Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B12: Vortices in Superconductors II |
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Sponsoring Units: DCMP Chair: Laura Greene, UIUC Room: LACC 402A |
Monday, March 21, 2005 11:15AM - 11:27AM |
B12.00001: Investigation of vortex-matter states in Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ B. J. Taylor, D. J. Scanderbeg, M. B. Maple, C. Kwon The magnetic field - temperature (H - T) phase diagram of the doped high-temperature superconducting cuprate system Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ [x = 0.0 - 0.4] has been extended to high magnetic fields previously unexplored for this system. The vortex-matter in thin film samples has been investigated via transport measurements in slow swept and static magnetic fields up to 45 tesla and to temperatures as low as 1.4 K. Analysis of the vortex-glass melting line (irreversibility line), H$_{g}$(T) (H$_{irr}$(T)), indicates that it follows the empirical scaling form (H$_{g}$/H$^{*}$) $\sim$ (1-T/T$_{c}$)$^{n}$ for all concentrations x, and is broken into three distinct regions with n $\sim$ 1.2, 2.0, 3.1 with increasing field. Furthermore, the field values at which the change in behavior occur are at H$_{1}$$^{*}$ $\sim$ Hg(T = 0.6 Tc) and H$_{2}$$^{*}$ $\sim$ H$_{g}$(T = 0.3 Tc) $\sim$ 3H$_{1}$$^{*}$. All measurements above 9 tesla were performed at the National High Magnetic Field Laboratory (NHMFL) facilities in Tallahassee FL, USA. This work was supported by the U. S. Department of Energy (DE-FG02-04ER46105) [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B12.00002: Evolution of Depinning Force Density of $Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7-\delta}$ Single Crystals with Temperature and Magnetic Fields P. Gyawali, T. Katuwal, V. Sandu, C.C. Almasan, B.J. Taylor, M.B. Maple We measured the zero field cooled magnetic hysteresis of the $Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7-\delta}$ single crystals with x = 0.14, 0.34, 0.47, 0.53 at various temperatures and magnetic fields. The critical current density $J_{c}$ was determined using the Bean's critical state model. The magnetic field and temperature dependences of the depinning force density $F_{dp}$ were then calculated. The $F_{dp}$ changes nonmonotonically with magnetic field. Specifically, it increases linearly with H to a peak value and then decreases with further increasing magnetic field. The position of the peak shifts to lower H values with increasing temperature. Also, for all measured H, the value of $F_{dp}$ decreases with increasing temperature. While previous studies have shown that small Pr amounts lead to an increase in the pinning energy of $YBa_{2}Cu_{3}O_{7-\delta}$, our results show that a higher concentration of Pr in $YBa_{2}Cu_{3}O_{7-\delta}$ decreases the pinning energy. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B12.00003: Manipulating vortex motion by thermal and Lorentz force in high temperature superconductors Zhi Wang, Lei Shan, Yingzi Zhang, Fang Zhou, Jiwu Xiong, Wenxin Ti, Hai-Hu Wen By using thermal and Lorentz force, the vortex motion is successfully manipulated in the mixed state of La$_{2-x}$Sr$_{x}$CuO$_{4}$ single crystals and YBa$_{2}$Cu$_{3}$O$_{7}$ thin films. In the normal state, in order to reduce the dissipative contribution from the quasiparticle scattering and enhance the signal due to the possible vortex motion, a new measurement configuration is proposed. It is found that the in- plane Nernst signal ( H $\vert \vert $ c) can be measurable up to a high temperature in the pseudogap region, while the Abrikorsov flux flow dissipation can only be measured up to T$_{c}$. This may point to different vortices below and above T$_{c}$ if we attribute the strong Nernst signal in the pseudogap region to the vortex motion. Below Tc the dissipation is induced by the motion of the Abrikosov vortices. Above Tc the dissipation may be caused by the motion of the spontaneously generated vortex anti-vortex pairs. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B12.00004: Magnetism near Vortex Cores of Cuprate Superconductors J.C. Lee, K. Prudchenko, B. Launspach, E.J. Ruiz, C. Boekema We examined muon-spin-resonance ($\mu $SR) vortex data of Bi2212, Tl2223, and YBCO to search for antiferromagnetism (AF) near the vortex cores. [1] Field distributions were obtained from $\mu $SR data using Maximum-Entropy analysis. The grainboundary and vortex signals were fitted by Gaussian and Lorentzian curves, the latter suggestive of extra AF ordering. Narrow Gaussians fit the grainboundary signals well, independent of temperature. For T $<$ 0.4T$_{c}$, Lorentzians fit much better than Gaussians on the high-field side associated with the vortex core. Such results suggest that magnetism exists near the vortex cores. [1,2] The field dependence of the YBCO AF Lorentzian width is discussed. An AF presence near vortex cores supports theories that predict spin ordering for cuprate superconductivity. Research supported by REU-NSF, WiSE@SJSU {\&} SJSU College of Science. [1] J. Lee \textit{et al}, J Appl Phys 95 (2004) 6906, and \textit{Virtual J Appl of Superconductivity, June 2004 V6 Issue11;} K Prudchenko \textit{et al}, \textit{www.jyi.org/volumes/volume10/issue6/articles/prudchenko.html} [2] C. Boekema \textit{et al}, Int J Modern Phys B17 (2003) 3436. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B12.00005: Correlation of Vortex Pinning with Topography in a Thin Film YBCO Sample N.C. Koshnick, O.M. Auslaender, J.E. Hoffman, E. Straver, K.A. Moler, R.A. Hughes, J.S. Preston Understanding the local effects that lead to vortex pinning is an important challenge for the development of high temperature superconductor technologies. We have developed a magnetic force microscope (MFM) that can image an individual vortex and place a bound on its pinning strength. By moving the magnetized cantilever tip closer to the surface, we can enter a regime where atomic forces dominate over the magnetic signal, allowing for the correlation of the pinned vortex locations with a map of the topographic features. We will describe the implementation of this technique on a superconducting YBCO film. [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B12.00006: Scanning hall probe microscopy of AC losses in YBCO coated conductors Rafael Dinner, George Daniels, David Larbalestier, Brady Gibbons, Vladimir Matias, Kathryn Moler, Malcolm Beasley Magnetic imaging of current-induced vortex movement in superconducting films yields detailed information about dissipation and the path of an applied current. In our large-area scanning hall probe microscope, a flow cryostat cools a sample while a micro-Hall probe is rastered near its surface using a 3-axis stepper-motor-based stage with submicron resolution and centimeter scan range. Hall probe time traces taken at each point are assembled into movies of the flux penetration as a function of time over a cycle of AC sample current. YBCO films grown on several substrates are examined, including bicrystal substrates that induce a single grain boundary across the current path and metal tapes that give rise to a grain boundary network. An extended Bean model allows us to extract pinning forces and critical currents of the intragrain film and its grain boundaries. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B12.00007: Comparative Study of Vortex Phase Transitions in Bi-2212 Single Crystals in Tilted Magnetic Fields Jovan Mirkovic, Atsushi Nakano, Hirokazu Sato, Takashi Yamamoto, Kazuo Kadowaki The vortex phases in high-quality Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$single crystals have been studied by means of the in-plane resistivity measurement in the Corbino electric contacts geometry, and the local ac-magnetic permeability measurements by using the miniature coils. For the first time, the nontrivial structural first-order phase transition has been found by both techniques, deep in the crossing lattice state. The transition separates the Abrikosov dominant (strong pinning) phase from the Josephson dominant (weak pinning) phase. The peak-effect of resistance was observed as a precursor of the lock-in vortex state in the critical angular range near the \textit{ab}-plane followed by the change from the first-order to the second-order phase transition. While at lower temperatures the columnar defects affect strongly the $H_{c}-H_{ab}$ phase diagram associated with the crossing lattice structure, at higher temperatures, the fingerprint of the crossing lattice is maintained even in a wider angular range than in the pristine samples. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B12.00008: Resonance-like behaviors of Josephson vortex flow resistance in mesoscopic intrinsic junctions of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Itsuhiro Kakeya, Miyako Iwase, Takuya Yamazaki, Takashi Yamamoto, Kazuo Kadowaki We have investigated dynamical nature of the Josephson vortex (JV) lattice in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) by measuring the Josephson flux flow $R_{ff} (H)$ and the current-voltage ($I-V$) characteristics along the $c$ axis as a function of magnetic field parallel to the $ab$ plane. The field dependence of the critical current $J_c(H)$ derived from the $I- V$ measurements oscillates as a function of $H$, similar to $R_{ff}(H)$ at a low current ($\sim$ 5 A/cm$^2$), indicating the dynamical formation of triangular lattice in low field and the square lattice in high field. In higher currents, $R_{ff}(H)$ strongly depends on the $c$ axis currents: at a field where $J_c(H)$ shows a maximum, $J_c(H)$ shows a minimum under a certain current and vice versa. This phenomenon can be interpreted as the formation of the resonance-like mode of the Josephson vortices, and is attributed to the phenomenon like Fiske resonance. This implies that Josephson vortices driven by the static current generate electromagnetic waves which resonate with Josephson vortex lattices in a mesoscopic junction. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B12.00009: Josephson flux flow resistance in single crystalline $\mathrm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ in various rectangular geometries Kazuo Kadowaki, Itsuhiro Kakeya, Miyako Iwase, Takuya Yamazaki, Takashi Yamamoto Josephson flux-flow resistance $\rho_c(H,I,\theta)$ has been studied in single crystalline $\mathrm{Bi_2Sr_2CaCu_2O_ {8+\delta}}$ with various rectangular geometries (more than 20 samples) as functions of magnetic field intensity $H$, current $I$ and the magentic field orientation angle $\theta$ with respect to the $\mathrm{CuO_2}$ plane. We observed a clear evidence of the lock-in behavior of the Josephson vortex parallel to the $\mathrm{CuO_2}$ planes. It is found that this lock-in angle $\theta$ strongly depends on the sample dimension, especially, on the length $\ell$ parallel to the magnetic field and is inversly propotional to the $\sqrt{\ell}$. This can be understood by considring the torque energy and energy to create pancake vortices very well. The flux-flow resistance $\rho_c$ shows a characteristic oscillatory behavior as a function of $H$ with a well defined peariodicity of $H=\phi_0/2sw$ in low fields and $H_0=\phi_0/sw$, where $s$ is the layer distance and $w$ is the width of the sample perpendicular to the magnetic field. This oscillatory behavior changes dramatically as a function of $I$ and can be explained by the dynamical ordering of the Josephson vortices in the restricted rectangular dimensions, which impose a potential well to the Josephson vortices. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B12.00010: Josephson Vortex State in Strongly Anisotropic Superconductor Bi-2212 Kazuto Hirata, Shuuichi Ooi, Shan Yu, El Hadi Smail Sadki, Takashi Mochiku We have reported that the periodic oscillations, observed in the Josephson-vortex (JV) flow resistance in Bi-2212 as a function of the parallel magnetic field, provide useful information on JV states. The 3D long-range-ordered state has been suggested from the ``beats'' phenomenon (Physica \textbf{C412-414}, 454(2004)in the oscillations. The 3D phase ranges from the lower magnetic field boundary ($H_{l})$ to the higher boundary ($H_{h})$, which is determined from the beginning and the end of the oscillations, respectively. The boundary $H_{l}$ is independent of temperature and almost constant, and only depends on the anisotropy parameter $\gamma $. Crossing the boundary $H_{h}$, the JV flow resistance without any oscillations suggests 2D-quasi-long-range ordered state, proposed by Hu{\&}Tachiki (PRB\textbf{70}, 064506(2004). [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B12.00011: Matching Effect of Vortex liquid in Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$ with Artificial Periodic Defects Shuuichi Ooi, Takashi Mochiku, Shan Yu, El hadi Smail Sadki, Kazuto Hirata To study the influence of artificial periodic pinning centers on the vortex matter in high-$T_c$ superconductors, we have measured the local magnetization and the electrical resistance of Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$ single crystals with sub-micron holes, which are distributed as a triangular lattice. Both measurements show matching effects in multiples of the matching field calculated from the lattice spacing of the defects array. The maximum field in which the matching effect is observed is larger than the field of the vortex-lattice melting transition in samples without artificial defects. There are several possibilities that is the existence of a kind of vortex liquid phase, which has an in-plane order, or a density matching between vortex liquid and defects lattice. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B12.00012: Kosterlitz-Thouless phase of interlayer Josephson vortices in high-Tc superconductors Qing-hu Chen, Xiao Hu, Masashi Tachiki Interlayer Josephson vortices induced by a magnetic field parallel to the CuO2 layers are investigated by Monte Carlo and dynamical simulations based on the anisotropic, frustrated XY model. At high magnetic field and/or high anisotropy, an intermediate Kosterlitz-Thouless phase between liquid and lattice is found, which is characterized by interlayer short-range and intralayer (thus 2D) quasi long-range crystalline orders of Josephson vortex lines [1]. The I-V characteristics of current applied parallel the CuO2 layers in this intermediate phase display nice non-Ohmic power-law behaviors [2]. It is also found that the resistivity in this geometry does not depend on the angle between the current and the magnetic field, which explains an interesting experimental observation made 15 years ago. [1] X. Hu and M. Tachiki: Phys. Rev. B, vol. 70 064506 (2004). [2] Q.-H. Chen et al., in preparation for submission. [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B12.00013: Critical Currents and the Peak Effect in YBCO J.W. Farmer, D.L. Cowan, M. Kornecki We have studied the peak effect (PE, a maximum in j(B)) through creep and hysteresis measurements in single crystals of YBCO, variously grown, doped, and in two cases neutron irradiated. Three sets of crystals show clear signs of cooperative fluxoid pinning by many weak defects, and all crystals from these sets exhibit a pronounced PE near 0.7 T$_{c}$. The other three sets of crystals, including the irradiated ones, are all strongly pinned at low T, and do not exhibit a PE. Low T pinning data alone suffice to predict the PE. We believe the PE is a consequence of thermally driven fluctuations in the fluxoid lattice, averaging and weakening the pinning potential. FFH[1] and others have shown the mean square displacement of a fluxoid is given by W $\alpha $ T$\lambda ^{2}$(T)/B$^{1/2}$ where $\lambda $ is the magnetic screening length. We test the hypothesis by comparing dj/dB and dj/dT in the region where j rises toward a peak, using the relation $\lambda ^{2}=\lambda _{GL}^{2}=\lambda _{o}^{2}$/(1-T/T$_{c})$. The agreement is very good below B$_{p}$, the value of B where j crests. A Lindemann-type melting model provides a good description of the peak field B$_{p}$(T).$\backslash $pard$^{1}$D.S. Fisher, M.P.A. Fisher, and D.A. Huse, Phys Rev B \textbf{43}, 130 (1991). [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B12.00014: Enhanced Flux Pinning in YBCO Films by Nano-Scaled Substrate Surface Roughness Qiang Li, Zuxin Ye, Weidong Si, Yufeng Hu Nano-scaled substrate surface roughness is shown to strongly influence the critical current density ($J_{c})$ in YBCO films, We prepared high quality c-axis oriented YBCO thin films by pulsed laser deposition on the single crystal LaAlO$_{3}$ substrates consisting of two separate twin-free and twin-rich regions. The nano-scaled corrugated substrate surface was created in the twin-rich region during the deposition process. Using magneto-optical imaging techniques coupled with optical and atomic force microscopy (AFM), we observed an enhanced flux pinning in the YBCO films in the twin-rich region, corresponding to $\sim $ 30{\%} increase in $J_{c}$. This result suggests that some nano-scaled substrate surface roughness may be beneficial for increasing $J_{c}$ in YBCO coated conductors. [Preview Abstract] |
Monday, March 21, 2005 2:03PM - 2:15PM |
B12.00015: Vortex Dynamics in Superconducting Ratchet in Niobium Thin Film Observed by Lorentz Microscopy Yoshihiko Togawa$^{1}$, Ken Harada$^{1,2}$, Tetsuya Akashi$^{2,3}$, Hiroto Kasai$^{2}$, Tsuyoshi Matsuda$^{1,4}$, Atsutaka Maeda$^{1,5}$, Akira Tonomura$^{1,2}$ Vortex dynamics in the superconducting ratchet was investigated in a Niobium thin film by direct imaging of Lorentz microscopy. Vortices were field-gradient driven by an ac magnetic field. Microscopic channels for vortex motion with arrow-shaped cages were created by Focused Ion Beam irradiation, where the potential energy distributed in a spatially asymmetric way. With increasing magnetic field, vortices penetrated inside the channel directed toward the thicker part of the sample, while vortices were not willing to intrude the oppositely-oriented channel. On the other hand, with decreasing field, in both channels, large portion of vortices were expelled through the bottle-neck of front cage of the channel toward the outside region. This observation indicated that field-gradient driven vortices were rectified in the asymmetric potential. For further demonstration of vortex motion control, a closed loop of the channel was fabricated and vortex motion was investigated in a two-dimensional circuit. [Preview Abstract] |
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B12.00016: Flux turbulence in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals Zuxin Ye, Qiang Li, Genda Gu We present magneto-optical imaging studies of flux turbulence in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystals. When magnetic flux is trapped by pinning in a Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystal and a moderate field of reverse direction is subsequently applied, a boundary of zero flux density will divide the regions of flux and antiflux. The propagation of this boundary shows turbulent behavior in narrow temperature ranges around 25 K. This is the first time that the flux turbulence is observed in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystals. In addition, the flux turbulence was found to be strongly correlated with the second-peak flux penetration. The implication of this study to the available theoretical models will be discussed. [Preview Abstract] |
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