Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H34: Superconductivity: Transport Properties |
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Sponsoring Units: DCMP Chair: J.R. Thompson, University of Tennessee Room: 404 |
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H34.00001: Resistivity and superfluid density measurements on under- and over-doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ films. Thomas Lemberger, Iulian Hetel, A. Tsukada, Michio Naito We have measured the resistivities and superfluid densities (or, penetration depths, $\lambda )$ of a series of LSCO films with a wide range of Sr concentrations. Films are grown by MBE on LaSrAlO$_{3}$ substrates under nominally identical conditions. Due to substrate mismatch, films are under compression. Resistivities decrease smoothly as Sr concentration increases, and resistive transitions are sharp. T$_{c}$ has a maximum at x = 0.15, while superfluid density 1/$\lambda ^{2}$(0) has a maximum at x $\approx $ 0.18. Interesting features in the T-dependence of 1/$\lambda ^{2}$ will be discussed. Absolute values of resistivity and superfluid density in these films indicates quality comparable to bulk materials. Falloff of superfluid density with overdoping, together with a smooth decrease in resistivity, is consistent with an interpretation in terms of a mesoscopically inhomogeneous superconducting state. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H34.00002: Strong Reduction of T$_{c }$Suppression by Magnetic Field in YBa$_{2}$Cu$_{3}$O$_{7+x}$ Films with Dispersed Nanoparticles E. Cimpoiasu, J. D. Feldmann, C. V. Varanasi, T. J. Haugan, P. N. Barnes, G. A. Levin Improvements in the critical current density $J_{c}$ in applied magnetic fields are of great importance for applications of the YBa$_{2}$Cu$_{3}$O$_{7-x}$ coated conductors. Nanosize inclusions have shown to be effective in increasing $J_{c}$, but the precise physical mechanisms of their action remains elusive. A broader range of experiments is needed in order to elucidate the physics of this phenomenon. Here we discuss the magnetic field H- and temperature T-dependence of the resistivity of thin films in the normal state and near T$_{c}$. Pure YBCO films will be compared with those that contain either dispersed Y$_{2}$O$_{3 }$nanoparticles or BaSnO$_{3}$ nanorods. The resistance of highly c-axis oriented YBCO films was measured by the Montgomery method in the range 20 K $<$ T $<$ 300 K and in fields up to 9 T. The films with inclusions show a much sharper and less broadened in-field transition (smaller T$_{c}$ suppression by field) than pure YBCO. This correlates well with increased $J_{c}$ measured by conventional methods and indicates increased pinning strength at all temperatures. In order to further identify the signatures of the nano-inclusions, the samples were annealed in air at 420 deg C. The changes induced by the annealing will be discussed. \textit{This work was partially supported by AFOSR and the AFRL Propulsion Directorate. } [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H34.00003: Quantum Oscillations and Hall Resistivity in YBCO and Tl-2201: Exploring the Fermi Surface of the Cuprates Brad Ramshaw The field of high temperature superconductivity has enjoyed something of a Renaissance in the past two years with the discovery of quantum oscillations in Shubnikov de Haas and de Haas-van Alphen measurements preformed on YBCO and Tl-2201. DC transport measurements around the one eighth hole doping region of high Tc phase diagram have shown a temperature and doping dependence to the Hall coefficient. In this doping region the Hall coefficient changes sign from positive to negative as temperature goes to zero, indicating a competition of mobilities between holes and electrons. In high magnetic fields these measurements also exhibit oscillations in one over the field strength, indicating the existence of small pockets of Fermi surface. This information, coupled with the Hall data, gives rise to the interpretation that the large hole like Fermi surface found in the overdoped region on Tl-2201 reconstructs into smaller electron like Fermi pockets on the underdoped side. Combined with other theoretical and experimental techniques, these experiments are allowing us to develop an understanding of the electronic structure of the cuprates across the entire phase diagram. This understanding is crucial to uncovering the underlying mechanism that gives rise to high temperature superconductivity. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H34.00004: Longitudinal and Transverse Transport properties of disordered graphene Vincent Ugarte, Vivek Aji We present results of calculations of the properties of thermal and electrical transport coefficients of disordered graphene in a weak magnetic field. In particular, we are interested in the effect of vanishing density of states and unitary scattering on transport coefficients near the Dirac point. The effect of impurity states is included within a self-consistent t-matrix approximation. We find a peak in both the Nernst coefficient and thermopower around the chemical potential which grows as one approaches the Dirac point. We compare our results to recent experimental data and discuss the importance of unitary scattering in various limits of temperature and chemical potential. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H34.00005: Linear-T resistivity and change in Fermi surface at the pseudogap critical point of a high-T$_c$ superconductor Ramzy Daou, Nicolas Doiron-Leyraud, David LeBoeuf, Shiyan Li, Francis Laliberte, Olivier Cyr-Choiniere, Y.J. Jo, Luis Balicas, J.-Q. Yan, J.-S. Zhou, John Goodenough, Louis Taillefer A fundamental question of high-temperature superconductors is the nature of the pseudogap phase which lies between the Mott insulator at zero doping and the Fermi liquid at high doping p. Here we report on the behaviour of charge carriers near the zero-temperature onset of that phase, namely at the critical doping p* where the pseudogap temperature $T^*$ goes to zero, accessed by investigating a material in which superconductivity can be fully suppressed by a steady magnetic field. Just below $p^*$, the normal-state resistivity and Hall coefficient of La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_{4}$ are found to rise simultaneously as the temperature drops below $T^*$, revealing a change in the Fermi surface with a large associated drop in conductivity. At $p^*$, the resistivity shows a linear temperature dependence as $T \rightarrow 0$, a typical signature of a quantum critical point. These findings impose new constraints on the mechanisms responsible for inelastic scattering and Fermi-surface transformation in theories of the pseudogap phase. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H34.00006: Transport Properties in Electron-Doped La$_{2-x}$Ce$_{x}$CuO$_{4}$ Thin Films Kui Jin, Xiaohang Zhang, Paul Bach, Richard Greene The electron-doped high-Tc cuprate La$_{2-x}$Ce$_{x}$CuO$_{4}$ (LCCO) is quite different from other members, such as Pr$_{2-x}$Ce$_{x}$CuO$_{4}$ (PCCO) and Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ (NCCO). One distinct difference is that the optimal Ce doping in LCCO is $\sim $0.10, compared to Ce $\sim $0.15 in PCCO and NCCO. Here, we will present a detailed and systematic study of the magnetic field and temperature dependence of the transport properties of LCCO, including the low-temperature Hall effect and in-plane angular magnetoresistance. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H34.00007: Evidence for individual quantum phase-slip events in homogeneous superconducting nanowires Mitrabhanu Sahu, Myung-Ho Bae, Andrey Rogachev, David Pekker, Nayana Shah, Tzu-Chieh Wei, Paul Goldbart, Alexey Bezryadin We report strong evidence for individual quantum tunneling events undergone by the superconducting order-parameter field in homogeneous Mo$_{79}$Ge$_{21}$ nanowires. We obtain this via measurements of the distribution of switching currents, whose width exhibits a rather counter-intuitive, monotonic increase with decreasing temperature. We outline a stochastic model of phase-slip kinetics, which relates the basic phase- slip rates to switching rates. Comparison with this model indicates that the phase predominantly slips via thermal activation at high temperatures, but at sufficiently low temperatures switching is caused by individual topological tunneling events of the order-parameter field, i.e., quantum phase slips (QPS). Importantly, measurements on several wires show that quantum fluctuations tend to dominate over thermal fluctuations at larger temperatures in wires having larger critical currents. This fact supports the view that the anomalously high switching-rates observed at low temperatures are indeed due to QPS, and are not consequences of extraneous noise or inhomogeneity of the wire. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H34.00008: Collapse of peak effect by alternating current and its frequency dependence in MgCNi3 single crystal Dong-jin Jang, Hyun-Sook Lee, H-G Lee, M-H Cho, Sung-Ik Lee The peak effect, which appears as sharp rise in critical current near superconductor-normal transition of a superconductor, is first order phase transition. However, if vortices happen to move across sample by direct current (DC), sharp transition nature becomes blunted by edge contamination as intensively studied in NbSe$_{2}$. This edge contamination has been shown to be removed by alternating current (AC) or by using edgeless Corbino geometry. Among few superconducting materials showing peak effect, MgCNi$_{3}$ exhibits fairly sharp peak effect even in DC strip geometry. And remarkably, critical current measured by using AC is greatly suppressed as frequency of AC increases. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H34.00009: Analytical Procedure for Measuring Electrical Resistivity of Anisotropic Materials C.A.M. dos Santos, B.S. de Lima, C.Y. Shigue, A. de Campos, M.S. da Luz, A.T. Rice, B.D. White, J.J. Neumeier The Montgomery method is used to determine the resistivity tensor of anisotropic materials [1] such as high-T$_{C}$ and FeAs superconductors, 2-layer Mn oxides, organic conductors, and quasi-1D conductors. It uses the Wasscher transformation [2], which calculates an isotropic equivalent sample of the anisotropic sample. This is a timing-consuming task because it is a numerical method based upon graphical analyses obtained from calculations by Logan, Rice, and Wick [3]. In this work we report a simplification of the Montgomery method. Analytical equations are derived and applied to several isotropic and anisotropic samples (Cu, Al, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$, Graphite, SrNbO$_{x}$, $\gamma $-Mo$_{4}$O$_{11})$. Comparisons with results obtained by using the standard four-probe method demonstrate the quality and simplicity of the procedure, which can easily be extended to data acquisition systems. This material is based upon work supported by FAPESP (grant No. 07-04572-8), NSF (grants Nos. DMR-0504769 and 0552458), and CNPq (grant Nos. 301334/2007-2 and 201439/2007-7). [1] H. C. Montgomery, J. Appl. Phys. \textbf{42}, 2971 (1971). [2] J. D. Wasscher, Philips Res. Repts. \textbf{16}, 301 (1961). [3] B. F. Logan, S. O. Rice, and R. F. Wick, J. Appl. Phys. \textbf{42}, 2975 (1971). [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H34.00010: Doping dependence of the dynamic and static critical exponents in Pr$_{2-x}$Ce$_x$CuO$_4$ M.C. Sullivan, R. Isaacs, J.B. Olson, J. Sousa, M. Salvaggio, R.L. Greene Scaling analysis of voltage vs.\ current isotherms is a favorite tool to study the normal-superconducting phase transition in cuprate superconductors. This measurement has never been performed on the electron-doped cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$, due in part to difficulties which result from finite-thickness effects, even in thick ($d \approx 3000$\AA) films.\footnote{ Phys. Rev. B \textbf{69}, 214524 (2004)} If finite-thickness effects are taken into consideration, we can find the critical isotherm and the dynamic critical exponent, and we can use small magnetic fields to find the static critical exponent. Similar measurements have been made on the more familiar hole-doped cuprates such as YBa$_2$Cu$_3$O$_{7-\delta}$.\footnote{ arXiv:0803.0969} We present our results of the dynamic critical scaling exponent $z$ and static critical exponent $\nu$ in our Pr$_{2-x}$Ce$_x$CuO$_4$ films as a function of doping. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H34.00011: Experimental study of superconductivity in single crystal few-layer NbSe$_2$ and the effect of high electric fields Neal Staley, Linjun Li, Zhuan Xu, Ying Liu There have been many studies on superconducting properties in two dimensional films. However, a detailed study of superconducting properties in the two-dimensional limit when crystallinity is still retained, which will allow the probing of band dependent superconductivity in 2D, has not been performed. Due to concerns over defects in ultra thin films deposited in the usual methods, we use the methods developed in preparing micromechanically exfoliated graphene devices. In these samples the band structure is present while maintaining extremely low defect density. Inspired by this simple process that created single crystal single sheet graphite we fabricated ultra thin single crystalline NbSe$_2$ flakes ranging from single to many sheets as estimated using an optical technique correlated to AFM and Raman spectroscopy measurements. Transport and planar tunnel junction devices were fabricated using standard ebeam lithography techniques. We will also study the behavior of of these devices in high electric fields. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H34.00012: Size effects in the nonlinear resistance in a virtual Berezinskii-Kosterlitz-Thouless state of superconducting films Alex Gurevich, Valerii Vinokur We show that the size effects strongly affect the nonlinear electric field-current ($E-I$) relation of superconducting films. We calculate $E(J)$ due to thermally-activated hopping of single vortices driven by current $I$ across the film in a magnetic field $H$, taking into account interaction of free vortices with their antivortex images and peaks in the Meissner currents at the film edges. Unbinding of virtual vortex-antivortex pairs not only mimics the transport uniform BKT behavior, it can dominate the observed $E(J)$ and result in the field-dependent ohmic resistance at small $I$. We show that $E(I)$ can be tuned by changing the film geometry and propose experimental tests of this theory. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H34.00013: Geometric Effects on the Tunneling Apparent Barrier Height Aran Garcia-Lekue, Thomas Frederiksen, Andres Arnau An experimental quantity which may help understanding the mechanism of electron tunneling, in STM experiments or across broken nanojunctions for example, is the apparent barrier height. In order to extract information from this experimental observable one can consider a simple one-dimensional tunneling model, where the apparent barrier height is the rate of change of the logarithm of the conductance with the tip-apex separation or vacuum gap. Theoretically, a faithful analysis of the apparent barrier height requires a precise description of the tunneling conductance in the vacuum region. However, most of the conductance calculations are performed using atom centered localized basis sets, which cannot adequately describe the tunneling current crossing the vacuum gap and can therefore lead to erroneous results. In this work, we present tunneling conductance calculations obtained using the transport calculation method introduced in Ref.\,[1]. Since this method employs a plane-wave basis set, it provides accurate results for the electron tunneling across the vacuum gap and, consequently, for the apparent barrier height. Here, we report results for broken Au nanojunctions with different geometries, which allows us to thoroughly investigate geometric effects on the apparent barrier height. 1. A. Garcia-Lekue and L.W. Wang, Phys. Rev. B. {\bf 74}, 245404 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H34.00014: Thermopower across the pseudogap critical point of La(1.6-x)Nd(0.4)Sr(x)CuO(4): Evidence for a quantum critical point in a hole-doped high-Tc superconductor Olivier Cyr-Choiniere, Ramzy Daou, Francis Lalibert\'e, David LeBoeuf, Nicolas Doiron-Leyraud, Jiaqiang Yan, Jianshi Zhou, John B. Goodenough, Louis Taillefer The thermopower S of the high-Tc superconductor La(1.6-x)Nd(0.4)Sr(x)CuO(4) was measured as a function of temperature T near its pseudogap critical point, the critical hole doping p* where the pseudogap temperature T* goes to zero. Just above p*, S/T varies as ln(1/T) over a decade of temperature. Below p*, S/T undergoes a large increase below T*. As with the temperature dependence of the resistivity, which is linear just above p* and undergoes a large upturn below T*, these are typical signatures of a quantum phase transition. This suggests that p* is a quantum critical point below which some order sets in, causing a reconstruction of the Fermi surface, whose fluctuations are presumably responsible for the linear-T resistivity and logarithmic thermopower. We discuss the possibility that this order is the ``stripe'' order known to exist in this material. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H34.00015: Low-Temperature Thermal Conductivity in a $d$-Wave Superconductor with Coexisting Checkerboard Charge Order Philip Schiff, Adam Durst Given the experimental evidence of charge order in the underdoped cuprate superconductors, we consider the effect of coexisting checkerboard charge order on low-temperature thermal transport in a d-wave superconductor. We compute the quasiparticle excitation spectrum in the presence of both order parameters and perform a diagrammatic Kubo formula calculation of the zero-temperature thermal conductivity tensor as a function of the magnitude and wave vector of the charge order. Results depend on disorder, indicating that, in the presence of charge order, zero-temperature thermal transport is no longer universal. [Preview Abstract] |
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