Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J10: Superconductivity: Vortex Imaging |
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Sponsoring Units: DCMP Chair: Timir Datta, University of South Carolina Room: Colorado Convention Center Korbel 1E |
Tuesday, March 6, 2007 11:15AM - 11:27AM |
J10.00001: Low Temperature STM Study of Vortex Motion on Fe doped NbSe2 Hui Wang, Jonghee Lee, Michael Dreyer, Barry Barker We investigated the vortex motion around magnetic Fe impurities on type II superconductor NbSe2 by a home built low temperature STM. Using Scanning Tunneling Spectroscopy Maps we recorded the movie of the motion at 4 K with a very slow decaying rate of the magnetic field ($\sim $ 5 nT/s). The map images were taken with a 400 nm by 400 nm field of view and in a 0.75 T magnetic field to start with. Each frame of the movie has $\sim $ 109 vortices and takes $\sim $ 8 min to acquire. Scanning tunneling spectroscopy data show that the superconductivity is destroyed at the impurity sites, which indicates that they serve as attractive pinning centers for the vortex lattice. The behavior of the overall motion of the vortex lattice can be explained by the Larkin-Ovchinnikov collective pinning theory. The average speed of the motion is $\sim $ 5 pm/s. Our STS movie data display the pinning and depinning events of a single vortex around the pinning center. A flux creep model will be exploited to understand the effect of the pinning centers on the vortex motion. [Preview Abstract] |
Tuesday, March 6, 2007 11:27AM - 11:39AM |
J10.00002: Local vortex--defect interaction in moving vortex lattices observed by STM Michael Dreyer, Jonghee Lee, Hui Wang, Barry Barker When applying a magnetic field to a type II superconductor, part of the magnetic flux penetrates the sample forming a current vortex. At high enough fields and low enough defect concentration the vortices form a 2D triangular lattice. We observed the vortex lattice on NbSe$_2$ single crystals using STM ($B=250-750$ mT, $T=4.2$ K). Due to a slow decay of the magnetic field of our superconducting magnet ($\sim -5$ nT/s) the vortices collectively move at an average speed of about 5 pm/s. The motion was observed by tracking the center of a vortex across consecutive images of the vortex lattice. The motion shows distinct acceleration/deceleration cycles we associate with collective pinning events on nearby defect sites. A more subtle observation was the deviation of the vortex positions from their `expected' location within the lattice of up to 3 nm. A similar effect was found in 2D simulations of a moving vortex lattice near defect sites. Since it takes an additional force to move a vortex out of position, we can identify subsurface defects and analyze the defect--vortex interaction. Results of the analysis will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 11:39AM - 11:51AM |
J10.00003: Lorentz Imaging of Superconducting Flux Vortices with a Commercial Transmission Electron Microscope James Loudon, Paul Midgley Magnetic flux penetrates type II superconductors along normal channels called flux vortices, each containing a single quantum of flux. It is beneficial to image these vortices and study their response to external stimuli as they determine the performance of many superconducting devices. Tonomura's research group have demonstrated that vortices can be imaged by transmission electron microscopy because of the deflection the electrons suffer as they pass through the magnetic flux within the vortices (Harada K. \textit{et al.}, Nature 360, 51, 1992). This technique offers spatial resolution superior to other techniques, real-time imaging and is sensitive to magnetic flux throughout the material, not simply surface fields. To our knowledge, Tonomura's is the only group to have successfully employed this technique and their experiments required custom-built high voltage microscopes. Here we demonstrate that flux vortices can be imaged with a commercially available electron microscope, opening the field for other researchers. We show images of flux vortices in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ and analyze their arrangements as a function of applied magnetic field and temperature. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J10.00004: Single Vortex Resolution Imaging of the Flux Front in a YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ Single Crystal Lan Luan, O. M. Auslaender, K. A. Moler, D. A. Bonn, Ruixing Liang, W. N. Hardy We have imaged the vortex state in an optimally doped, detwinned YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ single crystal, using magnetic force microscopy (MFM). The structure of the flux front, the boundary between vortices and anti-vortices, is studied with single vortex resolution, as it evolves with applied magnetic field. We find the front to be corrugated on a scale of several microns. On a smaller scale, the front is composed of alternating ``fingers'' of vortices and anti-vortices. We also observe stable vortex-antivortex pairs. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J10.00005: Magnetic Induction Profile in Superconductor/Ferromagnet Bilayers Qiang Li, Zuxin Ye Strong suppression of flux density peaks at the edge of a superconducting film was observed by magneto-optical imaging the magnetic induction profiles of an YBCO superconducting film on a magnetic substrate in perpendicular magnetic fields, The observed induction profile is in a striking contrast to the case of superconducting films on a non-magnetic substrate that display sharp flux density peaks at the edges of the films. The influence of magnetic substrate on the flux distribution in superconductor films may be modeled by considering the formation of a virtual infinite stack of superconducting films due to the magnetic mirror effect. We also found that the flux patterns in the magnetic substrate were strongly influenced by the flux distribution in the superconductor. These observations, results of computer simulations, and their implications to the transport and magnetization properties of superconducting films will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J10.00006: Dragging Individual Vortices to Probe the Dimensionality of Pinning in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ O. M. Auslaender, Lan Luan, K. A. Moler, R. A. Hughes, J. S. Preston, D. A. Bonn, Ruixing Liang, W. N. Hardy We have used a magnetic force microscope (MFM) to image and to manipulate individual vortices in optimally doped YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ samples: a 200nm film and a detwinned single crystal. In the film, if the force exerted by the MFM tip is strong enough to overcome the pinning potential, a pinned vortex jumps as a whole to a new pinning site. We find a wide spread of depinning forces, attesting to the importance of point pinners as opposed to pinning along one-dimensional defects. The behavior in the single crystal is very different. Even when a vortex is pinned the shape of its image is distorted, perhaps indicating meandering of the vortex line to take advantage of pinning centers as it traverses the crystal. When we drag a vortex, it tilts significantly before depinning, as signified by pronounced stretching of its image. This effect is highly anisotropic and depends on the pulling direction. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J10.00007: Manipulation of the magnetic flux in superconductor by the ferromagnetic domains in SC/FM hybrid Vitalii Vlasko-Vlasov, Ulrich Welp, Goran Karapetrov, Valentin Novosad, Andrei Belkin, Daniel Rosenmann, Wai Kwok We studied magneto-optically the magnetic flux entry and exit in SC/FM hybrid of a ferromagnetic permalloy film sputtered on the superconducting NbSe2 single crystal. The FM film had growth induced perpendicular anisotropy and the labyrinth equilibrium domain structure. However, we could align the domain walls in a desired direction by application of a strong enough in-plane field. Thus formed stripe domains introduce a pronounced directionality for the vortex motion in the underlying superconductor. The effect persists up to the fields of the stripe domain collapse and does not depend on the temperature at which the domain walls were polarized. It does not change at heating the sample and cooling back below Tc. We discuss the effect in terms of the domain wall pinning of vortices in conditions when the domain size is larger than the coherence length and propose a scheme for manipulating the transport properties of superconductors by the ferromagnetic domains. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J10.00008: Magnetic Force Microscopy of Superconducting Vortices in an Ordered Array of Artificial Pinning Centers Joel Keay, Preston Larson, Kevin Hobbs, Matthew Johnson, Ophir Auslaender, Kathryn Moler, John Kirtley Ordered arrays of artificial pinning centers were fabricated in Nb thin films using anodic aluminum oxide (AAO) as a template. These artificial pinning arrays have a triangular lattice parameter of 105 nm and antidot diameters of about 50 nm. The nanohole arrays show only a small decrease in the superconducting transition temperature, Tc$\simeq$7.1 K, from comparable unprocessed Nb thin films. Enhancement of the magnetization at the first, second and third matching fields (matching field = 2170 Oe) were observed in the magnetization half-loops of these arrays at 5 K. Magnetic Force Microscopy (MFM) was used to image the nanohole arrays above and below Tc . These images clearly show the nanohole lattice. Individual vortices have been imaged at low fields and their movement within the lattice is being explored. Further, domain rings have been imaged at low and high fields at temperatures between 5 and 5.5 K. Their properties are currently being investigated. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J10.00009: Vortex dynamics in mesoscopic weak-pinning superconducting channels with a Corbino geometry. T.W. Heitmann, K. Yu, C. Song, B.L.T. Plourde, M.B.S. Hesselberth, P.H. Kes We report transport measurements of vortex flow dynamics in mesoscopic weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. The channels are arranged in circular patterns on a Corbino disk geometry, thus eliminating the influence of edge barriers to vortex entry on the dynamics. The number of vortices which can be detected at particular flow velocities is limited by the method for measuring the flux flow voltage and the channel configuration. We discuss potential applications of this system for guiding vortices around nanofabricated structures free from edge barriers. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J10.00010: Structured pinning potentials for guiding vortex motion in superconductors. K. Yu, T.W. Heitmann, C. Song, B.L.T. Plourde, M.B.S. Hesselberth, P.H. Kes Nanofabricated pinning structures can be used to guide vortices in superconductors through various potential energy landscapes. We report transport measurements of vortex flow dynamics in structured weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. By arranging the channels in circular patterns on a Corbino disk geometry, we eliminate the influence of edge barriers to vortex entry on the dynamics. Patterning channel edges with different shapes allows us to explore the influence of the confinement potential on the vortex dynamics. We discuss one such pattern with channel edges in an asymmetric sawtooth configuration for investigations of vortex ratchet dynamics. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J10.00011: Dynamic flux-quantum phases in weak-pinning V$_{3}$Si and Re$_{3}$W A. A. Gapud, J. D. Hebert, A. Moradmand, P. Shrestha, A. Khan, Y. Zuev, D. K. Christen, V. Kuznetsova, J. R. Thompson The dynamics of transport-driven flux quanta in the \textit{absence }of pinning is a fundamental phenomenon little understood and studied by few. This is mainly because of a rarity of highly homogeneous type II samples with few pinning defects, combined with the technical challenge of passing high levels of transport current through such samples. These issues are addressed by the use of ultrasonically soldered leads and pulsed currents, in addition to the availability of relatively defect-free samples of the low-temperature superconductors, V$_{3}$Si and Re$_{3}$W. This enabled the study the critical-current ``peak effect'' in V$_{3}$Si [\textit{PRB }\textbf{67}, 104516], which also included the observation of metastable phases in connection with the peak effect, still to be reported in greater detail. Another observation is that of dissipative flux flow phases, in both V$_{3}$Si and Re$_{3}$W, along with evidence of an \textit{approach} towards the highly-ordered Bardeen-Stephen phase of free flux flow. The field dependence of free flux flow resistivity is also of interest in probing vortex \textit{core size} effects [\textit{PRB }\textbf{71}, 134505]. All of these are to be discussed in detail. \textit{Research at ORNL supported by DOE Office of Electricity Delivery and Energy Efficiency and DOE Office of Science, Basic Energy Sciences.} [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J10.00012: Vortex-antivortex molecules in superconducting films with magnetic dot arrays M.V. Milosevic, J.S. Neal, S.J. Bending, A. Potenza, C.H. Marrows Following earlier works [Milosevic and Peeters, PRB (2003), PRL (2004)], we studied the vortex-antivortex stabilization in a superconducting film under a square array of magnetic dots of variable size. The theoretical side of the investigation was done within the Ginzburg-Landau theory, and main findings comprise: (i) multi-shell vortex-antivortex structures, (ii) the profound interaction between neighboring vortex-antivortex molecules through exchange of ``valence'' antivortices, and (iii) dual interaction of stabilized vortex-antivortex pairs and magnetic dots with excess flux-lines of the applied homogeneous magnetic field. On experimental side, the results are corroborated by scanning Hall probe measurements, performed on a 80nm thick Pb film, on top of a square array (period 5$\mu $m) of magnetic dots of four sizes - R=0.522, 0.738, 0.808, and 0.902$\mu $m, etched out of a [2nm Pt] [0.6nm Co/1.0nm Pt]$_{10}$ multilayer film with perpendicular magnetization. A 20nm thick Ge layer was evaporated on top of the dots to avoid the proximity effect. In measurements performed at T=5K, direct SHPM images showed the structure of antivortices between the magnetic dots, whereas the successive difference images revealed the positioning of additional vortices in applied homogeneous magnetic field. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J10.00013: Vortex-antivortex phenomena in superconductors with antidot arrays Golibjon Berdiyorov, Milorad Milosevic, Roeland Geurts, Francois Peeters We investigated in detail the vortex configurations in superconducting films with regular antidot-arrays within the non-linear Ginzburg-Landau theory, where demagnetization effects and overlapping vortex cores are fully taken into account (contrary to the London approach). In addition to the well-known matching phenomena, we predict: (i) the \textit{nucleation of giant-vortex states} at interstitial sites; (ii) the \textit{combination of giant- and multi-vortices} at rational matching fields; and (iii) for particular interstitial vorticity, the symmetry imposed creation of \textit{vortex-antivortex configurations}. As a consequence of (iii), we predict \textit{resistance maxima }at particular matching fields, opposite to the expected minima due to commensurability effects. Using the same principle, we stabilized vortex-antivortex molecules in finite submicron superconducting polygons by strategically placed nanoholes. Compared to earlier predictions, we \textit{enhanced} the stamina of the antivortex with respect to temperature, applied fields and geometrical defects in the sample. Further, increased vortex-antivortex spacing and pronounced amplitudes of the local magnetic field in our system make these fascinating structures \textit{observable} by e.g. Scanning Tunneling or Hall probe microscopy. [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J10.00014: Novel Commensurability Effects and Enhanced Pinning at Nonmatching Fields for Vortices Interacting with Diluted Periodic Pinning Arrays Charles Reichhardt, Cynthia J. Olson Reichhardt Using numerical simulations, we demonstrate that periodic pinning arrays that have been diluted by removing some fraction of the pinning sites at random have pronounced commensurability effects at the {\it same} field strength as undiluted pinning arrays. The commensuration can occur at fields significantly higher than the field corresponding to one-to-one matching between the diluted pinning array and the vortices, and the effect persists for periodic arrays with up to 90 percent dilution. We show that samples with diluted periodic pinning arrays produce a considerable enhancement of the critical current for fields above the first matching field compared to samples with purely random pinning arrangements. These results suggest that diluted periodic pinning arrays may be a promising geometry to increase the critical current in superconductors over a wide magnetic field range. [Preview Abstract] |
Tuesday, March 6, 2007 2:03PM - 2:15PM |
J10.00015: Influence of Au layer on the morphology and superconductivity of the ultra-thin Pb film using Low-temperature STM/S Shengyong Qin, Alexander Khajetoorians, Chih-Kang Shih Thin film superconductivity is a subject of great scientific importance. Recently by using epitaxial thin Pb films, two papers reported the observation of quantum oscillations of thickness-dependent superconductivity. By using ex situ transport measurements on Pb thin films grown on Si(111) substrate and subsequently covered with 2 ML of Au, Guo et al. reported Tc oscillation between 22 and 28 MLs and a rapid decrease of Tc below 20 ML. On the other hand, Eom et al., by using in situ tunneling spectroscopy to measure the superconducting gap directly, reported persistent quantum oscillations of superconductivity from 18 ML down to 5 ML without any sign of quenching. One explanation of such apparent inconsistency is the existence of the Au capping layer used in the ex-situ transport measurements. Here we explore the role of Au capping layer on superconductivity of Pb thin film directly using STM/S. We show that the Au capping layer induces significant roughening of the Pb thin films. Moreover, we found that the deposition of Au first induces the formation of AuPb alloy followed by Au overlayer. Direct measurement of superconducting gaps on the film at different stages of Au deposition are also performed. The details of how Au overlayer impact the superconductivity of thin Pb films will be presented. [Preview Abstract] |
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