Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D11: Inhomogeneous Superconductors and Transport |
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Sponsoring Units: DCMP Chair: Don Gubser, Naval Research Laboratory Room: Morial Convention Center RO9 |
Monday, March 10, 2008 2:30PM - 2:42PM |
D11.00001: Evidence of Spatially Inhomogeneous Pairing on the Insulating Side of a Disorder-Tuned Superconductor-Insulator Transition K. H. Sarwa B. Tan, Kevin A. Parendo, Yen-Hsiang Lin, Allen M. Goldman The effect of a perpendicular magnetic field on disordered, amorphous and insulating indium oxide thin films has been investigated. The temperature dependence of the resistance in zero magnetic field, the magnetoresistance as a function of temperature, and the nonlinear current-voltage characteristics have been interpreted as evidence of the presence of superconducting islands on the insulating side of a chemical and/or structural disorder-tuned superconductor-insulator transition. The behavior is very similar to that observed in granular films which are composed of macroscopic grains of superconductor embedded in an insulator. However, characterization studies indicate that the films are fully connected, structurally homogeneous, and not granular. These results support theoretical models in which the destruction of superconductivity by disorder produces spatially inhomogenous pairing with a spectral gap. This work was supported in part by the National Science Foundation under grant NSF/DMR-0455121. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D11.00002: Disorder, Metal-Insulator crossover and Phase diagram in high-T$_{c}$ cuprates Florence Rullier-Albenque, Henri Alloul, Fedor Balakirev, Cyril Proust We have studied the influence of disorder induced by electron irradiation on the normal state resistivities $\rho $(T) of optimally and underdoped YBa$_{2}$Cu$_{3}$O$_{x}$ single crystals, using pulsed magnetic fields up to 60T to completely restore the normal state. We evidence that point defect disorder induces low $T$ upturns of $\rho $(T) which saturate in some cases at low $T$ in large applied fields as would be expected for a Kondo-like magnetic response. Moreover the magnitude of the upturns is related to the residual resistivity, that is to the concentration of defects and/or their nanoscale morphology. These upturns are found quantitatively identical to those reported in lower $T_{c}$ cuprates, which establishes the importance of disorder in these supposedly pure compounds. We therefore propose a realistic phase diagram of the cuprates, including disorder, in which the superconducting state might reach the antiferromagnetic phase in the clean limit. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D11.00003: Drag resistance in bilayer disordered superconducting thin films Yue Zou, Gil Refael, Jongsoo Yoon Highly disordered superconducting thin films exhibit a variety of novel phenomena, such as a possible metallic phase intervening the superconducting and the insulating state, and a huge peak in the magnetoresistance curve. Different theories have been proposed, including the quantum-vortex theory, the percolation picture of superconducting islands embedded in a normal metal, and the Bose metal theory. We propose that a drag resistance measurement in a bilayer setup would easily be able to determine which of the models applies. In such an experiment, two thin film superconductors are fabricated parallel to each other, separated by a thin insulator. A current bias is applied in one layer, and a voltage appears in the other due to the interaction between vortices (as in a Giaever transformer), or charge carriers (e.g., Coulomb drag), in different layers. Our calculation of the drag resistance in the various pictures will be discussed. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D11.00004: Granularity-induced field-hysteresis of transport critical current in patterned coated conductors A. A. Gapud, A. Khan, D. K. Christen, F. A. List III, R. Feenstra In superconducting coated conductors such as RABiTS and IBAD films, intergranular misorientations have been effectively minimized, but a small number of local, higher-angle misorientations remain. One important effect of such weak links is the hysteresis of the critical current density $J_{c}$ with respect to applied field $H$, brought about when large circulating currents trapped within adjacent grains produce a focused field within the grain \textit{boundaries }(GB's) which can partially cancel out $H$ when applied field is \textit{decreasing}, thus shifting the maximum $J_{c}$ from zero $H $to a finite field where the local field at the GB is at minimum. This effect has been seen recently in measurements of magnetization (\textit{induced}) currents, but has not been documented using transport (\textit{applied}) current. However, in samples that are \textit{patterned} into conduits 200 $\mu $m wide or less, the hysteretic effect on transport $J_{c}$ is clearly seen. This discrepancy between `magnetization $J_{c}$' and `transport $J_{c}$' may be due to differences in voltage criterion between the two types of measurement, as will be discussed. Systematic measurements and analyses will be presented, along with ramifications for applications. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D11.00005: Disorder induced resitivity upturns in metallic cuprates Brian M. Andersen, Wei Chen, Peter J. Hirschfeld We propose that experimentally observed resistivity upturn of cuprates at low temperature may be explained by properly accounting for the effects of disorder in a strongly correlated metallic host. Calculating DC transport using real space exact diagonization of a Hubbard model treated in an inhomogeneous Hartree-Fock approximation, we find that correlations induce magnetization around impurities with screening length which increases strongly as temperature decreases, giving rise to additional magnetic scattering which causes the resistivity upturn. This paramagnetic response together with the electronic band structure effect is capable of explaining the magnetoresistance as observed in disordered optimally doped YBCO. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D11.00006: Interrelations between superconducting properties and defect evolution in neutron irradiated YBa$_{2}$Cu$_{3}$O$_{7}$. Viorel Sandu, Gheorghe Aldica, Elena Sandu, Petru Nita We investigated the effect of neutron irradiation on the electric and magnetic properties, including the enhancement of the critical current density, of ceramic YBa$_{2}$Cu$_{3}$O$_{7}$(LiF)$_{x}$ samples. The superconducting properties of the virgin samples reach the optimal values for $x$ = 0.04. Up to the same $x$ value, the neutron irradiation up to 5$\times $10$^{17}$ neutrons/cm$^{2}$ enhances the superconducting response (critical temperature, transition width, etc) and double up the magnetic irreversibility. For $x \ge $ 0.08, all the superconducting properties are depressed whereas the increase of the irreversible magnetization is still present but less spectacular. The analysis of this behavior suggests a self-organization of the defects at low LiF content while for $x \ge $ 0.08 they are uniformly distributed within sample. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D11.00007: Negative Nernst effect in simulations of granular superconductors Andreas Andersson, Jack Lidmar The Nernst effect has recently become an important probe of superconducting fluctuations in high-Tc superconductors. The sign of the Nernst coefficient $\nu=E_y/(-B\partial_x T)$ is positive for ordinary vortex motion down an applied temperature gradient. Here we consider simulations of granular superconducting thin films in the vortex liquid regime. We find that the Nernst coefficient can become negative for certain magnetic fields. We attribute this observation to the motion of vortex vacancies in an otherwise pinned vortex solid. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D11.00008: Large oxygen-isotope shift above the quantum critical point of Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ John Mann, Pieder Beeli, Guo-meng Zhao We have studied the oxygen-isotope effect on the superconducting transition temperature $T_{c}$ in overdoped Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ with $x$ = 0.10, 0.20, and 0.25. We find the oxygen-isotope exponent $\alpha_{O}$ to be small ($\sim$0.02) for $x$ = 0.10 but substantial ($\sim$0.1) for $x$ = 0.20 and 0.25. The doping level above which $\alpha_{O}$ increases sharply coincides with a quantum critical point where the normal-state pseudogap starts to diminish. The present isotope-effect experiments provide direct and quantitative constraints on the pairing mechanism of high-temperature superconductivity in cuprates. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D11.00009: Isotope Effect in High-Tc Superconductors Dale Harshman, John Dow, Anthony Fiory For high-Tc superconductors in which transition temperatures, Tc, are reduced by doping, the oxygen isotope effect (OIE) coefficient in Tc is shown to increase systematically with the pair-breaking rate and with the valence difference between the substituted and native ions. Moreover, the OIE tends to zero as one approaches optimum (or ideal) stoichiometry at which the quality of the superconducting condensate is maximized. In materials with isovalent substitutions, e.g., Sr for Ba or Zn for Cu in YBCO, the small OIE of the parent compound is magnified, owing to pair-breaking disorder. In materials with heterovalent substitutions, e.g., La or Pr for Ba, where carrier densities are necessarily changed, pair breaking induces a much larger OIE. A seminal case is Pr-doped YBCO, where the decrease in Tc observed with Pr doping arises from pair-breaking caused by Pr-on-Ba-site defects. Without the defects, Tc is invariant, providing strong evidence against phononic mechanisms. The fact that Tc drops when Pr substitutes for Ba, but not for Y, indicates that the superconducting hole condensate resides in the BaO layers, where pair-breaking degrades Tc and dramatically increases the OIE. Superconductive pairing modeled on Coulomb coupling between the hole and the electron layers is shown to resolve the shortcomings in electron-phonon interactions. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D11.00010: Rich Behavior of the Critical Currents of Magnetically-Doped Superconducting Films in Applied Magnetic Fields Jeffrey Wasserman, Nina Markovic Films of MoGe were grown with varying dopant levels of cobalt contamination. Critical currents were measured in the presence of magnetic fields applied parallel to the film plane and perpendicular to current flow. Critical current curves reveal significantly different behavior depending on the polarity of the current with respect to the applied field. At sufficiently-high dopant concentrations, the critical current increases with increasing magnetic field intensity for one polarity of current. We will discuss these results in terms of pair-breaking effects of magnetic fields and magnetic impurities. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D11.00011: Analytic description of the transport $J_{c}(B)$ dependence of HTS thin films in small magnetic fields Jens H\"anisch, Francesco Grilli, Sebastian Engel, Bernhard Holzapfel Often, e.g. for deconvolution processes and field distribution calculations, an analytical function for the $J_{c}(B)$ dependence of high-$T_{c}$ thin films is needed. The parameters of these functions should still have a physical meaning regarding the intrinsic and extrinsic sample properties. Starting with the modified Kim model, described by Xu \textit{et al}., we found an excellent function by introducing a sharpness parameter $\beta $. This parameter describes the shape of $J_{c}(B)$ between the low-field plateau (single vortex pinning regime) and the power-law dependence at higher fields. The temperature dependence of all fitting parameters will be discussed. Furthermore, the importance of the field dependence of the $n$ value for distinguishing different pinning regimes will be illustrated. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D11.00012: Investigation of Vortex Pinning Anisotropy in the High Temperature Superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ Andra Petrean-Troncalli, Lisa Paulius, Heather Quantz, Valentina Tobos, Wai -K Kwok Columnar defects have proven to be highly effective at pinning vortices, but most studies have been performed with the defects oriented either \textit{perpendicular} or at large angles relative to the superconducting Cu-O planes. These studies have shown that the intrinsic pinning anisotropy of the crystal can actually be reversed by sufficiently strong columnar defects oriented \textit{perpendicular} to the Cu-O planes. We have preliminary data that indicate that the pinning anisotropy is actually enhanced for columnar defects introduced \textit{parallel} to the superconducting Cu-O planes. A single crystal of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ was polished down to a narrow width of 27 $\mu $m, allowing heavy ions to penetrate the crystal along the ab-plane. The crystal was irradiated with 1.4 GeV $^{208}$Pb$^{56+}$ ions to a dose matching field of 1T. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D11.00013: Conductance of d-wave superconductor/normal metal/d-wave superconductor junctions Dmytro Pesin, Anton Andreev, Boris Spivak We develop a theory of the low-temperature conductance of superconductor/normal metal/superconductor junctions in which the superconductors have d-wave pairing symmetry. We show that at low temperatures the conductance of the junction is determined by the inelastic relaxation time of quasiparticles in the bulk of d-wave superconductors, $G_{DND}\propto \sqrt{\tau^ {(d)}_{\epsilon}}$. Thus it greatly exceeds the conductance of the normal metal part of the junction, which is controlled by the elastic mean free path. This dependence of $G_{DND}$ on the inelastic relaxation time should be contrasted with that of the low-temperature conductance of the junction in the case of the s- wave superconductor leads, $G_{SNS}$. In the latter case the conductance is proportional to the first power of the inelastic electron relaxation time in the normal metal part of the junction, $G_{SNS}\propto \tau_{\epsilon}^{(n)}$ [1]. \newline [1] S. V. Lempitskii, Sov. Phys. JETP {\bf{58}}, 624 (1983); U. Gunsenheimer and A. D. Zaikin, Phys. Rev. B{\bf{50}}, 6317 (1994); F. Zhou and B. Spivak, JETP Lett. {\bf{65}}, 369 (1997). [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D11.00014: Interface superconductivity in bi-layers of insulating and overdoped metallic La$_{2-x}$Sr$_{x}$CuO$_{4 + \delta}$ Adrian Gozar, Gennady Logvenov, Anthony Bollinger, Ivan Bozovic We report on properties of thin superconducting (SC) sheets obtained in La$_{2-x}$Sr$_{x}$CuO$_{4 + \delta}$ bi-layers of overdoped, non-superconducting $(x = 0.45)$ and insulating $(x = 0)$ films grown by molecular beam epitaxy. Superconductivity is confined to a thickness of $\approx$ 2 nm from the interface. The observed transition temperatures have values $T_{c} \approx 15$~K, 30 K and 50 K depending on the layering sequence and oxidation state of the insulating material. Transport measurements are used to determine the screening properties of the quasi-2D SC sheets and the data in bi-layers are compared to results from single-phase films and bulk crystals. [Preview Abstract] |
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