Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N12: Vortices in Superconductors IV |
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Sponsoring Units: DCMP Chair: V.Z. Kresin, LBL, University of California Room: LACC 402A |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N12.00001: Studies of quantum fluctuations and competing orders on vortex dynamics in cuprate superconductors A.D. Beyer, C.R. Hughes, N.-C. Yeh, V.S. Zapf, M.-S. Park, K.-H. Kim, S.-I. Lee The existence of competing orders (CO) and the proximity to quantum criticality (QC) in cuprate superconductors create unconventional low energy excitations and significant quantum fluctuations (QF) which can alter the low temperature vortex dynamics of cuprates. We report studies on the effect of QF and CO on vortex dynamics in cuprates at low temperatures, focusing on the four-layer, hole-doped HgBa$_{2}$Ca$_{3}$Cu$_{4}$O$_{x}$ (Hg-1234). Hg-1234 has two underdoped inner layers that are anti-ferromagnetic and two optimally doped outer layers that are superconducting. Vortex phase diagrams, derived from 3$^{rd}$ harmonic AC hall probe and high-field DC cantilever magnetization measurements, allow comparison of Hg-1234 with other cuprates such as YBa$_{2}$Cu$_{3}$O$_{7-x}$ and La$_{0.1}$Sr$_{0.9}$CuO$_{2}$. Comparison plots of the ab-plane reduced fields (normalized by the paramagnetic field, H$_{para})$, h$_{irr.}$(t)=H$_{irr.}$(t)/H$_{para }$and h$_{C2}$(t)=H$_{C2}$(t)/H$_{para }$ versus reduced temperature, t, demonstrate that QF and CO indeed affect Hg-1234 more than other cuprates, with Hg-1234 having the smallest extrapolated value of h*$\equiv $ h$_{irr.}$(0)$\approx $0.12, indicating its closest proximity to QC. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N12.00002: Single Superconducting Vortex Depinning Force Measured with a Magnetic Force Microscope Jenny E. Hoffman, Andrew Whitehead, Nick C. Koshnick, Ophir M. Auslaender, Eric W. J. Straver, Rob A. Hughes, John S. Preston, Dan Rugar, Kathryn A. Moler The pinning of vortices plays an important role in superconductor applications, allowing transport currents to flow without dissipation. Measurements of vortex pinning to date have mostly been bulk measurements, yielding average properties without specific information about individual vortex pinning sites or strengths. We demonstrate the use of a magnetic force microscope to measure the depinning forces of single vortices in a superconducting YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ film. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N12.00003: Super-Hard Superconductivity Philip Adams, David Young, Monica Moldovan, Ruslan Prozorov We present the magnetic response of Type-II superconductivity in the extreme pinning limit, where screening currents within an order of magnitude of the Ginzburg-Landau depairing critical current density develop upon the application of a magnetic field. We show that this ``super-hard'' limit is well approximated in highly disordered, cold drawn, Nb wire whose magnetization response is characterized by a cascade of Meissner-like phases, each terminated by a catastrophic collapse of the magnetization. Direct magneto-optic measurements of the flux penetration depth in the virgin magnetization branch are in excellent agreement with the exponential model in which $J_c(B)=J_{co}\exp(-B/B_o)$, where $J_{co}\sim5\rm{x}10^6$ A/cm$^2$ for Nb. The implications for the fundamental limiting hardness of a superconductor will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N12.00004: Experimental Evidence for Giant Vortex States in a Mesoscopic Superconducting Disk Akinobu Kanda, Ben Baelus, Francois Peeters, Kazuo Kadowaki, Youiti Ootuka Response of a mesoscopic superconducting disk to perpendicular magnetic fields is studied by using the multiple-small-tunnel- junction method, in which transport properties of several small tunnel junctions attached to the disk are measured simultaneously. This allows us for the first experimental distinction between the giant vortex states and multivortex states. Moreover, we experimentally find magnetic field induced rearrangement and combination of vortices. The experimental results are well reproduced in numerical results based on the nonlinear Ginzburg-Landau theory. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N12.00005: Observation of the spontaneous vortex phase in the weakly ferromagnetic superconductor ErNi$_{2}$B$_{2}$C: A penetration depth study Ee Min Elbert Chia, Tuson Park, Myron Salamon, Heon-Jung Kim, Sung-Ik Lee The coexistence of weak ferromagnetism and superconductivity in ErNi$_{2}$B$ _{2}$C suggests the possibility of a spontaneous vortex phase (SVP) in which vortices appear in the absence of an external field. We report evidence for the long-sought SVP from the in-plane magnetic penetration depth $\Delta \lambda (T)$ of high-quality single crystals of ErNi$_{2}$B$_{2}$C, using a high-precision tunnel-diode based, self-inductive technique at 21~MHz.. In addition to a slight depression of superconductivity at the N\'{e}el temperature $T_{N}$ = 6.0~K and at the weak ferromagnetic onset at $T_{WFM}$=2.3~K, $\Delta \lambda (T)$ rises to a maximum at $T_{m}=0.45$ K before dropping sharply down to $\sim $0.1~K. We assign the 0.45 K-maximum to the proliferation of spontaneous vortices. A model proposed by Koshelev and Vinokur explains the increasing $\Delta \lambda (T)$ as the vortex density increases, and its subsequent decrease below $T_{m}$ as defect pinning suppresses vortex hopping. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N12.00006: Finite-size Intrinsic Josephson Junctions under Layer Parallel Magnetic Field Takeshi Hatano, Shinya Urayama, Sunmi Kim, Huabing Wang, Masanori Nagao, Kunihiro Inomata, Yoshihiko Takano, Tsutomu Yamashita, Masashi Tachiki, Masahiko Machida Oscillations of Josephson vortex flow resistance have been studied in finite-size Bi-2212 intrinsic Josephson junctions as a function of magnetic field applied parallel to the junctions. The lengths of junctions fabricated are around one micrometer so as to enhance pinning effects of the Josephson vortex lattice to the junction edges and thus to enhance the formation of rectangular vortex lattice which would lead an \textit{in-phase} mode of the junctions as a possible candidate for the THz generator application. The observed Josephson vortex flow resistance showed a periodic oscillation with a period ($H_{P})$ corresponding to one flux quantum enters per junction, namely, corresponding to the rectangular vortex lattice. The peaks in the oscillations were found at the fields $H$=\textit{nH}$_{P}$, here n shows an integer number. Therefore, the Josephson vortex lattice flow speed is maximum when the outermost vortex rows geometrically match to the edges of the junction. Contrary to this, minimum of flow speed, namely pinning of the vortex lattice, was observed at $H$=($n$+1/2)$H_{P}$. The pinning configuration will be discussed based on the strong edge effects in finite-size intrinsic Josephson junctions. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N12.00007: Anisotropic vortex structure in tilted magnetic fields in the spin triplett superconductor Sr$_{2}$RuO$_{4}$ Voicu Octavian Dolocan, Ying Liu, Pascal Lejay, Dominique Mailly, Klaus Hasselbach Using a $\mu$SQUID microscope we imaged magnetic flux above the ab surface in the unconventional anisotropic superconductor Sr$_{2}$RuO$_{4}$, at temperatures between 0.4 K and 1.3 K and magnetic fields between 0 to 70 gauss at various angles. We observed vortex chains as well as coexistence of vortices and chains for tilted fields. The distance between the chains varies as 1/B. The mass anisotropy expressed as the ratio of the penetration depth ($\lambda_{c}/\lambda_{ab}$) is about 20 for Sr$_{2}$RuO$_{4}$, situating Sr$_{2}$RuO$_{4}$ in respect of anisotropy between YBCO and BSCCO. We'll discuss the different origins for vortex chain formation for these three superconductors. In the case of Sr$_{2}$RuO$_{4}$ we can successfully describe the ordering of the flux into vortex chains using Ginzburg-Landau theory. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N12.00008: Phase Diagram for a Hiden Competing Order in the Mixed State of YBa$_2$Cu$_4$O$_8$ CRYSTALS UP TO 150 kG Takekazu Ishida, Koji Sato, Yuzo Yoshida, Shuichi Kawamata, Takahiko Sasaki, Norio Kobayashi, Seiji Adachi, Takato Machi Torque measurements on YBa$_2$Cu$_4$O$_8$ crystals show multiple peaks as a function of $\theta_{ca}$ between the c axis and the magnetic field $H$ in the c-a plane. We propose that the first peaks at $\theta_{ca} \simeq 80$ and 100 degrees arise from a possible long-range spiral spin density wave through the matching pinning between vortices and spins. We construct a contour map for a competing order parameter in YBa$_2$Cu$_4$O$_8$ which starts at a finite $H$ and is located far from Hc2 line. The regime of the proposed spin density wave goes to higher temperatures when field increases. The first peak does not appear when the field direction is scanned in the c-b plane. The NMR line width of the c-axis aligned YBa$_2$Cu$_4 $O$_8$ powders in H perpendicular c shows broadening at temperatures below 30 K. These features might be intrinsic in the cleanest underdoped cuprate. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N12.00009: AC current rectification in Nb films with or without symmetrical Nb/Ni periodic pinning arrays in perpendicular magnetic field Vladimir Pryadun, Arkadi Levanyuk, Raul Villar, Farkhad Aliev Rectification of AC current has been observed in plain superconducting Nb films and in Nb/Ni films with symmetric periodic pinning centers. The rectified DC voltage appears for various sample geometries (cross or strip) both along and transverse to the alternating current direction, is nearly anti-symmetric with perpendicular magnetic field and strongly dependent on temperature below T$_{c}$. Analyses of the data at different temperatures, drive frequencies from 100kHz to 150MHz and at the different sample sides [1] shows that not far below Tc the rectification phenomena can be understood in terms of generation of electric fields due to local excess of critical current. Further below T$_{c}$ anisotropic pinning effects could also contribute to the rectification. [1] F.G.Aliev, et al., Cond. Mat.405656. Supported by Comunidad Autonoma de Madrid -CAM/07N/0050/2002 [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N12.00010: Vortex Dynamics in Confined Systems Studied by Scanning Goran Karapetrov, Jan Fedor, Maria Iavarone, Daniel Rosenmann, Wai-Kwong Kwok We report on a study of vortex arrangements in mesoscopic superconductors using scanning tunneling microscopy and spectroscopy. Using NbSe$_{2}$ single crystals and conventional ion beam lithography we defined different mesoscopic systems that confine vortex motion. The spatial confinement of vortices introduces novel collective behavior that strongly depends on the size of the superconductor, vortex density, and temperature. We will compare the experimental results with existing theoretical models based on Ginzburg-Landau theory. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N12.00011: Vortex-glass transitions in Low-Tc superconductors Javier E. Villegas, Elvira M. Gonzalez, Maria P. Gonzalez, Jose L. Vicent We have measured I-V characteristics in the mixed state of superconducting plain Nb thin films, Nb thin films with artificial periodic arrays of magnetic pinning centers, and Nb/Cu superlattices. Using a scaling analysis of I-V characteristics, we have found clear evidence of a vortex-glass transition in those Low-Tc systems. Such glass transitions belong to the same universality than those earlier observed in High-Tc systems, since similar critical exponents have been found. We have studied the effect of the presence of periodic pining centers on the glass transition, the effect of the artificial anisotropy in the dimensionality on the glass transition of Nb/Cu superlattices, as compared to the behavior of Nb plain films. Among the most remarkable results, a dimensional crossover in the glass transition is found, from a three-dimensional VG transition in Nb films to a two-dimensional one in Nb/Cu superlattices. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N12.00012: AC-Susceptibility and Ultrasonic Attenuation Measurements of Vortex Dynamics in the Vicinity of the Peak Effect in V-Ti Alloys - Multicriticality Revisited I.K. Dimitrov, N.D. Daniilidis, V.F. Mitrovi{\'c}, C. Elbaum, X. S. Ling In-situ SANS and ac-susceptibility measurements have provided evidence for a first-order Bragg glass transition into a disordered vortex state in a Nb single crystal. This transition manifests itself in the peak effect (PE) in the critical current density, widely believed to be associated with the sudden softening of the vortex lattice. Subsequent studies mapping the full phase diagram in the same sample have suggested the existence of four distinct phase boundaries meeting at a single multicritical point (MCP). The natures of the transition lines combined with simple thermodynamic requirements suggest that the MCP is a bicritical point. This would rule out either the bulk transition line T$_{c2}$(T) or the surface superconducting transition H$_{c3}$(T) as being related to the MCP. Mutual inductance magnetic ac-susceptibility and ultrasonic attenuation measurements in V-21at.{\%}Ti have unequivocally established the presence of a PE in this alloy. The H-T phase diagram for this sample will be presented and vortex dynamics in the vicinity of the PE will be discussed. We are indebted to Prof. Shapira of Tufts University for providing us with the sample. This work was supported by the NSF under Grant No. DMR-0406626. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N12.00013: Specific Heat Measurements at the Bragg Glass Disordering Transition N.D. Daniilidis, I.K. Dimitrov, Vesna Mitrovi{\`c}, C. Elbaum, X.S. Ling We will report specific-heat measurements on a Nb single crystal, in which previous neutron scattering studies showed that the peak effect (PE) coincides with an order-disorder transition of the vortex lattice. The PE transition shows metastability and is believed to be first order. In addition, the PE transition line has been uncovered to merge with the low-field continuous transition H$_{c2}$(T) line at a multicritical point. Thermodynamic considerations imply that this is a bicritical point. Measurements of specific heat and latent heat throughout the H-T phase diagram are essential for understanding the nature and relevance of the observed transition lines and the different phases of vortex matter in the presence of quenched disorder. In specific, the measurements will elucidate the nature of the disordered phase, vortex liquid or vortex glass, for bulk type-II superconductors. The measurements are performed in a home-built calorimeter cryostat. The cryostat allows for continuous-heating temperature measurements in vacuum with a heat-leak regulating stage included between the sample and the helium bath. [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N12.00014: Effect of the misalignment between the applied and internal magnetic fields on the critical currents in tilted superconducting thin films B. Maiorov, T.G. Holesinger, B.J. Gibbons, S. Kreiskott, Q.X. Jia, V. Matias, L. Civale The analysis of critical current density ($J_c$) as a function of the orientation of the applied magnetic field (\textbf{H}) provides a very effective way to identify and discriminate the various pinning mechanisms in high temperature superconductors. In thin and/or anisotropic superconductors, the vortices may decrease their free energy by tilting their orientation with respect to \textbf{H}, therefore producing a misalignment between the applied and internal magnetic fields. We present $J_c$ angular studies in films with the crystallographic axis tilted with respect to the sample surface. This results in a rich behavior, characterized by an asymmetric angular dependence and a shift of the $J_c$ $ab$-planes maxima position as function of the magnetic field. We show under which conditions the misalignment must be taken into account and quantify the magnitude of the shift. [Preview Abstract] |
Wednesday, March 23, 2005 10:48AM - 11:00AM |
N12.00015: NMR spin-echo measurement of driven vortex motion in the superconducting phase of the electron-doped HTSC Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$. W.G. Clark, Guoqing Wu, S.E. Brown, R.L. Greene, H. Balci We report modulation of the $^{63}$Cu spin echo as a function of arrival time in the superconducting phase of a single crystal of Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$. It occurs when the sample is immersed in liquid He, but not in He vapor. Our interpretation is that the RF pulses that form the spin echo excite an ultrasonic oscillation in the NMR coil which is transmitted to the sample via the He liquid. It generates an oscillatory motion of the vortex lattice, which causes the oscillation of the local magnetic field oscillation responsible for the spin echo modulation (near 20 kHz). Two features that support this interpretation are: (1) Adding mass to the NMR coil changes the spin echo oscillation frequency, and (2) the echo modulation disappears when the sample leaves the superconducting phase because the magnetic field is rotated away from the $a-b$ plane. We thank L. Bulaevskii for important comments and the NSF for support from Grants DMR-0334869 (WGC) and DMR-0203806 (SEB). [Preview Abstract] |
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