Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J11: Focus Session: Engineering Vortex Matter in Superconductors I |
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Sponsoring Units: DMP Chair: Ulrich Welp, Argonne National Laboratory Room: 007B |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J11.00001: Observing real time motion of nano-scale objects Joris Van de Vondel, Matias Timmermans, Tom\'as Samuely, Bart Raes, Lise Serrier-garcia, Victor Moshchalkov The dynamics of nanoscale objects is a very interesting field of research with a strong technological impact. Still, the combination of a technique resolving (sub)nanometer particles within a time frame relevant to observe dynamics is a very challenging task. Due to the inherent atomic-scale resolution, scanning tunneling microscopy (STM) is an ideal candidate to achieve this goal. Nevertheless, in most physical systems the dynamic events of the objects under investigation cannot be resolved by conventional STM image acquisition and will only reveal an average trace of the moving object. This is why a strong drive exists to develop new functionalities of STM, which allow studying dynamic events at the nanoscale. We address this issue, for vortex matter in NbSe$_{\mathrm{2}}$, by driving the vortices using an ac magnetic field and probing the induced periodic tunnel current modulations [1]. Our results reveal different dynamical modes of the driven vortex lattice. In addition, by extending a known functionality of STM, ( i.e. the `Lazy Fisherman' technique) we can use single pixel information to obtain the overall dynamics of the vortex lattice with submillisecond time resolution and subnanometer spatial resolution. [1] M. Timmermans et al., ACS NANO, 8 (3), 2782 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J11.00002: Vortex cutting in superconductors Vitalii K. Vlasko-Vlasov, Alexei E. Koshelev, Andreas Glatz, Ulrich Welp, Wai-K. Kwok Unlike illusive magnetic field lines in vacuum, magnetic vortices in superconductors are real physical strings, which interact with the sample surface, crystal structure defects, and with each other. We address the complex and poorly understood process of vortex cutting via a comprehensive set of magneto-optic experiments which allow us to visualize vortex patterns at magnetization of a nearly twin-free YBCO crystal by crossing magnetic fields of different orientations. We observe a pronounced anisotropy in the flux dynamics under crossing fields and the filamentation of induced supercurrents associated with the staircase vortex structure expected in layered cuprates, flux cutting effects, and angular vortex instabilities predicted for anisotropic superconductors. At some field angles, we find formation of the vortex domains following a type-I phase transition in the vortex state accompanied by an abrupt change in the vortex orientation. To clarify the vortex cutting scenario we performed time-dependent Ginzburg-Landau simulations, which confirmed formation of sharp vortex fronts observed in the experiment and revealed a left-handed helical instability responsible for the rotation of vortices. [Preview Abstract] |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J11.00003: Detecting vortices in superconductors: Extracting one-dimensional topological singularities from a discretized complex scalar field Carolyn Phillips, Tom Peterka, Dmitry Karpeyev, Andreas Glatz In type-II superconducting material, the dynamics of the vortices play a critical role in determining the performance of the material. ~In Ginzburg-Landau simulations of superconducting materials, vortices correspond to topological singularities in a discretized complex scalar field. Visualizing the vortices to understand their behavior is a key step in using simulations to engineer optimized pinning landscapes. ~In the past, vortices have been visualized by examining contour plots and isosurfaces of the magnitude of the field. ~However, these methods, primarily used for small-scale simulations, blur the fine details of the vortices, scale poorly to large-scale simulations, and do not easily enable isolating and tracking individual vortices. ~We present a method for exactly finding the vortex core lines from a complex order parameter field. With this method, the vortices can be easily described at a resolution even finer than the mesh itself. ~The precise determination of the vortex cores allows the interplay of the vortices inside a model superconductor to be visualized in higher resolution than has previously been possible. ~ [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J11.00004: Improved critical current in confined superconductors in parallel field configuration Andreas Glatz, Igor Aronson, Yonglei Wang, Zhili Xiao We present results on the re-entrance of the superconducting state in systems placed into a magnetic field parallel to the applied current. In experiments it was observed that the magneto-resistance first increases with magnetic field, but at higher field drops again such that superconductivity is recovered. This effect is strongly temperature dependent and can lead to a suppression of resistance below the measurable threshold over a range of a few kG. We study the vortex dynamics and magneto-resistance in this situation in the framework of a large-scale time-dependent Ginzburg Landau simulation. A small external current as well as the magnetic field are applied in the x-direction, the latter is then ramped up. Our simulations reproduce this effect and reveal the mechanism for the observed behavior: the intermediate resistive state is due to a vortex instability leading to an unwinding of twisted vortex configurations. This leads to a periodic dynamic resistive state. When the field increases these instabilities get stabilized due to a higher vortex density and the resistance drops upon increasing the magnetic field. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J11.00005: Interplay between vortex matter phases and arrays of pinning centers in low temperature superconductors Jose L. Vicent, Javier del Valle, Alicia Gomez, Manuel Rodriguez, Daniel Granados, Fernando Galvez, Elvira M. Gonzalez We have studied vortex matter phases in Nb films grown on Si substrates with arrays of Cu nanodots. The symmetry of the pinning arrays rules the presence of vortex matter phases. Four-fold symmetry arrays enhance the vortex glass transition temperature, at matching fields, in comparison with plain Nb films. This is a similar effect that obtained using arrays of magnetic pinning centers (Villegas et al. PRB72, 174512). Breaking the symmetry of the pinning array, such that the array mimic a smectic crystal, leads to a new phase, in a very narrow temperature window between the liquid and glassy phases, which can be identified with a vortex smectic phase. Remarkably, the smectic vortex phase is enhanced increasing the array symmetry. Increasing the number of vortices vanishes this smectic phase. (H,T) phase diagrams will be presented for different types of arrays. [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J11.00006: Vortex-Antivortex coexistence in Nb based Superconductor/Ferromagnet heterostructures C. Di Giorgio, F. Bobba, A. Scarfato, M. Longobardi, M. Iavarone, S.A. Moore, G. Karapetrov, V. Novosad, V. Yefremenko, A.M. Cucolo Superconductor/Ferromagnet thin film heterostructures, based on Niobium/Permalloy (Nb/Py), have been studied by low temperature Magnetic Force Microscopy. The experimental observation of spontaneous Vortex-Antivortex in these systems depends on the Nb penetration depth and thickness, on the intensity of the Py stray field induced by the small, alternating up-and-down, out-of-plane component $M_0$, and the width of the Py stripe-like magnetic domains. Comparison with the available theoretical models allows to estimate the threshold of the local $M_0$ for different Py thicknesses. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 4:18PM |
J11.00007: Development of strong vortex pinning and very high $J_{c}$ in iron based superconductors Invited Speaker: Chiara Tarantini Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ (Ba122) is the most tunable of the Fe-based superconductors (FBS) in terms of its acceptance of high densities of secondary phases capable of acting as effective pinning centers without depressing the properties of the superconducting matrix. It has been demonstrated that self-assembled nanorods made of Ba-Fe-O generate a strong correlated pinning along the c-axis, enhancing the critical current density, $J_{c}$, in this direction and reducing the $J_{c}$ anisotropy [1, 2]. However, when 20{\%} of secondary phases are introduced, the reduction of the cross-section becomes significant, decreasing the low field performance. In order to overcome this issue, artificially introduced pinning centers can be added by multilayer deposition producing an almost isotropic increase of $J_{c}$ [2]. Moreover, FBS are very sensitive to strain, allowing an important enhancement in the critical temperature, $T_{c}$, of the material. It will be shown that strain induced by the substrate can further improve $J_{c}$ of both single and multilayer films by more than expected because of the $T_{c}$ increase. The multilayer deposition of Ba122 on CaF$_{2}$ increases the pinning force density, $F_{p}$, by more than 60{\%} compared to a single layer film, reaching a maximum of 84 GN/m$^{3}$ at 22.5T and 4.2 K, the highest value ever reported in any 122 phase. This work shows that the in-field performance of Ba122 widely exceeds that of Nb$_{3}$Sn above 10T, attracting attention for possible applications. \\[4pt] [1] C. Tarantini, S. Lee, Y. Zhang, J. Jiang, C.W. Bark, J.D. Weiss, A. Polyanskii, C.T. Nelson, H.W. Jang, C.M. Folkman, S.H. Baek, X.Q. Pan, A. Gurevich, E.E. Hellstrom, C.B. Eom, D.C. Larbalestier, Appl.Phys.Lett. 96, 142510 (2010) \\[0pt] [2] C. Tarantini, S. Lee, F. Kametani, J. Jiang, J.D. Weiss, J. Jaroszynski, C.M. Folkman, E.E. Hellstrom, C.B. Eom, D.C. Larbalestier, Phys.Rev.B 86, 214504 (2012) \\[0pt] [3] C. Tarantini, F. Kametani, S. Lee, J. Jiang, J.D. Weiss, J. Jaroszynski, E.E. Hellstrom, C.B. Eom, D.C. Larbalestier, Scientific Reports (2014) [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J11.00008: Defects and critical current in REBCO films by ion irradiation Qiang Li, Toshinori Ozaki, Cheng Zhang We will present our recent studies on the defects and critical current density (J$_{\mathrm{c)}}$ in superconducting ReBa$_{\mathrm{2}}$Cu$_{3}$O$_{7}$ films (Re $=$ Y, or rare earth element) irradiated by several types of ions at energy level between hundreds of KeV and tens of MeV. We observed remarkable enhancement of J$_{\mathrm{c}}$ in some of the irradiated films at low temperature and at high magnetic fields up to 35 T. We examined the ion irradiation induced defects by using advanced transmission electron microscopy. It was found that the ion irradiation at this kinetic energy range produces defects that are rather small ($\sim$ a few nanometers) in physical size. However, these defects were found to create a substantially large strain field in the vicinity that depresses the pair potential and produce effective flux pinning at low temperatures. As the temperature approaching $T_{\mathrm{c}}$, the irradiation induced pinning was found less effective, presumably due to the softening of vortex line. A correlation between the nano-structures of the defects and critical current will be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J11.00009: Enhanced critical currents of commercial 2G superconducting coated conductors through proton irradiation Ulrich Welp, M. Leroux, K.J. Kihlstrom, W.-K. Kwok, A.E. Koshelev, D.J. Miller, M.W. Rupich, S. Fleshler, A.P. Malozemoff, A. Kayani We report on magnetization and transport measurements of the critical current density, Jc, of commercial 2G YBCO coated conductors before and after proton irradiation. The samples were irradiated along the c-axis with 4 MeV protons. Proton irradiation produces a mixed pinning landscape composed of pre-existing rare earth particles and a uniform distribution of irradiation induced nm-sized defects. This pinning landscape strongly reduces the suppression of Jc in magnetic fields resulting in a doubling of Jc in a field of $\sim$ 4T. The irradiation dose-dependence of Jc is characterized by a temperature and field dependent sweat spot that at 5 K and 6 T occurs around 20x10$^{16}$ p/cm$^2$. Large-scale time dependent Ginzburg-Landau simulations yield a good description of our results. 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 (KK, ML, AEK) and by the D.O.E, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357 (UW, WKK). [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J11.00010: Optimization of vortex pinning by nanoparticles using numerical simulations Alexei Koshelev, Ivan Sadovskyy, Carolyn Phillips, Andreas Glatz Vortex pinning by self-assembled nanoparticles has been established as an efficient route to enhance current-carrying capability of practical superconductors. We explore vortex pinning by randomly distributed metallic spherical nanoparticles using large-scale numerical simulations of time-dependent Ginzburg-Landau equations. We found optimal size and density of particles at which the highest critical current realizes for fixed magnetic field. For every particle size the critical current reaches maximum value at certain particle density, typically corresponding to 15-22\% of the volume fraction filled by the particles, which is close to the percolation concentration. This optimal particle density increases with the magnetic field. We also found that the optimal particle diameter is close to 4 coherence lengths. Our results provide guidance for pinning optimization in practical superconductors. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J11.00011: Highly effective mixed pinning landscape produced by combined proton and heavy-ion irradiations in commercial coated conductors Leonardo Civale, Maxim Leroux, Karen Kihlstrom, Ulrich Welp, Wai-Kwong Kwok, Marty Rupich, Steven Fleshler, Alex P. Malozemoff, G. Ghigo, A. Kayani Particle irradiation is a very useful method to enhance the critical current density (J$_{c})$ in high T$_{c}$ superconductors. As the nature of the damage produced under given irradiation conditions is well studied, it also provides a valuable tool to engineer controlled pinning landscapes to improve our understanding of vortex matter. Recently, it has been shown that proton irradiation can produce significant further J$_{c}$ increase in commercial coated conductors (CC) with already high J$_{c}$. Here we report a further step towards J$_{c}$ design, by combining 4 MeV proton and 250 MeV Au irradiations on the same CC. We show that the J$_{c}$ improvement is better than what results from each individual irradiation, with columnar and random defects being dominant at low and high fields, respectively. Flux creep rates provide additional information about the vortex dynamics and depinning mechanisms in different regions of the Temperature-Field-Orientation phase diagram. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J11.00012: Towards critical current by design Ivan Sadovskyy, Alexei Koshelev, Andreas Glatz Understanding dynamic behavior of vortex matter in complicated pinning landscapes is a major challenge for both fundamental science and energy applications. In particular, critical current can be significantly enhances by optimizing type, size and density of inclusions. We present the large-scale simulations for vortex dynamics. We analyzed a number of inclusion types and found optimal configurations corresponding to the maximal critical current. Particularly, we studied the interplay between vortex-vortex and vortex-inclusion interactions in the presence of columnar defects (e.g., irradiated by heavy ions at an angle), chemically grown nanorods, spherical inclusions (e.g., irradiated by protons or self-assembled). We compared our simulations to several experimental results and found a good agreement. [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J11.00013: Temperature dependent pinning landscapes in REBCO thin films Jan Jaroszynski, Anca-Monia Constantinescu, Xinbo Paul Hu The pinning landscapes of REBCO (RE=rare earth elements) thin films have been a topic of study in recent years due to, among other reasons, their high ability to introduce various phases and defects. Pinning mechanisms studies in high temperature superconductors often require detailed knowledge of critical current density as a function of magnetic field orientation as well as field strength and temperature. Since the films can achieve remarkably high critical current, challenges exist in evaluating these low temperature (down to 4.2 K) properties in high magnetic fields up to 30 T. Therefore both conventional transport, and magnetization measurements in a vibrating coil magnetometer equipped with rotating sample platform were used to complement the study. Our results clearly show an evolution of pinning from strongly correlated effects seen at high temperatures to significant contributions from dense but weak pins that thermal fluctuations render ineffective at high temperatures but which become strong at lower temperatures Support for this work is provided by the NHMFL via NSF DRM 1157490 [Preview Abstract] |
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