Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L12: Superconducting Properties |
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Sponsoring Units: DCMP Chair: Stan Tozer, NHMFL Room: LACC 402A |
Tuesday, March 22, 2005 2:30PM - 2:42PM |
L12.00001: Superstructures in superconductors: The case of YBCO Zahirul Islam, X. Liu, S.K. Sinha, J.C. Lang, S.C. Moss, D. Haskel, G. Srajer, B.W. Veal, U. Welp, D. Wermeille Superstructures characterized by {\bf q}=(q$_x$, 0, 0) are observed throughout the phase diagram of yttrium-barium cuprates (YBa$_2$Cu$_3$O$_{6+x}$, YBCO); {\bf q} decreases with doping from $\frac12$ (2-unit-cell) in the heavily un-derdoped compound to $\frac15$ in the overdoped material. A 4-unit-cell superstucture is stable in the vicinity of optimal doping. The superstructures in YBCO correspond to short-range ordered regions of coupled atomic displacements on neighboring CuO, BaO, and CuO$_2$ planes, respectively. T-dependent measurements suggest that these ``nanodomains'' experience anharmonic thermal motion. These regions induce a long-range strain in the host, which manifests as ``bow-tie''-shape Huang diffuse scattering below $\sim$200 K. X- ray diffuse scattering results will be presented within the context of the oxygen ordering and the phase diagram. [Preview Abstract] |
Tuesday, March 22, 2005 2:42PM - 2:54PM |
L12.00002: Measuring the London Penetration Depth in Aniso\-tropic Superconductors Charles C. Agosta, C. Martin, I. Mihut, C. Gatete, S.W. Tozer, H.A. Radovan, E.C. Palm, T.P. Murphy We will show that by measuring the penetration depth in the conducting planes of an anisotropic superconductor and applying a magnetic field parallel to the conducting planes we can get rid of the signal coming from the vortices, and directly measure the London penetration depth, $\lambda_L$. Using a tunnel diode oscillator (TDO) in a dilution refrigerator, we have measured $\lambda_L$ verses magnetic field in CeCoIn$_5$ and found it to be linear, which is consistent with a d- wave order parameter. In the layered organic superconductor $\alpha$- (ET)$_2$HN$_4$Hg(SCN)$_4$, $\lambda_L$ verses magnetic field follows the shape predicted by BCS theory and an s-wave order parameter. The same measurement in $\kappa$-(ET)$_2$Cu(NCS)$_2$ is also consistent with an s-wave order parameter. The first two results are supported by other types of measurements, but the results for $\kappa$-(ET)$_2$Cu(NCS)$_2$ are puzzling because most other measurements suggest that there are nodes in its order parameter. We will discuss the possible reasons why $\lambda_L$ is not linear as a function of magnetic field in $\kappa$-(ET)$_2$Cu(NCS)$_2$. We will also discuss how the same measurements under pressure will sort help sort out the roles of impurities and inhomogeneity in these materials. In this context we will describe a new pressure cell we have designed for these TDO experiments, and our preliminary results. Work at Clark supported by NSF-DMR-0331272. [Preview Abstract] |
Tuesday, March 22, 2005 2:54PM - 3:06PM |
L12.00003: Penetration Depth and Isotope Effect in Highly Overdoped YBCO Andrei Baikalov, Yu-Yi Xue, Ru-Ling Meng, Ching-Wu (Paul) Chu Magnetic penetration depth was measured for Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7}$ samples in the range from slightly to highly overdoped carrier concentrations covering Quantum Critical Point (QCP) doping level p=0.2. Together with oxygen isotope exchange experiment in progress, the data should indicate the role of phonons in understanding of the QCP phenomena. Separation of each sample onto different particle size powders along with appropriate model allowed us to extract temperature dependences for in-plane and out-of-plane penetration depth thus defining anisotropy coefficient behavior for the range of highly overdoped compounds. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:18PM |
L12.00004: Measurements of penetration depth anisotropy in MgB$_2$ J. Fletcher, A. Carrington, S.M. Kazakov, J. Karpinski The presence of multiple gaps in MgB$_2$ leads to a temperature dependent anisotropy of both superconducting length scales, the London penetration depth, $\lambda$, and the Ginzberg-Landau coherence length, $\xi$. Using a sensitive rf technique, the temperature dependence of both $\lambda_a$ and $\lambda_c$ is measured in single crystals of MgB$_2$. The temperature dependent anisotropy of the penetration depth, $\gamma_\lambda(T)=\lambda_{c}/\lambda_{a}$, calculated from measurements is in approximate agreement with that expected from calculations based on the measured superconducting gaps and band structure calculations. Torque and specific heat measurements are used to determine the anisotropy in $H_{c2}$ and the predicted convergence in the anisotropy of $\lambda$ and $\xi$ near $T_c$ is examined. [Preview Abstract] |
Tuesday, March 22, 2005 3:18PM - 3:30PM |
L12.00005: Superconducting parameters of aluminum-lithium alloys M.N. Ou, B.J. Chen, Y.Y. Chen, J.C. Ho Superconducting transition temperatures T$_{c}$ near 1 K of single-phase fcc aluminum-lithium alloys, with 0 to 10 at.{\%} Li, have been determined through ac susceptibility data. Earlier calorimetric measurements above 2 K on the same samples yielded the Debye temperature $\theta _{D}$ and the density of states at Fermi level N(0) from the lattice specific heat coefficient and the electronic specific heat coefficient, respectively. Following the modified BCS expression, T$_{c}$ = 0.85$\theta _{D}$exp[-1/N(0)V], such experimentally derived T$_{c}$, $\theta _{D }$and N(0) values for each sample provide a measure of the electron-phonon interaction parameter V, which plays central roles in inducing the traditional superconductivity. Its value increases monotonically from 0.611 to 0.710 eV for 0 to 10 at.{\%} Li. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L12.00006: Specific Heat of Superconducting HfV$_{2}$: Effect of Magnetic Fields J.L. Smith, F.R. Drymiotis, T. Kimura, G. Lawes, D.J. Thoma, J.C. Lashley, R.A. Fisher, N.E. Phillips Specific-heat ($C)$ measurements on a single crystal of HfV$_{2} $ were made from 1 to 300 K in magnetic fields ($H)$ to 14 T along the [110] axis. The HfV$_{2}$ has a martensitic transition at $T_{M}$ = 118 K and becomes superconducting (SC) at $T_{c} \quad \sim $8 K. The $T_{c}$ and $C$ vary following repeated cooling cycles through the $T_{M}$ from room temperature. This variation is probably related to strains or fracturing or both from the cubic-to-orthorhombic transition at $T_{M}$. For zero field, $\Delta C(T_{c})$/\textit{$\gamma $T}$_{c}$ = 2.06, which indicates strong coupling, and is nearly independent of variations in \textit{$\gamma $}$_{n}$, $T_{c}$, and $C$ that result from the thermal cycling. The SC state $C$ can be fitted with the alpha model for strongly-coupled superconductors using an energy gap \textit {$\Delta $}(0)/k$_{B}T_{c}$ = 2.1; the associated electron-phonon coupling constant \textit {$\lambda $} = 1.5. Both of these parameters are similar to those of Pb. In the normal state, the Sommerfeld constant (\textit{$\gamma $}$_{n})$ depends on the thermal history: for $T_{c}$ = 8.0 K, \textit{$\gamma $}$_{n}$ = 42.1 mJ K$^{2}$ mol$^{-1}$ after the first cooling. From fits above $T_{c}$, the Debye theta \textit{$\Theta $}$_{D}$, characterizing the low-temperature lattice $C$ is 177 K following the first cooling through $T_{M}$. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L12.00007: Unconventional Spin-Fluctuation Mediated Superconductivity in PuMGa$_5$ (M=Co, Rh) John Sarrao, E.D. Bauer, N.J. Curro, L.A. Morales, J.D. Thompson, M.J. Graf, A.V. Balatsky, F. Wastin, J. Rebizant, P. Boulet, E. Colineau, J.C. Griveau, G.H. Lander The discovery of superconductivity at $T_c$=18.5 K in PuCoGa$_5$ and at $T_c=9$ K in PuRhGa$_5$ has generated renewed interest in Pu-based intermetallic compounds. The thermodynamic properties of PuMGa$_5$ (M=Co, Rh), such as the initial slope of the upper critical field and specific heat jump at $T_c$, are consistent with an electronic specific heat coefficient $\gamma \sim$ 50-100 mJ/mol-K$^2$ indicating moderately heavy fermion behavior in these materials. The normal and superconducting states of these two Pu-based superconductors are remarkably similar to those of the well-known heavy-fermion CeMIn$_5$ (M=Co, Rh, Ir) superconductors, in which superconductivity is mediated by antiferromagnetic spin fluctuations. We present electrical resistivity and nuclear spin lattice relaxation measurements on PuMGa$_5$ that indicates a single (spin-fluctuation) energy scale dominates the physical properties, suggesting that spin fluctuations control both the superconducting and normal states. We contrast these behaviors with those observed in isostructural UMGa$_5$ and NpMGa$_5$. [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L12.00008: Weak ferromagnetic order and possible high-T$_{c}$ superconductivity in the new RuCa$_{2}$PrCu$_{2}$O$_{8+\delta}$ ruthenium-cuprate H.C. Ku Weak ferromagnetic order with ordering temperature T$_{m} \quad \sim $ 47 K and possible superconducting transition with T$_{c} \quad \sim $ 37 K are observed for the new ruthenium-cuprate RuCa$_{2}$PrCu$_{2}$O$_{8+\delta }$ with the orthorhombic distortion of the tetragonal RuSr$_{2}$GdCu$_{2}$O$_{8+\delta }$-type (with T$_{m} \quad \sim $ 136 K and T$_{c}$(max) $\sim $ 65 K) phase. Anomalous temperature dependent magnetization M$_{m}$(T) in both field-cooled (FC) and zero-field-cooled (ZFC) modes and isothermal magnetic hysteresis M$_{m}$(B$_{a})$ below and above T$_{m}$ and T$_{c}$ indicates a very complicated coexistence and interplay between weak-ferromagnetic order and possible superconductivity. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L12.00009: Inversion symmetry breaking superconductors: Re$_3$W and Re$_3$Mo Valentina Kuznetsova, Ivan Sergienko, Maxim Lobanov, James Thompson, David Mandrus Superconductors that break inversion symmetry and have strong spin-orbit coupling are theoretically predicted to have many anomalous properties, including the development of a two-gap structure in the superconducting density of states. One recent example of such a material is CePt3Si, which has attracted much recent attention. We have begun to examine two other materials that meet these criteria, Re3Mo and Re3W, both of which crystallize in the $\alpha $-Mn structure that breaks inversion symmetry. Here we present X-ray diffraction, magnetization, resistivity, and specific heat data on both Re3Mo and Re3W. Characteristic parameters of the superconductivity are extracted, and the data are closely examined for any deviation from ordinary BCS behavior. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L12.00010: Elastic anomalies in HoNi$_2$B$_2$C single crystals A. Knigavko, V.D. Fil, A.N. Zholobenko, E.-M. Choi, S.-I. Lee Temperature and magnetic field behavior of elastic properties of HoNi$_2$B$_2$C single crystal have been experimentally determined. The main result is a huge softening the velocity of C$_{66}$ mode in a very wide tempearture range, which can be adequately explained in terms of a theoretical model with the Jahn-- Teller interaction. This interaction is shown to be the driving force of the tetragonal- orthorhombic structural phase transition observed in HoNi$_2$B$_2$C previously. The H--T phase diagrams of this compound with magnetic field oriented along principal crystal directions have been revised by means of analysis of anomalies in the sound velocity and attenuation of the C$_{66}$ mode. New features of the phase diagrams are the critical points where several phase transition lines, separating magnetic phases, merge or maybe intersect. On the other hand, our experiments did not find manifestations of the superconducting phase transition in the acoustic properties. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L12.00011: Co-existence of ferromagnetism and superconductivity in Ni/Bi bilayers Patrick LeClair, Jagadeesh Moodera, Don Heiman, John Philip Thin films of Bi on ultra-thin Ni seed layers (2-15ML) exhibit a novel superconducting (SC) phase, with the ferromagnetism (FM) of Ni quenched for $<$ 2nm. We have prepared Al/Al$_2$O$_3$/Ni/Bi tunnel junctions with varying Ni thicknesses to study the competition between FM and SC. We unambiguously demonstrate that by tuning Ni thickness, the competition between FM and SC in Ni/Bi can be tailored. For narrow range of Ni thickness, the coexistence of a SC energy gap and conduction electron spin polarization are visible on the Ni side of the Ni/Bi bilayers is directly observed. We believe this represents one of the clearest observations of SC and FM coexisting. We have also performed extensive structural and electrical characterization of the Bi. Hall and resistivity measurements show a much higher carrier concentration and lower mobility compared to bulk Bi, and metallic R(T) behavior. These facts support the idea that an increased DOS at the Fermi level in this structure of Bi is responsible for the observed superconductivity. Supported by NSF grants. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L12.00012: Effects of fluctuations and Coulomb interaction on the transition temperature of granular superconductors Andrei Lopatin, Igor Beloborodov, Konstantin Efetov, Valerii Vinokur We investigate the suppression of superconducting transition temperature in granular metallic systems due to (i) fluctuations of the order parameter (bosonic mechanism) and (ii) Coulomb repulsion (fermionic mechanism) assuming large tunneling conductance between the grains $g_T \gg 1$. We find the correction to the superconducting transition temperature for 3 $d$ granular samples and films. We demonstrate that if the critical temperature $T_c > g_T \delta$, where $\delta$ is the mean level spacing in a single grain the bosonic mechanism is the dominant mechanism of the superconductivity suppression, while for critical temperatures $T_c < g_T \delta$ the suppression of superconductivity is due to the fermionic mechanism. [Preview Abstract] |
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