-axis were studied by angle resolved magnetoresistivtiy and high resolution AC specific heat measurements. Results from transport measurements confirm an enhancement in the ab-plane critical current for magnetic fields aligned along the columnar defects induced by HII. Surprisingly, specific heat measurements reveal a reduction in the thermodynamic upper critical field anisotropy of YBCO by about one half in the B$_{\mathrm{\Phi }} \quad =$6T crystal. Moreover, for the B$_{\mathrm{\Phi }}=$1T crystal, we found the formation of an anomalous peak in the critical temperature near the direction of HII which may be associated with the Bose-glass transition. [Preview Abstract] |
Wednesday, March 4, 2015 9:36AM - 9:48AM |
L11.00009: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 9:48AM - 10:00AM |
L11.00010: Single vortex pinning and penetration depth in superconducting NdFeAsO$_{1-x}$F$_x$ Jessie T. Zhang, Jeehoon Kim, Magdalena Huefner, Cun Ye, Stella Kim, Paul Canfield, Ruslan Prozorov, Ophir M. Auslaender, Jennifer E. Hoffman We use a magnetic force microscope (MFM) to investigate single vortex pinning and penetration depth in NdFeAsO$_{1-x}$F$_x$, one of the highest-$T_c$ iron-based superconductors. In fields up to 20 Gauss, we observe a disordered vortex arrangement, implying that the pinning forces are stronger than the vortex-vortex interactions. By manipulating the vortices using the MFM tip, we obtain single vortex depinning forces for vortices in NdFeAsO$_{1-x}$F$_x$. Moreover, our MFM measurements allow the first local and absolute measurement of the superconducting penetration depth in NdFeAsO$_{1-x}$F$_x$ to nanometer scale resolution. [Preview Abstract] |
Wednesday, March 4, 2015 10:00AM - 10:12AM |
L11.00011: Structural Studies of Metastable and Ground State Vortex Lattice Domains in MgB$_2$ E.R. De Waard, S.J. Kuhn, C. Rastovski, M.R. Eskildsen, A. Leishman, C.D. Dewhurst, L. DeBeer-Schmitt, K. Littrell, J. Karpinski, N.D. Zhigadlo Small-angle neutron scattering (SANS) studies of the vortex lattice (VL) in the type-II superconductor MgB$_2$ have revealed an unprecedented degree of metastability that is demonstrably not due to vortex pinning, [C. Rastovski $et$ $al.$, Phys. Rev. Lett. {\bf 111}, 107002 (2013)]. Application of an AC magnetic field to drive the VL to the ground state revealed a two-step power law behavior, indicating a slow nucleation of ground state domains followed by a faster growth. The dependence on the number of applied AC cycles is reminiscent of jamming of soft, frictionless spheres. Here, we report on detailed structural studies of both metastable and ground state VL domains. These include measurements of VL correlation lengths as well as spatially resolved SANS measurements showing the VL domain distribution within the MgB$_2$ single crystal. We discuss these results and how they may help to resolve the mechanism responsible for stabilizing the metastable VL phases. [Preview Abstract] |
Wednesday, March 4, 2015 10:12AM - 10:24AM |
L11.00012: Enhancement of critical current through oxygen irradiation in optimized 2G superconducting coated conductors Karen Kihlstrom, M. Leroux, U. Welp, W-K. Kwok, A.E. Koshelev, G.W. Crabtree, M.W. Rupich, S. Fleshler, A.P. Malozemoff, A. Kayani We demonstrate the strong enhancement of the critical current density, Jc, of commercial coated conductors (CC) in high magnetic fields using 3 MeV oxygen irradiation. The rapid suppression of Jc of coated conductors in magnetic fields remains a barrier for application of CCs in motors, generators, transformers, solenoids, and MRI systems. Using TRIM simulations, we determined that 3-MeV O-ions produce an essentially uniform defect distribution in bare CC at a rate that is $\sim$ 2500 faster than that achieved with proton irradiation. Irradiating with 3 MeV O-ions to a dose of 1x10$^{13}$ O-ions/cm$^{2}$ generates a near doubling of the critical current at low temperatures: at 5K, 5T, we enhanced the critical current from 6.4 MA/cm$^{2}$ to 12.2 MA/cm$^{2}$. This dose can be achieved in a couple of seconds, thus this irradiation technique could be incorporated into a viable reel-to-reel production process. This work supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. D.O.E., Office of Science, Office of Basic Energy Sciences (K.K, M.L.,A.K) and by the D.O.E, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357 (U.W., W.K.). [Preview Abstract] |
Wednesday, March 4, 2015 10:24AM - 10:36AM |
L11.00013: Magnetic Levitation Force Measurement System at Any Low Temperatures From 20K To 300K Sukru Celik, S. Baris Guner, Elvan Coskun Most of the magnetic levitation force measurements in previous studies were performed at liquid nitrogen temperatures. For the levitation force of MgB2 and iron based superconducting samples, magnetic levitation force measurement system is needed. In this study, magnetic levitation force measurement system was designed. In this system, beside vertical force versus vertical motion, lateral and vertical force versus lateral motion measurements, the vertical force versus temperature at the fixed distance between permanent magnet PM -- superconducting sample SS and the vertical force versus time measurements were performed at any temperatures from 20 K to 300 K. Thanks to these measurements, the temperature dependence, time dependence, and the distance (magnetic field) and temperature dependences of SS can be investigated. On the other hand, the magnetic stiffness MS measurements can be performed in this system. Using the measurement of MS at different temperature in the range, MS dependence on temperature can be investigated. These measurements at any temperatures in the range help to the superconductivity properties to be characterized. [Preview Abstract] |
Wednesday, March 4, 2015 10:36AM - 10:48AM |
L11.00014: Controlling microwave driven vortex avalanches by superconductivity stimulation Antonio Lara, Farkhad G. Aliev, Alejandro V. Silhanek, Victor V. Moshchalkov The response of superconducting devices to electromagnetic radiation is a core concept implemented in diverse applications, ranging from the currently used voltage standard to single photon detectors in astronomy. Surprisingly, a sufficiently high power subgap radiation may stimulate superconductivity itself. Recently we have demonstrated the possibility of stimulating also type II superconductors, in which the radiation may cause nonlinear response of the vortex core [1]. This effect opens the possibility of effective control over vortex avalanches. Here we report on a detailed study of superconductivity enhancement and avalanche control by GHz radiation in type II superconducting Pb films in the presence of vortices. The magnetic field dependence of the vortex related microwave losses in a film with periodic pinning reveals a reduced dissipation of mobile vortices in the stimulated regime due to a reduction of the core size. Results of numerical simulations support the validity of this conclusion. Moreover, we demonstrate that microwave stimulated superconductivity induces a notable increase of microwave depinning power needed to trigger avalanches in the proximity of vortex depinning frequencies. Our findings open new ways to control the vortex dissipation and depinning induced avalanches by using superconductivity stimulation. \\[4pt] [1] A. Lara et al., submitted [Preview Abstract] |
Wednesday, March 4, 2015 10:48AM - 11:00AM |
L11.00015: Observation of Sinusoidal -- Like Regular Voltage Oscillations in Silver Doped YBa$_{2}$Cu$_{3}$O$_{7-x}$ Atilgan Altinkok, Murat Olutas, Atilla Kilic, Kivilcim Kilic The influence of bi-directional square wave (BSW) current was investigated on the evolution of the $V -t$ curves at different periods ($P)$, temperatures and external magnetic fields. It was observed that slow transport relaxation measurements result in regular sinusoidal voltage oscillations which were discussed mainly in terms of the dynamic competition between pinning and depinning.The symmetry in the voltage oscillations was attributed to the elastic coupling between the flux lines and the pinning centers along grain boundaries and partly inside the grains. This case was also correlated to the equality between flux entry and exit along the YBCO/Ag sample during regular oscillations. It was shown that the voltage oscillations can be described well by an empirical expression $V(t)$ $\sim$ sin(\textit{wt}$+\varphi )$. We found that the phase angle $\varphi $generally takes different values for the repetitive oscillations. Fast Fourier Transform analysis of the $V-t$ oscillations showed that the oscillation period is comparable to that ($P_{I})$ of the BSW current. This finding suggests a physical mechanism associated with charge density waves (CDWs), and, indeed, the weakly pinned flux line system in YBCO/Ag resembles the general behavior of CDWs. At certain values of $P_{I}$, amplitude of BSW current, $H$ and $T$, the YBCO/Ag sample behaves like a double-integrator, since it converts the BSW current to sinusoidal voltage oscillations in time. [Preview Abstract] |
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