Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L25: Superconductivity: Vortex Phenomena II |
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Sponsoring Units: DCMP Chair: Milind Kunchur, University of South Carolina Room: D166 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L25.00001: Vortex Pinning in MoGe Thin Films Containing Periodic Hole Arrays Michael Latimer, Zhili Xiao, Wai-Kwong Kwok, Alexandra Joshi-Imre, Castro Abughayada Resistivity measurements on MoGe thin films containing periodic hole arrays were carried out to study the effects of the lattice symmetry and the size of the pinning centers. Thin films of MoGe were prepared with holes drilled using focused-ion-beam milling to create pinning sites for the vortex lattice. We investigate periodic arrays with hexagonal, square and triangular geometry to determine the change in transport properties with varying magnetic fields. Hole sizes from 50nm to 100nm were tested determine the effects of single and multiple vortices in a single pinning site. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L25.00002: Flux dynamics across MoGe bridges in the parallel field orientation Milind Kunchur, Manlai Liang, Alexander Gurevich We have investigated flux motion in amorphous molybdenum-germanium (MoGe) film bridges with the magnetic field B parallel to the film plane but perpendicular to the current direction. In a temperature range close to the transition temperature Tc we observe an exponential dependence of the Ohmic resistance R on B at low values of the current I, and a nonlinear R(I) at higher currents. In this regime, the diameter of the vortex is approximately equal to the film thickness and the applied magnetic field is comparable to the lower critical field for the parallel field orientation. Dissipation presumably occurs by thermally activated flux jumps over the thickness of the film involving nucleation and expansion of vortex kinks. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L25.00003: Vortex instability in molybdenum-germanium superconducting film Manlai Liang, Milind Kunchur We studied the high driving force regime of the current-voltage transport response in the mixed state of amorphous molybdenum-germanium (MoGe) superconducting films to the point where the flux flow becomes unstable. The observed nonlinear response conforms with the classic Larkin-Ovchinikov picture with a quasiparticle energy-relaxation rate dominated by the quasiparticle recombination process. The measured energy relaxation rate was found to have a magnitude and temperature dependence in agreement with theory. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L25.00004: Small Angle Neutron Scattering Studies of the Vortex Lattice in CeCoIn$_5$ with $H \perp c$ M.R. Eskildsen, P. Das, A.T. Holmes, E.M. Forgan, A.D. Bianchi, J.S. White, S. Gerber, M. Kenzelmann, J.L. Gavilano, M. Zolliker, C. Wang, E.D. Bauer, J.L. Sarrao, C. Petrovic We report on small-angle neutron scattering measurements on the vortex lattice (VL) in the mixed state of CeCoIn$_5$ with the magnetic field ($H$) along $[100]$ and $[110]$. For both field orientations a distorted hexagonal VL is observed, reflecting the penetration depth anisotropy of the screening current plane. With $H \parallel [100]$ the VL is oriented with Bragg reflections along the [001]-axis at all fields. For $H \parallel [110]$ the same VL orientation is observed at low fields, followed by a $90^{\circ}$ first-order reorientation transition as $H$ is increased. We attribute this behavior to Fermi surface anisotropy coupled with non-local effects. For $H \parallel [100]$ we obtain the field dependence of the form factor ($|F|^2$) both within (50~mK) and outside (350~mK) the magnetic $Q$-phase. At both temperatures $|F|^2$ varies with $H$ in a manner similar to $H \parallel [001]$ [J.S. White {\em et al.}. New J. Phys. {\bf 12}, 023026 (2010)], due to the competition between Pauli paramagnetism and the antiparallel spin alignment of $d$-wave pairing giving rise to ``magnetized" VL cores. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L25.00005: Low temperature enhancement of the remanent magnetization in CeCoIn$_{5}$ C.F. Miclea, M. Nicklas, A.C. Mota, F. Steglich, M.M. Altarawneh, N. Harrison, I. Vekhter, J.D. Thompson, R. Movshovich We investigated the vortex dynamics together with RF penetration depth measurements in the heavy fermions compound CeCoIn$_{5}$ down to~50~mK. No strong pining is observed and the relaxation curves are logarithmic as expected from Kim-Anderson theory. The temperature dependence of the relaxation rate, $S$, with a small but finite residual value indicate that quantum tunneling plays a role in the vortex creep only at very low temperatures. Remarkably, a new phase transition marked by a strong increase in the remnant magnetization, M$_{rem}$ is observed around $T$~=~0.3~K in very low magnetic fields. $M_{rem}$ increases roughly by a factor of two at 50 mK and we discuss if this can be caused solely by the change in the vortex lattice symmetry or underling magnetism has to play a role. Moreover, this anomaly is corroborated by the RF measurements at very low fields. We extended the vortex dynamics investigation to Pb irradiated CeCoIn$_{5}$. While the defects created by irradiation have a clear effect on the relaxation rates the enhancement of $M_{rem}$ still takes place at the same temperature. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L25.00006: Vortex Core Size Measurements in YNi$_2$B$_2$C and TmNi$_2$B$_2$C P. Das, C. Rastovski, K. Schlesinger, M.R. Eskildsen, J.M. Densmore, S.L. Bud'ko, P.C. Canfield The vortex core size in type-II superconductors is typically determined from measurements of a related quantity combined with a theoretical model, with the best known example being the upper critical field and the GL-result: $\xi = \sqrt{\phi_0/2\pi H_{c2}}$. However, for many non-conventional superconductors such an approach is problematic, as for example in the case of TmNi$_2$B$_2$C and CeCoIn$_5$ where $H_{c2}$ is suppressed by coexistence with magnetism. In such instances a direct, model independent determination of the vortex core is desirable, and can be obtained by small-angle neutron scattering (SANS) measurements of the vortex lattice (VL) if a sufficient number of reflections are recorded [J. M. Densmore {\em et al.}, Phys. Rev. B {\bf 79}, 174522 (2009)]. Here we report on VL SANS studies on two members of the borocarbide superconductors, YNi$_2$B$_2$C and TmNi$_2$B$_2$C. Non-magnetic Y1221 measurements at $0.2$ and $0.5$~T show clear evidence of a vortex squeezing effect. In magnetic Tm1221 the vortex core size was found to be $\xi = 10.8$~nm, roughly a factor of two smaller than the value estimated by the measured $H_{c2}$ ($21$~nm). Supported by NSF award no. DMR-0804887 (Notre Dame) and DOE BES contract No. DE-AC02-07CH11358 (Ames). [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L25.00007: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L25.00008: Revealing the effect of edge contamination on vortex matter structure in a Nb single crystal with neutron diffraction techniques Helen Hanson, Xi Wang, Michael Luk, Jing Shi, Xinsheng Ling, Brian Maranville, Charles Majkrzak The vortex matter of type II superconductors provides a model system to study the effect of quenched random disorder on an elastic lattice, particularly in the framework of Bragg glass theory. Neutron scattering techniques are used to examine the structure of the vortex matter and to quantify the phase diagram. After measuring various thermal-magnetic histories, our data provided evidence for the edge contamination model in a Nb single crystal. Since surface oxidation is known to suppress the Bean-Livingston Surface barrier and the inhomogeneous distribution of surface impurities in Nb, we oxidize our sample surface and repeat our measurements. By comparing the data, we are able isolate the dynamic impact of the edge disorder from the static influence of bulk pinning. We discuss the various experimental obstacles in measuring the predicted Bragg glass state. We also report on Reverse Monte Carlo Refinement simulations modeling possible structures of our vortex matter. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L25.00009: Exploring the vertical variation of the flux line lattice angular orientation using a novel neutron diffraction technique Xi Wang, Helen Hanson, Jing Shi, Xinsheng Ling, Brian Maranville, Charles Majkrzak, Mark Laver, Uwe Keiderling, Margarita Russina We use a slicing neutron diffraction technique, employing neutron reflectometry collimation, to study the orientational order of the flux line lattice in a Nb single crystal. We are able to reveal the spatial variation of the different orientation distributions along the length of the flux lines. The results are strongly dependent on the magnetic history of the vortex matter, suggesting various interactions with the disorder in the system. After thermally annealing the different initial states, memory of the growth procedure is removed from the data and a possible ground state is reached. In this final state, the novel vertical slicing reveals the persistence of a domain splitting. We suggest that this domain splitting is due to the quenched disorder in the underlying Nb atomic lattice. We believe that this new insight will be instrumental in growing a true Bragg glass, the theoretically predicted ground state with topological order. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L25.00010: Vortex activation energy in high-Tc superconductors from transport measurements: new systematics Yuri L. Zuev, John Sinclair, James R. Thompson, Sung Hun Wee, Claudia Cantoni, David K. Christen From electrical transport measurements on thin films of YBCO with and without nano-precipitate ``columnar defects,'' we extract information about the activation energy for vortex creep. We also obtain new scaling between temperature, critical current density $J_C$, and the power law index $n$ describing the voltage-current relation $V\propto I^n$. This scaling occurs in the range of field and temperature where $J_C$ decays as a power-law of magnetic field. In this regime the $n$-value can unexpectedly increase as applied field increases. We will discuss implications of these observations and compare systems with and without such scaling. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L25.00011: Comparison of free flux flow in two single crystals of V$_{3}$Si with slightly different pinning strengths Ozarfar Gafarov, Albert A. Gapud, Sunhee Moraes, James R. Thompson, David K. Christen, Arneil P. Reyes Results of recent measurements on two very clean, single-crystal samples of the A15 superconductor V$_{3}$Si are presented. Magnetization and transport data confirm the ``clean'' quality of both samples, as manifested by: (i) high residual resistivity ratio, (ii) low critical current densities, and (iii) a ``peak'' effect in the field dependence of critical current. The (H,T) phase line for this peak effect is shifted in the slightly ``dirtier'' sample, which also has higher critical current density J$_{c}$(H). High-current Lorentz forces are applied on mixed-state vortices in order to induce the highly ordered free flux flow (FFF) phase, using the same methods as in previous work. A traditional model by Bardeen and Stephen (BS) predicts a simple field dependence of flux flow resistivity $\rho _{f}$(H), presuming a field-independent flux core size. A model by Kogan and Zelezhina (KZ) takes core size into account, and predicts a deviation from BS. In this study, $\rho _{f}$(H) is confirmed to be consistent with predictions of KZ, as will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L25.00012: Magnetic-field-induced stripe order and a 2D vortex glass phase in La$_{1.905}$Ba$_{0.095}$CuO$_4$ John Tranquada, Jinsheng Wen, Qing Jie, Su Jung Han, Qiang Li, Markus Huecker, Zhijun Xu, Liyuan Zhang, Genda Gu, M.V. Zimmermann, D.K. Singh We have measured the resistivity parallel and perpendicular to the CuO$_2$ planes in La$_{1.905}$Ba$_{0.095}$CuO$_4$ ($T_c = 32$ K) as a function of perpendicular magnetic field. We have discovered a significant regime of field and temperature where the perpendicular resistivity is finite (and large) but the parallel resistivity is zero. This regime appears to correspond to a quasi-two-dimensional vortex glass phase, a state that theory predicts cannot exist at finite temperature. It seems to be stabilized by field-induced charge and spin stripe order, which we have detected with x-ray and neutron diffraction, respectively. [Preview Abstract] |
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