Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X41: Focus Session: Search for New Superconductors - Carbides, Borides and Organics |
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Sponsoring Units: DMP Chair: Zach Fisk, University of California, Irvine Room: F152 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X41.00001: Superconducting Transition in New Thorium-Nickel-Carbon Ternary Compound ThNi$_{4}$C Ted Grant, Antonio Jefferson S. Machado, Cigdem Capan, Zachary Fisk Since the discovery of high temperature superconductivity in iron pnictides, there is an emphasis on finding new Co, Ni, or Fe based superconductors. We have synthesized for the first time ThNi$_{4}$C that is in the hexagonal CaCu$_{5}$ prototype structure. We discovered bulk superconductivity in ThNi$_{4}$C that has previously been unreported in the thorium-nickel-carbon ternary system. The data from magnetic susceptibility, electrical resistivity, and heat capacity indicate bulk superconductivity with T$_{c}$= 5.5 K. Results from thorium substitution with Y, Lu, La, and Gd will also be presented. [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X41.00002: Effect of disorder on the electronic band structure of CaC$_6$: a first-principles study Tom Berlijn Recent ARPES measurements [1] raise serious questions on the Fermi surface of superconducting [2] CaC$_6$. Specifically, the heavily discussed Ca band was not observed, and the charge transfer from Ca is found largely complete, contrary to previous theoretical studies [3-6]. Here we investigate the effects of potential Ca disorder on the electronic band structure, using a newly developed Wannier function-based disorder method. In particular, quasi-localization of the Ca carriers will be examined via the one-particle spectral function.\\[4pt] [1] T. Valla et al, Phys. Rev. Lett. {\bf 102}, 107007 (2009)\\[0pt] [2] T.E. Weller et al, Nat. Phys. {\bf 1}, 39 (2005)\\[0pt] [3] G. Csanyi et al, Nat. Phys. {\bf 1}, 42 (2005)\\[0pt] [4] I.I. Mazin, Phys. Rev. Lett. {\bf 95}, 227001 (2005)\\[0pt] [5] M. Calandra et al, Phys. Rev. Lett. {\bf 95}, 237002 (2005)\\[0pt] [6] L. Boeri et al, Phys. Rev. B {\bf 76}, 064510 (2007) [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X41.00003: Phonon Softening and Instability of MgCNi$_{3}$; Phonon dispersion measurements Hawoong Hong, Mary Upton, Ayman Said, Hyun-Sook Lee, Sung-Ik Lee, Ruqing Xu, Tai-Chang Chiang Inelastic x-ray scattering from a single crystal MgCNi$_{3}$ was carried to determine phonon dispersion on the three major axes. Our results agree qualitatively with the most recent theoretical calculation. The dispersion shows quite strong softening of the longitudinal acoustic (LA) modes toward zone boundaries along $\Gamma $-M and $\Gamma $-R. However, phonon instabilities predicted by earlier theories were not observed. Our results would motivate the rethinking of MgCNi$_{3}$ superconductivity based on unstable phonons. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X41.00004: Unconventional superconductivity in noncentrosymmetric Mo$_{3}$Al$_{2}$C Amar Karki, David Young, Philip Adams, Ilya Vekhter, Dana Browne, Julia Chan, Edem Okudzeto, Ruslan Prozorov, Hyunsoo Kim We have presented experimental results of resistivity, magnetic susceptibility, penetration depth and specific heat measurements for a polycrystalline sample of the noncentrosymmetric superconductor Mo$_{3}$Al$_{2}$C. All measurements confirm a bulk superconducting transition at 9 K in this compound. A high value of the upper critical field $H_{c2}$(0) suggests that pair breaking due to the Zeeman effect in Mo$_{3}$Al$_{2}$C is small. The specific heat jump $\Delta $\textit{C/$\gamma $T}$_{c}$ at \textit{Tc }was found to be 1.95, higher than the BCS value of 1.43 for a weakly coupled superconductor, suggesting that Mo$_{3}$Al$_{2}$C may be strongly coupled. In addition, evidence for unconventional superconductivity is clearly apparent in the electronic specific heat $C_{es}$ and the penetration depth $\lambda _{s}$ in the superconducting state, indicating an anomalous pairing mechanism and a possible nodal-gap structure in Mo$_{3}$Al$_{2}$C. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X41.00005: An Ultra-Thin Molecular Superconductor Made from Charge Transfer Complexes Kendal Clark, A. Hassenien, S. Khan, K.-F. Braun, H. Tanaka, S.-W. Hla A class of charge transfer molecular systems having a D$_{2}$A arrangement (D = donor, A = accepter) exhibit superconductivity in the bulk and are often termed ``unusual superconductors'' based on the different nature of their superconducting states as compared to convention BCS superconductors. In this study we have formed an ultra-thin (BETS)$_{2}$-GaCl$_{4 }$molecular superconductor consisting of a single sheet of layered molecules composed of individual GaCl$_{4}$ sandwiched between the chains of a double \textit{domino }stacked BETS on a Ag(111) surface. Amazingly, the superconducting gap can still be detected in such an ultra-thin molecular layer, and the shape of the gap reveals a d-wave pairing symmetry. Moreover, real space STM spectroscopic images provide direct evidence of the superconducting site as the BETS chains. In stark contrast to the high Tc superconductors, the spectroscopic maps clearly display nanoscale electronic order indicating robust superconducting properties at this extreme spatial limit.. [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X41.00006: Survey of Pauli Limiting and FFLO behavior in organic superconductors William A. Coniglio, Laurel E. Winter, C. C. Agosta Superconducting effects near the Pauli paramagnetic limit have drawn increased attention recently, with several suggestions of inhomogeneous superconductivity (Fulde-Ferrell-Larkin-Ovchinikov) at high magnetic fields and low temperatures. Many of these studies have been done on layered superconductors when the magnetic field is oriented exactly parallel to the conducting layers to suppress orbital limiting. We compare the normalized phase diagrams found by RF penetration depth measurements on $\kappa$-(ET)$_2$Cu(NCS)$_2$, CeCoIn$_5$, $\alpha$-(ET)$_2$NH$_4$Hg(SCN)$_4$, $\beta^{\prime\prime}$-(ET)$_2$SF$_5$CH$_2$CF$_2$SO$_3$, and $\lambda$-(BETS)$_2$GaCl$_4$. In most of these materials, we have found possible inhomogeneous phases below reduced temperatures $(T/T_c)$ in the range 0.1 to 0.3. We also inspect some measurable sample characteristics for clues to the formation or absence of the inhomogeneous state. We will recommend $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br, which seems not to have an inhomogeneous state, for further phase diagram study. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X41.00007: Upper critical field study in the organic superconductor $\beta''$-(ET)$_{2}$SF$_{5}$CH$_{2}$CF$_{2}$SO$_{3}$ : Possibility of Fulde-Ferrell-Larkin-Ovchinnikov state Kyuil Cho, Braunen Smith, William Coniglio, Laurel Winter, Charles Agosta, John Schlueter We report upper critical field measurements in the metal-free- all-organic superconductor $\beta''$-(ET)$_{2}$SF$_{5}$CH$_{2} $CF$_{2}$SO$_{3}$ obtained from measuring the in-plane penetration depth using the tunnel diode oscillator technique. For magnetic field applied parallel to the conducting planes the low temperature upper critical fields are found to exceed the Pauli limiting field calculated by using a semi-empirical method. Furthermore, we found a signature that could be the phase transition between the superconducting vortex state (SCVS) and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in the form of a kink just below the upper critical field and only at temperatures below 1.23\,K. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X41.00008: Mixed pairing symmetry in $\kappa$-(BEDT-TTF)$_2$X organic superconductors from ultrasonic velocity measurements Mario Poirier, Maxime Dion, David Fournier, Andre-Marie S. Tremblay, Kim D. Truong Discontinuities in elastic constants are detected at the superconducting transition of layered organic conductors $\kappa$-(BEDT-TTF)$_2$X by longitudinal and transverse ultrasonic velocity measurements. Symmetry arguments show that discontinuities in shear elastic constants can be explained in the orthorhombic compound only if the superconducting order parameter has a mixed character that can be of two types, either A$_{1g}$+B$_{1g}$ or B$_{2g}$+B$_{3g}$ in the classification of irreducible representations of the orthorhombic point group D$_{2h}$. Consistency with other measurements suggests that the A$_{1g}$+B$_{1g}$ d$_{xy}$+d$_{z(x+y)}$) possibility is realized. Such clear symmetry-imposed signatures of mixed order parameters have not been observed in other superconducting compounds. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X41.00009: Upper Critical Field of Carbon-doped MgB$_{2}$ Thin Films by HPCVD Using TMB Wenqing Dai, Dwight Rickel, Xiaojun Weng, Ke Chen, Joan Redwing, Qi Li, Xiaoxing Xi We report $H_{c2}$ measurements in high magnetic field up to 60 T on carbon-doped MgB$_{2}$ thin films. About 100 nm thick carbon-doped MgB$_{2}$ films were fabricated by the Hybrid Physical-Chemical Vapor Deposition (HPCVD) method using Trimethylboron (TMB) as the doping source. For heavily doped MgB$_{2}$ films, high slope $-dH_{c2}^{//ab} /dT$ near T$_{c}$, as large as 8.3 T/K, was obtained for a sample with the T$_{c}$ of 32.3 K. The $H_{c2}$(T) in ab-plane has a downward curvature and is above 60 T at low temperatures. The c-direction $H_{c2}$ is close to 20 T. The $H_{c2}$ anisotropy increases with carbon-doping, as opposed to the reported results from other carbon-doping techniques. The TEM study shows the MgB$_{2}$ forms many nm thick layers sandwiched in the MgB$_{2}$C$_{2}$ matrix. Using two-gap dirty limit theory, the temperature dependences of $H_{c2}$ and abnormal $H_{c2}$ anisotropy in the heavily doped samples are explained by the presence of many very thin MgB$_{2}$ layers. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X41.00010: Josephson and Multi-Gap Quasi-Particle Tunneling in Crystalline MgB$_{2}$-based Junctions with an MgO Sputtered Barrier Jean-Baptiste Laloe, J. S. Moodera MgB$_{2}$ is a multi-gap superconductor with a T$_{C}$ of 39K and a hexagonal structure. This simple and stable compound is very attractive for device applications. We have deposited and patterned micron-sized SIS tunnel junctions with highly textured MgB$_{2}$ electrodes grown by MBE co-evaporation with sputter-deposited MgO tunnel barriers, in an entirely in-situ process. This method enabled us to obtain low resistance junctions with very good oxide coverage. We present $I-V$ and \textit{dI/dV} data displaying Josephson pair tunneling as well as the quasi-particle tunneling signature of both the Pi- and Sigma-bands of the MgB$_{2}$. Our experimental gap values agree with theoretical calculations. Although our MgB$_{2}$ films were $c$-axis oriented, growth-related roughness of the bottom MgB$_{2}$ enable $a/b$-axis tunneling and thus explain the observed Sigma-band features. We link our data to a simple model assuming tunneling to occur from both the Pi- and Sigma-bands in parallel, proportionally weighted depending on the interfacial topography. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X41.00011: Microwave Resonant Activation of MgB2 Thin Film Josephson Junction Roberto Ramos, Joseph Lambert, Steven Carabello, Jerome Mlack, Zechariah Thrailkill While in the superconducting state, a current-biased Josephson junction behaves like a nonlinear resonator. Using the picture of a washboard potential, the phase particle oscillates within the well When the junction is biased near its critical current. Under microwave radiation, this phase particle can be resonantly activated and made to escape. We will report results of the first microwave resonant activation of MgB2 thin film Josephson junctions at sub-Kelvin temperatures. By manipulating microwave frequency and power, we are able to control the state of the system. [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X41.00012: Study of native oxide on MgB$_{2}$ thin films by electron tunneling Ke Chen, Qi Li, X. X. Xi A native oxide of a few monolayers uniformly covering a metal film surface can often serve as a good tunnel barrier. Good quality MgB$_{2}$/native oxide/Pb Josephson tunnel junctions have been made despite little is known about the barrier's composition, which is studied by electron tunneling in this work. Native oxides were grown naturally on MgB$_{2}$ thin films in air at room or elevated temperatures up to 400 \r{ }C right after the films were made by the hybrid chemical-physical vapor deposition method. $I$--$V$ and \textit{dI}/\textit{dV}--$V$ curves were measured on MgB$_{2}$/native oxide/Pb tunnel junctions, showing that the tunnel barrier has a typical thickness of about 1 nm and height of about 3 eV, more than 3 times higher than reports from other groups on the films made by other techniques. The phonon spectra of the native oxides were observed in voltages between $\pm $120 meV. The phonon structure is derived from the tunneling conductance curve and compared to that of MgO. Based on the barrier height, the phonon structure, and the photoemission spectra on the native oxide from Robert Buhrman's group at Cornell University, we conclude that the native oxide is mainly MgO doped with some boron. [Preview Abstract] |
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