Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y46: Focus Session: Superconductivity, Vortex Matter-III; Unconventional Structures and Transport |
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Sponsoring Units: DCMP Chair: Alexei Koshelev, Argonne National Laboratory Room: Mile High Ballroom 4E |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y46.00001: Transition from Vortex-Antivortex Pairs to Single Vortices in 2D SNS Josephson Junction Arrays Malcolm Durkin, Serena Eley, Nadya Mason SNS Josephson Junction arrays represent a model system for studying vortices in 2D systems.~ Despite long term interest in zero magnetic field effects, such as the Berezinsky-Kosterlitz-Thouless transition, the field tuned transition between a state dominated by vortex-antivortex pairs and one dominated by single vortices remains largely unstudied. Often, evidence for such a transition is mischaracterized as a large suppression of the single vortex energy barrier at low fields. Here we demonstrate, via transport measurements, a finite magnetic field transition between vortex-antivortex pair and single vortex excitations in a 2D superconductor. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y46.00002: Fractional Josephson vortices in two-gap superconductor long Josephson junctions Ju Kim We investigated the phase dynamics of long Josephson junctions (LJJ) with two-gap superconductors in the broken time reversal symmetry state. In this LJJ, spatial phase textures (i-solitons) can be excited due to the presence of two condensates and the interband Joesphson effect between them. The presence of a spatial phase texture in each superconductor layer leads to a spatial variation of the critical current density between the superconductor layers. We find that this spatial dependence of the crtitical current density can self-generate magnetic flux in the insulator layer, resulting in Josephson vortices with fractional flux quanta. Similar to the situation in a $YBa_2Cu_3O_{7-x}$ superconductor film grain boundary [1], the fractionalization of a Josephson vortex arises as a response to either periodic or random excitation of i-solitions. This suggests that magnetic flux measurements may be used to probe i-soliton excitations in multi-gap superconductor LJJs. \\ \\ 1. R. Mint and I. Papiashvili, Phys. Rev. B \textbf{64}, 134501 (2001). [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y46.00003: Dynamic vortex in a narrow superconducting film with Josephson junction Alex Gurevich A vortex moving along a planar Josephson junction perpendicular to a narrow thin film strip is investigated. Exact solutions of the equations of nonlocal Josephson electrodynamics describing the dynamic structure of the vortex driven by time-dependent current across an overdamped junction are obtained. It is shown that this problem reduces to two coupled nonlinear differential equations for the vortex position and core length. Using these equations, the critical film width below which the vortex turns into a phase slip is calculated. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y46.00004: Fractional Vortices in Multi-Gap Superconductors Yen Lee Loh, Monica Kim, Ju H. Kim Novel topological defects, known as fractional vortices, can occur in thin films of multi-gap superconductors. We study two-gap and three-gap superconducting films within a classical Ginzburg-Landau description, using numerical simulations and analytic approximations. In two-gap superconducting films, we find that the interband Josephson coupling $J_{12}$ leads to an effective attraction between half-vortices, whereas the permeability parameter $\mu$ leads to an effective repulsion between half-vortices. We locate the phase boundary in $(J_{12}, \mu)$ space that marks the onset of spontaneous vortex fractionalization. We describe how the size of a fractional vortex increases as one goes deeper into the fractionalized phase. Our results suggest that coating a multi-gap superconducting film with a paramagnetic overlayer will enhance the tendency towards vortex fractionalization. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y46.00005: Phase diagrams of vortex matter with multiple length scale pair interaction in layered superconductors Qingyou Meng, Christopher Varney, Hans Fangohr, Egor Babaev Recently, Romero-Isart {\em et al.} [Phys. Rev. Lett. 111, 145304 (2013)] proposed a new way of trapping ultracold atoms using the magnetic field generated by a vortex lattice, where the lattice is generated by pinning the vortices. Here, we show that the same effect can be achieved with layered superconductors without pinning the vortices. We utilize Langevin dynamics to determine the ground state phase diagrams for pair potentials that describe the vortex physics of layered superconducting systems. We also present two zero temperature phase diagrams of vortex matter with three and four short length scale pair interaction. In the first phase diagram, there are 10 phases such as hexagonal, dimer, stripe, void, like kagom\'e, square, dimer hexagonal, honeycomb, glass and cluster phases. And in the second one, there are 8 phases such as hexagonal, dimer, stripe, honeycomb, kagom\'e, glass, void and cluster phases. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y46.00006: Zero-Magnetic-Field Phase-Decoherence Transition in Underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ Paul Baity, Xiaoyan Shi, Zhenzhong Shi, Dragana Popovi\'{c} The two key prerequisites for superconductivity are electron pairing and phase coherence of the pair wave-function. We present an electrical transport study on underdoped La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO) films ($x=0.07$ and $0.08$) that suggests that, in zero magnetic field ($H=0$), superconductivity is destroyed by thermal unbinding of vortex-antivortex phase fluctuations at a temperature $T_{BKT}$. In particular, current-voltage ($I-V$) curves follow a power law $V \propto I^{\alpha(T)}$ with $\alpha (T) \geq 3$ for $T\leq T_{BKT}$. In addition, the contribution of the superconducting fluctuations to the conductivity, $\Delta\sigma_{SCF}(T,H=0)$, obtained by extrapolating the measured magnetoresistance from the normal state at high enough $H$ and $T$, increases monotonically with decreasing $T$ and diverges exponentially at $T_{BKT}$. These results suggest that the $H=0$ superconducting transition, where the Ohmic resistivity also vanishes, is due to the loss of phase coherence and manifests itself as a Berezinskii-Kosterlitz-Thouless transition. Our findings agree well with other experiments on LSCO with higher doping. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y46.00007: Transport Properties of Thin Films of The Noncentrosymmetric Superconductors ZrRe$_{6}$ and Re$_{3}$W Mojammel Alam Khan, Daniel Lepkowski, Ahmad Us Saleheen, Joseph Prestigiacomo, Amar Karki, Rongying Jin, Tijiang Liu, Shane Stadler, Phil Adams, David Young Thin films ($\sim$50{\AA} - 500{\AA}) of ZrRe$_{6}$ and Re$_{3}$W were grown by pulse laser deposition from arc-melted targets. The dependence of the film's critical temperatures, as well as the film's upper critical field, was determined as a function of film thickness and compared to bulk samples. A 500{\AA} film of ZrRe$_{6}$ had a transition temperature near 6.5 K. The thermal conductivity and thermo-electric power of bulk samples were also measured. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y46.00008: Quenched Randomness, the Imry-Ma theorem, and Topology Thomas Proctor, Eugene Chudnovsky, Dmitry Garanin In 1975, Imry and Ma made the analytical prediction that an exchange model in $d$ dimensions under the influence of a weak random field of strength $h_r$ will have a correlation length of $R_f \propto h_r^{-\frac{2}{4-d}}$ . However, numerical results since then have not given strong support to this analytical result. In our numerical studies, we have found that models that support topological structures, such as vortices or skyrmions, show spin states that have hysteresis, are highly dependent on initial conditions, and do not follow Imry-Ma prediction. Meanwhile, models that do not support these topological structures follow the Imry-Ma prediction, implying that the Imry-Ma state is not reached because of topological effects. These findings have implications for random magnets and flux lattices. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y46.00009: Magnetoresistance, Hall effect, and Point Contact Tunneling Spectroscopy of Superconducting LiTi$_{2}$O$_{4}$ Thin Films X.H. Zhang, R. Suchoski, S. Maruyama, S. Yasui, J.M. Shin, Y.P. Jiang, R.L. Greene, I. Takeuchi, G. He, L. Shan, K. Jin Superconducting LiTi$_{2}$O$_{4}$ thin films with a transition temperature of 11 K have been epitaxially fabricated on MgAl$_{2}$O$_{4}$ substrates using pulsed laser deposition (PLD). Systematic studies of the transport properties and the tunneling spectroscopy of the films (t $\sim$ 180nm) have been performed. In the normal state, the Hall coefficient shows a nearly constant value with a positive sign over a broad temperature range, suggesting a single-band hole-like electronic transport. The magnetoresistance of the material shows an unexpected change in the sign at 50 K. Below this temperature, the resistance shows a conventional parabolic increase with field. However, above this temperature, an unusual negative magnetoresistance appears. In the superconducting state, an upper critical field of about 18 Tesla is found by both magnetotransport and point-contact tunneling spectroscopy (PCS). In addition, our PCS results suggest that the superconducting gap in LiTi$_{2}$O$_{4}$ is BCS-like. A possible cause of the unusual negative magnetoresistance will be discussed. Preliminary results on the field effect using ionic liquid gating will also be presented. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y46.00010: I-V characteristics of atomically thin superconducting $La_{2-x}Sr_xCuO_4$ films Scott Dietrich, William Mayer, Sergey Vitkalov, Andrey Sergeev I-V characteristics of $La_{2-x}Sr_xCuO_4$ thin films grown by Molecular Beam Epitaxy are studied in a wide range of temperatures in zero magnetic field. Strongly nonlinear response to applied electric current was observed near the superconducting state. At temperatures $T>T_{BKT}$ the observed characteristics have three distinct features: (1) at small currents voltage $V$ is proportional to the current $I$; (2) at higher currents a cubic dependence ($I \sim I^3$) is observed; (3) and even higher currents $V \sim I^\alpha$ with coefficient $\alpha(T)<$ 3, which decreases with the temperature increase. At $T=T_{BKT}$ the coefficient $\alpha=3$. Observed nonlinear response is in-line with Berezinsky--Kosterlitz--Thouless (BKT) theory. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y46.00011: $H_{c2}$ as a probe for multiband superconductivity in LAO/STO Jonathan Edge, Alexander Balatsky We investigate the temperature dependence of the upper critical field $H_{c 2} $ as a tool to probe the possible presence of multiband superconductivity at the interface of LAO/STO. The behaviour of $H_{c 2} $ can clearly indicate two-band superconductivity through its nontrivial temperature dependence. For the disorder scattering dominated two-dimensional LAO/STO interface we find a characteristic non-monotonic curvature of the $H_{c 2} (T) $. We also analyse the $H_{c 2} $ for multiband bulk STO and find similar behaviour. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y46.00012: Magnetotransport in extremely-high magnetic fields of La$_{2-x}$Sr$_{x}$CuO$_{4}$ with nearly continuous doping Zachary Stegen, Greg Boebinger, Fedor Balakirev, Albert Migliori, Jie Wu, Ivan Bozovic The doping dependence of the Hall effect and longitudinal magneto-resistance in the high-temperature superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ are examined. Samples were grown using Combinatorial Molecular Beam Epitaxy (COMBE), which allows electrical transport measurements with a doping resolution of $\Delta x \approx 0.0002$. This is an increase in doping resolution of about 50-fold compared to typical magneto-resistance measurements. The experiments were performed in pulsed magnetic fields up to 57 T with the goal of investigating transport properties of the magnetically-induced normal state near optimum doping. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y46.00013: Force balance on two-dimensional superconductors with a single moving vortex Chun Kit Chung, Emiko Arahata, Yusuke Kato We study forces on two-dimensional superconductors with a single moving vortex based on a recent fully self-consistent calculation of DC conductivity in an $s$-wave superconductor (E. Arahata and Y. Kato, arXiv:1310.0566). By considering momentum balance of the whole liquid, we attempt to identify various contributions to the total transverse force on the vortex. This provides an estimation of the effective Magnus force based on the quasiclassical theory generalized by Kita [T. Kita, Phys. Rev. B, {\bf 64}, 054503 (2001)], which allows for the Hall effect in vortex states. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y46.00014: Intermediate state: a new look at an old story Vladimir Kozhevnikov, Rinke Wijngaarden, Jesse de Wit, Chris Van Haesendonck One of the central problems of superconductivity is magnetic structure of vortices and elicitation of microscopic parameters from parameters of the mixed state (MS) in type-II superconductors. Similar problem, i.e. magnetic structure of normal (N) domains and elicitation of the microscopic parameters from parameters of the intermediate state (IS) in type-I materials, is the longest standing problem of superconductivity advanced by Landau in 1930s. We will report on our recent study of the IS in a high purity indium films using magneto-optical imaging, and transport and magnetization measurements. The least expected observation is that the magnetic flux density in N-domains can be as small as nearly 40\% of the thermodynamic critical field $H_c$. This fact contradicts and hence overthrows a paradigm stating that the N-phase is unstable in the fields less than $H_c$. We will present a new theoretical model of the IS for the first time consistently addressing this and $all$ other properties of the IS. Moreover, our model, based on rigorous thermodynamics of the equilibrium flux structure, allows for quantitative determination of the domain-wall parameter and the coherence length. Possible impact of our model on the vortex structure will be discussed. [Preview Abstract] |
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