Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F39: Superconductivity: Vortices and Pinning |
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Sponsoring Units: DCMP Chair: Tom Lemberger, Ohio State University Room: 386 |
Tuesday, March 14, 2017 11:15AM - 11:27AM |
F39.00001: Vortex in the maze Vadim B. Geshkenbein, Roland Willa, Gianni Blatter Recent advances in vortex imaging allow for tracing the position of individual vortices with high resolution. By pushing an isolated vortex with a transport current and measuring the linear $ac$ response of the vortex, its trajectory and the associated pinning energy has been found in selected regions of the pinning landscape [1]. Analyzing the full two-dimensional problem, we show that the `broken spring’ effect reported in [1] finds a natural explanation in terms of a vortex escape in the direction transverse to the applied force. Extending the analysis to include high-frequency response data, we show that the pinning potential can be systematically reconstructed. We introduce the Hessian, the determinant of second derivatives, as a new quantity characterizing a two-dimensional pinning landscape. The regions where vortices can assume an equilibrium position under the action of a homogeneous external force, and hence be observed in vortex imaging, involve positive Hessian. We calculate the area of such stable regions for different types of disorder potentials, that provides information on what part of the landscape can be mapped. [1] L.\ Embon {\it et al.}, Sci.\ Rep.\ {\bf 5}, 7598 (2015). [Preview Abstract] |
Tuesday, March 14, 2017 11:27AM - 11:39AM |
F39.00002: Model of vortex dynamics in superconducting films in two-coil measurements of the coherence length Thomas Lemberger, Yen Lee Loh In two-coil measurements on superconducting films, a magnetic field from a small coil is applied to the center of the film. When the amplitude of the ac field is increased, the film undergoes a transition from the ``Meissner'' state to a state with vortices and antivortices. Ultimately, the vortex density matches the applied magnetic field and field screening is negligible. Experimentally, the field at the transition is related to the superconducting coherence length, although a full theory of the relationship is lacking. We show that the mutual inductance between drive and pickup coils, on opposite sides of the film, as a function of ac field amplitude is well-described by a phenomenological model in which vortices and antivortices appear together in the film at the radius where the induced supercurrent is strongest, and then they move through a landscape of moderately strong vortex pinning sites. [Preview Abstract] |
Tuesday, March 14, 2017 11:39AM - 11:51AM |
F39.00003: Enhancing Pinning For Vortices in Hyperuniform Substrates and Emergent Hyperuniform Vortex States Minh Quan Le Thien, Danielle McDermott, Cynthia J. Olson Reichhardt, Charles Reichhardt Disordered hyperuniformity is a state which simultaneously has isotropic liquid like properties and crystalline like properties such as little variation in the density fluctuations over long distances. An open question is what possible applications could utilize properties of hyperuniformity. One of the major issues for applications of type-II superconductors is how to achieve high critical currents by preventing vortex depinning, so there is great interest in understanding which pinning site geometries will lead to the optimal vortex pinning. Using large scale computational simulations, we show that vortices in a type-II superconductor with a hyperuniform pinning arrangement exhibit enhanced pinning compared to an equal number of randomly placed pinning sites, and that the enhancement is robust over a wide range of parameters. The stronger pinning arises in the hyperuniform arrays due to the suppression of pinning density fluctuations, permitting higher pin occupancy and the reduction of weak links that lead to easy flow channeling. We also show that in general, in amorphous vortex states in the presence of either random or hyperuniform pinning arrays, the vortices themselves exhibit disordered hyperuniformity due to the repulsive nature of the vortex-vortex interactions. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:03PM |
F39.00004: Effect of hexagonal patterned arrays and defect geometry on the critical current of superconducting films Ivan Sadovskyy, Yonglei Wang, Zhili Xiao, Wai-Kwong Kwok, Andreas Glatz Understanding the effect of pinning on the vortex dynamics in superconductors is a key factor towards controlling critical current values. Large-scale simulations of vortex dynamics can provide a rational approach to achieve this goal. Here, we use the time-dependent Ginzburg-Landau equations to study thin superconducting films with artificially created pinning centers arranged periodically in hexagonal lattices. We calculate the critical current density for various geometries of the pinning centers -- varying their size, strength, and density. Furthermore, we shed light upon the influence of pattern distortion on the magnetic field dependent critical current. We compare our result directly with available experimental measurements on patterned molybdenum-germanium films, obtaining good agreement. Our results give important systematic insights into the mechanisms of pinning in these artificial pinning landscapes and open a path for tailoring superconducting films with desired critical current behavior. [Preview Abstract] |
Tuesday, March 14, 2017 12:03PM - 12:15PM |
F39.00005: A Dichotomy in the Metastable-to-Ground State Vortex Lattice Transition in MgB$_2$ M.R. Eskildsen, E.R. De Waard, A. Leishman, J. Archer, C.D. Dewhurst, L. DeBeer-Schmitt, J. Karpinski, N.D. Zhigadlo The vortex lattice (VL) in the type-II superconductor MgB$_2$ can serve as a model for out-of-equilibrium systems. Previous small-angle neutron scattering (SANS) studies of the VL in this material 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)]. We used a stop-motion SANS technique to image the VL between successive applications of a controlled number of AC magnetic field cycles. This allows us to study in detail how the metastable VL is driven back to the ground state. Our results show a dichotomy in the behavior for metastable configurations induced by crossing the equilibrium, second order phase transition in different directions. For a metastable state induced by super heating, the VL returns to the ground state through a continuous domain rotation. In contrast, in the super cooled case, VL ground state domains nucleate and grow at their final orientation. [Preview Abstract] |
Tuesday, March 14, 2017 12:15PM - 12:27PM |
F39.00006: High Field Structural Transition of Vortex Lattice Observed in Single Crystals of HgBa$_{2}$CuO$_{4+\delta}$ Jeongseop A. Lee, Ingrid Stolt, Yizhou Xin, W. P. Halperin, A. P. Reyes, M. K. Chan Vortex dynamics in HgBa$_{2}$CuO$_{4+\delta}$ changes at a temperature $T_{v}(H)$ corresponding to the formation of a vortex lattice (VL) in a field $H$. Above this onset temperature, the spatial fluctuations of local magnetic fields due to vortex supercurrents are motionally averaged to zero. Below $T_{v}$ the local field distribution is quasi-static. This change in the vortex timescale is marked by a small enhancement in the transverse relaxation rate and appearance of a characteristic NMR spectral lineshape due to the presence of VL. The $T_{v}$ was measured using relaxation measurements for three samples, two underdoped with $T_{c}$ = 87 and 79 K and one overdoped with $T_{c}$ = 78 K. We present our results from $^{17}$O NMR at the apical oxygen site as a function of external magnetic field up to 30 T. The vortex lineshape contribution can be separated from inhomogeneous broadening by deconvolution with the normal state spectra. We fit the spectra below $T_{v}$(H) to the field distribution of an idealized VL from Ginzburg-Landau theory to determine the VL symmetry, penetration depth, and coherence length and find evidence of a VL structural transition in high magnetic field. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 12:39PM |
F39.00007: Field and Temperature dependence of the Vortex Lattice Rotation in the Chiral phase of UPt$_{3}$ with ${\bf H} \parallel {\bf c}$} Keenan Avers, Morten Eskildsen, William Halperin, William Gannon, Jorge Gavilano, Gergely Nagy, Urs Gasser, Charles Dewhurst The unconventional, multi-phase, superconductor UPt$_3$ is suspected to have one chiral superconducting phase. We have performed small angle neutron scattering (SANS) from the vortex lattice (VL) in UPt$_3$ at the Paul Scherrer Institute, ILL, and Oak Ridge National Lab with the magnetic field parallel to the hexagonal $c$-axis. At low temperatures the diffraction pattern of the VL rotates away from a high symmetry direction producing two domains of different orientation. This rotation can be reduced or eliminated by increasing the temperature at constant field within the chiral state. We have found that there are two different rotated vortex states, one that is stable and one metastable, the former being realized by entering the chiral state by decreasing the magnitude of the applied field, and the latter generated starting from zero field. This rotation may be evidence of a non-singular, field history dependent, vortex core sturcture in UPt$_{3}$. [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F39.00008: Polycrystalline order and disorder in vortex lattices in SNS arrays Malcolm Durkin, Ian Mondragon-Shem, Taylor Hughes, Nadya Mason We present transport measurements on superconductor-normal-superconductor (SNS) arrays placed in a finite magnetic field, studying the introduction of disorder into the vortex lattice. SNS arrays provide a well-defined periodic potential for vortices, allowing a crystalline structure to form when the vortex lattice is commensurate with the SNS array. Disorder is introduced into the crystalline lattice when the field is shifted away from a commensurate value and the vortex lattice no longer matches the array. A recurring question is whether the disorder causes glassy, polycrystalline, or other arrangements of the lattice. We study this by applying a current to drive vortex motion for different commensurate/incommensurate field values. We observe a two-step transition at incommensurate fields consistent with a transition from pinned vortices to lattice defect flow to lattice flow. This suggests a polycrystalline structure with defects forming on the edges of crystalline domain. [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F39.00009: Strong pinning regimes explored with large-scale Ginzburg-Landau simulations Roland Willa, Alexei E. Koshelev Improving the current-carrying capability of superconductors requires a deep understanding of vortex pinning. Within the theory of (3D) strong pinning [1] an ideal vortex lattice is weakly deformed by a low density $n_p$ of strong defects. In this limit the critical current $j_c$ is expected to grow linearly with $n_p$ and to decrease with the field $B$ according to $B^{-\alpha}$ with $\alpha\approx 0.5$. In the small-field limit the (1D) strong pinning theory of isolated vortices predicts $j_c\propto n_p^{0.5}$, independent of $B$. We explore strong pinning by low defect densities using time-dependent Ginzburg-Landau simulations [2]. Our numerical results suggest the existence of a wide regime, where the lattice order is destroyed and yet interactions between vortices are important. In particular, for large defects we found an extended range of power-law decay of $j_c(B)$ with $\alpha\approx 0.3$, smaller than predicted. This regime requires the development of new analytical models. Exploring the behavior of $j_c$ for various defect densities and sizes, we will establish pinning regimes and applicability limits of the conventional theory. [1] G. Blatter {\it et al.}, Phys. Rev. Lett. {\bf 92}, 067009 (2004) [2] I. A. Sadovskyy {\it et al.}, J. Comput. Phys. {\bf 294}, 639 (2015) [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F39.00010: Simulations of geometrical vortex lattice pinning and melting in thin superconducting strips Andreas Glatz, Gian Paolo Papari, Francesco Tafuri, Valerii Vinokur Recent findings that geometric restrictions may induce self-arresting hypervortices recovering the dissipation-free state at high fields and temperatures made superconducting strips an important topic of superconductivity studies. Here we report on the geometrical melting of the vortex lattice in a thin strip preceded by magnetoresistance (MR) oscillations fingerprinting the underlying regular vortex structure. We compare our numerical simulations of these systems to experimental measurements, allowing us to relate observed resistance oscillations to the penetration of vortex rows with intermediate geometrical pinning and uncover the details of geometrical melting. Our findings offer a reliable and reproducible pathway for controlling vortices in geometrically restricted nanodevices and introduce a novel technique of geometrical spectroscopy, inferring detailed information of the structure of the vortex system through a combined use of MR curves and large-scale simulations. [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F39.00011: Effects of anisotropy and disorder-mediated nucleation of vortices on the superheating field of superconductors Danilo Liarte, Sam Posen, Mark Transtrum, Gianluigi Catelani, Matthias Liepe, James Sethna We provide a theory for the effects of disorder and materials anisotropy on the maximum parallel surface field $H_{\mathrm{sh}$ that a superconductor can sustain, important for accelerating cavities in current particle accelerators. (Current niobium cavities routinely operate above $H_{c1}$, in a metastable regime susceptible to vortex penetration). Dirt is discussed in an 'instanton' calculation of disorder-mediated vortex nucleation. The increased susceptibility to dirt due to the smaller coherence lengths in new materials (Nb$_3$Sn, NbN, MgB$_2$) is swamped by much stronger effects of the distance from the pure $H_{\mathrm{sh}$: Nb$_3$Sn should be as reliable at 0.92 Tesla as Nb at typical operating fields of 0.18 Tesla, according to a crude estimate. The effects of anisotropy in layered materials is calculated within Ginzburg-Landau theory, applicable near the critical temperature. For high-$\kappa$ materials like MgB$_2$, the anisotropy is negligible near $T_c$; we speculate about possible large anisotropies at lower temperatures. We briefly review current experimental development of Nb$_3$Sn cavities. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F39.00012: Crossing fields in thin films of isotropic superconductors F Colauto, V K Vlasko-Vlasov, A A Bouzdin, A A M Oliveira, A M H Andrade, D Carmo, W A Ortiz, D Rosenman, W -K Kwok We study magnetic flux cutting effects by imaging the vortex dynamics in Nb films of different thickness in the crossing in-plane (H$_{\mathrm{\vert \vert }})$ and normal fields. For H$_{\mathrm{\vert \vert }}=$1 kOe the motion of the normal vortices in a 200 nm film is found to be anisotropic. At T\textgreater T$_{\mathrm{c}}$/2 we observe a delay in the vortex propagation across H$_{\mathrm{\vert \vert }}$. At T\textless T$_{\mathrm{c}}$/2, when thermomagnetic instabilities occur, the vortex dendrites tilt perpendicular to the in-plane field direction. In a 100 nm film, the normal flux dynamics is isotropic and independent of H$_{\mathrm{\vert \vert }}$. Our calculations of the thermodynamic potential for the in-plane vortices predict their existence at H$_{\mathrm{\vert \vert }}=$1 kOe only in the 200 nm film. In the 100 nm sample, H$_{\mathrm{\vert \vert \thinspace }}$monotonously changes through the film thickness. Therefore, the observed delay of the normal flux motion across H$_{\mathrm{\vert \vert }}$ in the thicker film is due to the vortex cutting-reconnection of the normal and in-plane vortices. The enhanced pinning potential for motion across H$_{\mathrm{\vert \vert }}$ explains also the tilt of the dendrite branches at T\textless T$_{\mathrm{c}}$/2. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F39.00013: Triode for magnetic flux quanta. Vitalii Vlasko-Vlasov, Fabiano Colauto, Timothy Benseman, Daniel Rosenmann, Wai-Kwong Kwok We designed a magnetic vortex triode using an array of closely spaced soft magnetic Py strips on top of a Nb superconducting film. The strips act similar to the grid electrode in an electronic triode, where the electron flow is regulated by the grid potential. In our case, we tune the vortex motion by the magnetic charge potential of the strip edges, using a small magnetic field rotating in the film plane. The magnetic charges emerging at the stripe edges and proportional to the magnetization component perpendicular to the edge direction, form linear potential barriers or valleys for vortex motion in the superconducting layer. We directly imaged the normal flux penetration into the Py/Nb films and observed retarded or accelerated entry of the normal vortices depending on the in-plane magnetization direction in the stripes. The observed flux behavior is explained by interactions between magnetically charged lines and magnetic monopoles of vortices similar to those between electrically charged strings and point charges. We discuss the possibility of using our design for manipulation of individual vortices in high-speed, low-power superconducting electronic circuits. [Preview Abstract] |
Tuesday, March 14, 2017 1:51PM - 2:03PM |
F39.00014: Imprinting superconducting vortex trajectories in a magnetic layer J\'{e}r\'{e}my Brisbois, Maycon Motta, Jonathan I. Avila, Gorky Shaw, Thibaut Devillers, Nora M. Dempsey, Savita K. P. Veerapandian, Pierre Colson, Benoit Vanderheyden, Philippe Vanderbemden, Wilson A. Ortiz, Ngoc Duy Nguyen, Roman B. G. Kramer, Alejandro V. Silhanek We experimentally show that the principle of local polarization of a magnetic layer, a well-known method to store information namely in hard drives and credit cards, can be applied for imprinting into a soft magnetic layer of permalloy (Py) the trajectory of vortices moving in a superconducting film (Nb). In full analogy with a magnetic drawing board, vortices act as tiny magnetic scribers leaving a wake of polarized magnetic media in the Py layer. We have used the magneto-optical imaging technique to investigate the mutual interaction between superconducting vortices and ferromagnetic domains. In general, we observe that the flux propagation is delayed at the border of the magnetic layer. Interestingly, in thin Py layers without stripe domains, vortices leave clear imprints of locally polarized magnetic moments along their trajectories. Furthermore, the printings were found to be stable and could still be observed at room temperature, allowing for ex situ observation of the flux penetration in superconductors. We expect our findings to pave the way for further studies for optimizing magnetic recording of superconducting vortex trajectories. [Preview Abstract] |
Tuesday, March 14, 2017 2:03PM - 2:15PM |
F39.00015: Angular Momentum of Multiply Quantised Vortices in Fermionic Superfluids Abhinav Prem, Sergej Moroz, Victor Gurarie, Leo Radzihovsky We study vortex textures in weakly coupled $s$-wave fermionic superfuids at zero temperature. In the BCS limit, we find that for an axisymmetric multiply quantised vortex with vorticity $k>1$ the angular momentum is significantly reduced from the value $L_z = \hbar kN/2$, which naturally arises in the strongly-coupled BEC regime. This phenomenon is rooted in the presence of unpaired fermions in the BCS ground state of a superfluid with an axisymmetric multiply quantised vortex. The predicted deviation of the angular momentum can be understood from the spectral flow along the vortex sub-gap states. [Preview Abstract] |
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