Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D25: Superconductivity: Vortex Phenomena I |
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Sponsoring Units: DCMP Chair: Carmen Almasan, Kent State University Room: D166 |
Monday, March 21, 2011 2:30PM - 2:42PM |
D25.00001: Anomalous field-symmetric Nernst signal in striped cuprate La$_{2-x}$Ba$_x$CuO$_4$ N. Phuan Ong, Lu Li, J. M. Tranquada, Genda Gu Starting at the structural transition temperature $T_{d2}$ = 54 K, the striped cuprate La$_{2-x}$Ba$_x$CuO$_4$ ($x = \frac18$) displays a remarkable cascade of transitions$^1$ at the characteristic temperatures $T_{d2} > T_1^{**} > T_{BKT} >T_c$, before settling down to 3D superconductivity with long-range coherence at $T_c$= 5 K. The Nernst signal $e_N$ and thermopower $S$ have been investigated in detail in these multiple states. As in pure LaSrCuO, the Nernst coefficient $N = lim_{B\to 0} e_N/B$ (initially negative) acquires a positive vortex contribution at 120 K that grows rapidly. However, here, $N$ saturates in the interval $T_{d2}$ (54 K) $\to T_1^{**}$ (34 K). As the vortex liquid becomes increasingly stabilized below $T_1^{**}$, $N$ resumes increasing at an even steeper rate. Surprisingly, below 34 K, $e_N$ acquires a $B$-symmetric component that is very large and oscillatory in $B$. We have excluded $S$ and quasiparticles as the source of the anomalous term. We will discuss various origins including the possibility of vortex formation mechanisms that break time-reversal invariance.\\ 1) J. M. Tranquada \emph{et al.}, Phys. Rev. B 78, 174529 (2008). [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D25.00002: Torque measurements in underdoped Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$ single crystals T. Hu, H. Xiao, P. Gyawali, H.H. Wen, C.C. Almasan We report in-plane and out-of-plane angular-dependent torque measurements on underdoped Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$ single crystals both below and above the zero-field superconducting transition temperature $T_{c}(0)$ and in applied magnetic fields $H$ up to 14 T. The out-of-plane torque data show that a diamagnetic signal persists into the normal state, which is consistent with previous reports on Bi$_2$Sr$_2$Ca$_2$CuO$_{8+\delta}$. These data also reveal that the vortex matter in the superconducting state has lower dimensionality compared to the one in the normal state. Furthermore, in-plane torque measurements show that the presence of vortices above $T_c(0)$ destroys the phase coherence of the Cooper pairs. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D25.00003: Generics of the diamagnetism, Nernst signal, and finite size effects in superconductors above the transition temperature S. Weyeneth, T. Schneider Various superconductors exhibit peculiar features above the transition temperature $T_{c}$. In particular the observation of a large Nernst signal $N$ and a remarkable diamagnetism above $T_{c}$ in cuprate and conventional superconductors attracted considerable attention. Noting that in these materials the spatial extent of the homogeneous domains is limited, we explore the relevance of a zero dimensional (0D)-model, neglecting thermal fluctuations, to describe the isothermal magnetization curves $m_{\mathrm{d}}\left(H\right)$ in various superconductors above $T_{c}$. It is shown that for cuprates as well as for Pb nanoparticles, both, the full 0D-model as well as its Gaussian approximation, mimic the essential features of the magnetization curves above $T_{c}$ rather well. Furthermore, the isothermal Nernst signal of a superconducting Nb$_{0.15}$Si$_{0.85}$ film is fully consistent with this scenario. Accordingly, the observed diamagnetism above $T_c$ in Pb nanoparticles, in the cuprates La$_{1.91}$Sr$_{0.09}$CuO$_{4}$ and BiSr$_2$Ca$_2$CuO$_{8-\delta}$, as well as the Nernst signal in Nb$_{0.15}$Si$_{0.85}$ films, are all in excellent agreement with the scaling properties emerging from the here discussed 0D-model. Therefore, singlet Cooper pairs subjected to orbital pair breaking in a 0D-system are the main source of the observed diamagnetism and Nernst signal in an extended temperature window above $T_{c}$. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D25.00004: Effects of Vortex Charge Explored by NMR Spectroscopy in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Andrew Mounce, S. Oh, S. Mukhopadhyay, W.P. Halperin, A.P. Reyes, P.L. Kuhns, K. Fujita, M. Ishikado, S. Uchida We measure nuclear magnetic resonance (NMR) spectra for various dopings of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+y}$ to determine the variations of local magnetic fields generated by the vortex lattice at low temperatures. With increasing the external magnetic field H$_0$, the local field variations decrease to values lower than expected by Ginzburg-Landau calculations for an Abrikosov vortex lattice.\footnote{Brandt, E. H., Phys. Rev. Lett. \textbf{66}, 3213-3216 (1991).} Taking into account charge accumulation in vortex cores, which has been predicted by theory\footnote{Khomskii, D. I. \& Freimuth, A. Phys. Rev. Lett. \textbf{75}, 1384-1386 (1995).} and suggested by experiment,\footnote{Kumagai, \textit{et al.} Phys. Rev. B \textbf{63}, 144502 (2001).} we are able to identify the amount of charge needed to disturb the vortex lattice and decrease local field variations as in the observed spectra. The amount of charge is found to be $\sim$2x$10^{-3}$e and doping dependent in line with theoretical predictions.$^2$ This work is supported by DOE/BES: DE-FG02-05ER46248 and the NHMFL by NSF and the State of Florida. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D25.00005: Vortex dynamics in ferromagnetic / high T$_{c}$ superconducting heterostructures N. Haberkorn, J. Kim, M. Miura, B. Maiorov, P. Dowden, L. Civale In this work we explore the influence of the ferromagnetic landscape generated by magnetic phase separation in manganites on the vortex pinning of HTS/FM heterostructures. We have grown Gd$_{2/3}$Ca$_{1/3}$MnO$_{3 }$(GCMO), Y$_{2/3}$Ca$_{1/3}$MnO$_{3 }$(YCMO) and YBCO films, as well as GCMO/YBCO and YCMO/YBCO bilayers by Pulsed Laser Deposition. GCMO is a ferrimagnetic material with Curie temperature of $\sim $ 80 K and a compensation temperature of 15 K, whereas YCMO is ferromagnetic with Curie temperature $\sim $ 80 K. In both materials the saturation magnetization is smaller than the value expected from ferrimagnetic and ferromagnetic order, suggesting phase separation with small ferromagnetic domains. The magnetic domain size as a function of temperature for the magnetic films was investigated using magnetic force microscopy (MFM). We will present a comparison of the vortex pinning and dynamics in the YBCO single layers and the GCMO/YBCO and YCMO/YBCO bilayers that allows us to identify the temperature and field regimes where magnetic pinning is effective. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D25.00006: Hysteretic magneto-transport of a High-T$_{c}$ superconducting/ferromagnetic multilayer with tunable magnetic domain structure Javier E. Villegas, Cristina Visani, Peter J. Metaxas, Aurelie Collaudin, Baptiste Calvet, Rozenn Bernard, Javier Briatico, Cyrile Deranlot, Karim Bouzehouane The magneto-transport of a hybrid heterostructure combining a YBaCuO$_{7-\delta }$ thin film and a Co/Pt superlattice shows an unusual hysteretic behavior. Depending on the angle between the external applied field and the film plane, and on the magnetic history, either a increase or a decrease of the mixed-state resistance is observed. The combination of magneto-transport, magnetic force microscopy and anomalous Hall effect measurements allows us to correlate these effects to the magnetic domain structures in the Co/Pt superlattice. We unequivocally prove that the hysteretic magneto-transport is induced by the stray magnetic fields from tunable magnetic domain structures, which may induce vortices or produce vortex pinning, leading to the increase/decrease of the mixed-state resistance. Work supported by French ANR ``Superhybrids-II'' and RTRA ``Supraspin'' grants. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D25.00007: Vortex/Domain Coupling in Superconducting Films on Ferromagnetic Substrates with Different Domain Structures Vitalii Vlasko-Vlasov, Ulrich Welp, Daniel Rosenmann, Wai Kwok, Alexander Buzdin, Alexander Melnikov, Lyudmila Uspenskaya, Vincent Fratello In this work we address engineering of magnetic pinning in the superconducting/ferromagnetic hybrids. Using direct magneto-optical observations effects of interactions between superconducting vortices in Nb film and magnetization in domains of substituted iron garnet films are studied. Two garnet films with perpendicular anisotropy but different thickness-to-domain width ratios and one showing the reorientation from the in-plane to the normal easy axis are used as substrates. All three samples demonstrate strong domain/vortex coupling and reveal high pinning by domain walls, which persists up to temperatures close to the superconducting Tc. In turn, vortex motion modifies domain structures of garnets at T$<$Tc resulting in the alignment and strong contraction of wide domains but smaller width changes of narrow domains. A model is proposed explaining the results of our observations. This work was supported by DOE-BES under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D25.00008: Matching field effects in high-$T_{c}$ superconductors with self-assembled columnar defects J.W. Sinclair, Y.L. Zuev, J.R. Thompson, D.K. Christen, C. Cantoni, S.-H. Wee, A. Goyal, C. Varanasi Columnar defects (CDs) in a superconductor provide angularly selective vortex pinning, where the density of CDs provides a natural scale for the magnetic field, the ``matching field.'' While dramatic changes in the critical current density $J_{c}$ might be expected when the vortex density exceeds the CD density, observations of this phenomena has been elusive in systems with chemically produced, self-assembled CDs of BaSnO$_{3}$, BaZrO$_{3}$, SrZrO$_{3}$, etc. Here we describe studies of two RBa$_{2}$Cu$_{3}$O$_{\sim 7}$ systems containing these self-avoiding CDs, using either contact free magnetic or transport measurements. In magnetic measurements on a material with measured areal CD of $\sim $2.5 T, the $J_{c}$ decreased abruptly when the applied field $H$ exceeded this level; this feature was observed over a wide temperature range, from 77 to $\sim $ 40 K. All these features disappeared when the field was tilted away from the CD orientation. Research at ORNL sponsored by US DOE. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D25.00009: Superconducting vortex dynamics on asymmetric arrays with symmetric pinning centers Jose L. Vicent, David Perez de Lara, Alejandro Alija, Elvira M. Gonzalez, Maria Velez, Jose I. Martin Arrays of Ni nanodots embedded in Nb superconducting films have been fabricated by sputtering and electron beam lithography techniques. The arrays are periodic triangular lattices of circular Ni dots arranged in a kagom\'e-like pattern with broken reflection symmetry. DC magnetoresistance shows several fractional matching field minima below the first matching field for vortex motion parallel and perpendicular to the array reflection symmetry. AC magnetoresistance shows reversal ratchet effect when the vortex lattice moves parallel to the array reflection symmetry. These effects could be understood taking into account the vortex lattice density. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D25.00010: Enhanced superconducting vortex pinning in the Corbino geometry Yaniv Rosen, Ivan K. Schuller We probed a dynamic system of superconducting vortices with an artificial pinning landscape in the Corbino geometry. Current was applied from the center of the disc and propagated radially outward to produce a circular force with a strength proportional to 1/r on the vortices. This caused a shearing force on the vortex lattice and was studied with varying current densities and temperature. Matching minima in the magnetoresistance curves were still observed under the Corbino conditions for the square lattice pinning site geometry. Surprisingly the even numbered matching fields show enhanced pinning compared to the odd matching fields. Other interesting temperature and current density dependencies will also be discussed. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D25.00011: Proximity Effects and Vortex Dynamics in Nanostructured Superconductors Serena Eley, Nadya Mason We report transport measurements on triangular arrays of proximity-coupled superconducting islands placed on normal-metal substrates. The superconducting islands are well-understood coherent systems with long-range electron interactions, while the intervening normal metal channels introduce known dissipation into the system. We show how by changing the island spacing, we can tune characteristics such as the critical temperature and field. The arrays undergo a Kosterlitz-Thouless vortex-unbinding phase transition at zero-field, and we observe frustration-induced magnetoresistance oscillations at finite fields. We also observe unusual cusp-like behavior in resistance vs temperature at finite magnetic fields [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D25.00012: Vortex-induced dissipation in current-biased superconducting nanowires Lev Bulaevskii, Matthias Graf, Cristian Batista, Vladimir Kogan We study the dissipation due to vortex crossings in thin current-biased superconducting films with thickness on the order of the coherence length, and with width much narrower than the magnetic Pearl length in thin films. We find that for technologically relevant thin and narrow films or nanowires with width much larger than the coherence length, the barrier for phase slips by creation of temporary normal regions across the entire film width is too big. Thus phase slips become highly improbable. Instead, we propose the process of a vortex crossing the strip from one edge to the other, perpendicular to the bias current, as the dominant mechanism for generalized phase slips resulting in detectable voltage pulses. We derive phase-current relations and predict the amplitude and duration of voltage pulses induced by vortex motion due to thermal fluctuations and bias current. The consequnces for the current-voltage characteristics and the fundamental limitations for dark counts in superconducting nanowire single-photon detectors are discussed. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D25.00013: Synthesis and vortex dynamics of high-Tc superconducting nanoribbons with a periodic array of holes Qiong Luo, Michael Latimer, Zhili Xiao, Lei Fang, Wai-Kwong Kwok We fabricated nanoribbons of high-temperature superconductors YBCO-123 and BSCCO-2212. Experimental procedures for growing, manipulating and characterizing the nanoribbons will be presented. Furthermore, we introduced regular arrays of nanoscale holes into these nanoribbons through focused-ion-beam (FIB) milling to study the effects of periodic pinning on vortex dynamics. Resistive measurements reveal vortex matching effect and striking feature in the volage-current behavior associated with various driven regimes of the vortex matter related to vortex dynamics phase transitions [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D25.00014: Strong pinning of vortex lines by nanoparticles Alexei Koshelev, Alejandro Kolton Pinning of vortex lines by array of nanoparticles embedded inside superconductors became the most efficient practical way to achieve high critical currents. In this situation pinning occurs via trapping of the vortex-line segments and the critical current is determined by the typical length of trapped segment. To verify analytical estimates and develop a quantitative description of strong pinning, we use large-scale numerical simulations. We study the dependence of the critical force on the density of pins in the regime of independently pinned lines, statistical properties of trapped lines, and suppression of the apparent critical force by thermal fluctuations. [Preview Abstract] |
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