Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S11: Superconductivity: MgB2, Organics, Pd-Chalcogenides |
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Sponsoring Units: DCMP Chair: Michelle Johannes, Naval Research Laboratory Room: 007B |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S11.00001: A comparison of the superconducting states of Ta4Pd3Te16 and Nb3PdxSe7 Qiu Run Zhang, Bin Zeng, Daniel Rhodes, Luis Balicas We have measured the superconducting upper critical fields of a Ta$_{4}$Pd$_{3}$Te$_{16}$ sample and a Nb$_{3}$Pd$_{x}$Se$_{7}$ sample with very close values of $T_{c}$. The Maki parameter of Ta$_{4}$Pd$_{3}$Te$_{16}$ ($H_{c2}(T \rightarrow 0 \, \mathrm{K}) \sim 5.5 T$) is small and hence it is an orbital limited system. However, $H_{c2}$ shows an unconventional linear $T$ dependence in the whole temperature range. Even though they have similar crystalline structures, the upper critical fields of Nb$_{3}$Pd$_{x}$Se$_{7}$ are extremely high ($H_{c2}(T \rightarrow 0 \, \mathrm{K}) \sim 25 T$) and much more anisotropic. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S11.00002: High-field studies of the Pd-based Superconductor Ta$_4$Pd$_3$Te$_{16}$ Toni Helm, Philip J.W. Moll, Robert Kealhofer, James G. Analytis The layered Pd-based ternary chalcogenide Ta$_4$Pd$_3$Te$_{16}$ (TPT) has not gotten much of attention since its first synthesis in 1997. Recently, TPT was found to turn superconducting (SC) below a critical temperature of $T_c=4.5\,$K and up to $6.5\,$K under pressure. The layered material has an orthorhombic crystal structure and the main conduction channel is suspected to run along one dimensional (1D) PdTe-chains. Band structure calculations find multiple bands at the Fermi level including 1D sheets. One of the striking features in the family of M$_2$Pd$_x$Q$_5$ (M$=$Nb and Ta, Q$=$S and Se) is a very enhanced upper SC critical field $H_{c2}$. To understand the mechanism behind this enhancement TPT is of special interest since it has a similarly complex structure but much lower $H_{c2}$. Anomalous thermal transport properties and a significant anisotropy in $H_{c2}$ have been interpret in terms of an unconventional SC ground state present in TPT. Here we report studies of normal-state magnetotransport and magnetic torque in high fields that disclose details of TPT's electronic structure enabling us to speculate about the origin of SC in this compound. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S11.00003: Anisotropic Superconducting Gap and Elongated Vortices with Caroli-De Gennes-Matricon States in the New Superconductor Ta$_{4}$Pd$_{3}$Te$_{16}$ Huan Yang, Zengyi Du, Delong Fang, Zhenyu Wang, Yufeng Li, Guan Du, Xiyu Zhu, Hai-Hu Wen The superconducting state is formed by the condensation of a large number of Cooper pairs. The normal state electronic properties can give significant influence on the superconducting state. For usual type-II superconductors, the vortices are cylinder like with a round cross-section. For many two dimensional superconductors, such as Cuprates, 2H-NbSe$_{2}$ etc., albeit the in-plane anisotropy, the vortices generally have a round shape. In this paper we report the scanning tunneling microscopy/spectroscopy measurement results of a recently discovered new superconductor Ta$_{4}$Pd$_{3}$Te$_{16}$. The chain like conducting channels of PdTe$_{2}$ in Ta$_{4}$Pd$_{3}$Te$_{16}$ make a significant anisotropy of the in-plane Fermi velocity. We suggest at least one anisotropic superconducting gap with gap minima or possible node in this multiband system. In addition, elongated vortices are observed with an anisotropy of about $\xi_{\mathrm{\parallel b}}$/$\xi_{\mathrm{\bot b}} \approx $ 2.5. Clear Caroli-de-Gennes-Matricon states are also observed. Our results will initiate the study on the elongated vortices and superconducting mechanism in the new superconductor Ta$_{4}$Pd$_{3}$Te$_{16}$. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 8:48AM |
S11.00004: Tuning superconductivity in Nb$_{2}$Pd$_{0.81}$S$_{5}$ using applied pressure and uniaxial strain Kuan-Wen Chen, Qiu Run Zhang, Daniel Jackson, Andrew Gallagher, Naoki Kikugawa, Scott Riggs, David Graf, James Hamlin, Luis Balicas, Ryan Baumbach Nb$_{2}$Pd$_{0.81}$S$_{5}$ is a recently reported transition metal-chalcogenide superconductor (T$_{\mathrm{c}}$ $\sim$ 6.6 K) with unusually large upper critical fields (H$_{\mathrm{c}}$ \textgreater 37 T for H // b) [1]. We present electrical resistivity measurements under applied pressure for this compound, where a piston cylinder cell was used with Daphne 7474 oil as the pressure transmitting medium. These measurements reveal that the superconducting transition temperature abruptly increases to 8.5 K for P \textless 2 kbar, but additional pressure (up to 16 kbar) has little effect on T$_{\mathrm{c}}$. This result may indicate that while the electronic state of this compound is sensitive to strain, it is only weakly affected by hydrostatic pressure. This viewpoint is supported by subsequent experiments where application of Daphne oil or N-grease to the crystal surface results in an increase of T$_{\mathrm{c}}$ to 8 K. In order to systematically disentangle the influence of pressure and strain, we will present results from resistivity measurements where the sample is uniaxially strained using a piezo-stack (``elastoresistance'') along the b direction. \\[4pt] [1] Q.Zhang, et. Al.,''Superconductivity with extremely large upper critical fields in Nb$_{2}$Pd$_{0.81}$S$_{5}$,'' Scientific Reports(2013). [Preview Abstract] |
Thursday, March 5, 2015 8:48AM - 9:00AM |
S11.00005: MgB$_{2}$ Coated Ellipsoids as an Approach to Investigate the Possible Enhancement of the Vortex Penetrating Field of SRF Cavities Teng Tan, Matthaeus Wolak, Tsuyoshi Tajima, Xiaoxing Xi, Leonardo Civale Superconducting rf (SRF) cavities fabricated from bulk niobium (Nb) are a key component for modern particle accelerators. The magnetic field distribution on the inner wall of an SRF cavity is inversely similar to the field distribution on top of a superconducting ellipsoid when we put it in a magnetic field parallel to its axis. By measuring the vortex penetration into the magnetized superconducting ellipsoids, we can deduct the behavior of SRF cavities. Magnesium diboride (MgB$_{2})$ has potential to replace Nb as it has a higher $T_{c}$ of 39 K, a lower residual resistivity of $\sim$ 0.1 $\mu\Omega$ cm (at 42 K), and a higher thermodynamic critical field $H_{c}$ value compared to Nb. In this work, we successfully coated uniform MgB$_{2}$ layers on top of molybdenum and niobium ellipsoids. SQUID magnetometer measurements showed that the coated MgB$_{2}$ layer has a $T_{\mathrm{c}}$ above 38.5 K, and can provide a perfect magnetic shielding up to $\sim$ 500 Oe at 1.8K. By coating MgB$_{2}$ on Nb ellipsoids, we increased the vortex penetration field (the maximum field at which a cavity can be operated) by $\sim$ 500 Oe at 2 K. [Preview Abstract] |
Thursday, March 5, 2015 9:00AM - 9:12AM |
S11.00006: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S11.00007: Multiple Andreev Reflections in MgB$_{2}$/I/Pb Heterojunctions Cooled below 1 Kelvin Roberto Ramos, Steven Carabello, Joseph Lambert, Wenqing Dai, Qi Li, Ke Chen, Daniel Cunnane, C.G. Zhuang, X.X. Xi We have measured the I-V and dI/dV-V characteristics of two MgB$_{2}$/I/Pb heterojunctions below 1 Kelvin. In both junctions which were grown on c-axis substrates, we observed the characteristic pi peak corresponding to c-axis tunneling, consistent with theoretical predictions. Furthermore, we have observed sub-harmonic peaks in dI/dV that are consistent with Multiple Andreev Reflections (MAR) usually associated with high-transparency junctions. We also describe the temperature dependence of other sub-gap peaks observed. By systematically reducing noise in our setup and using sub-Kelvin temperatures, we were able to observe relatively sharp MAR peaks which are unusual for junctions with a large R$_{\mathrm{subgap}}$/R$_{\mathrm{n}}$ ratio. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S11.00008: The Effect of Argon Ambient Pressure and Annealing Time on Bulk MgB$_{2}$ Superconductor Murat Erdem, Ozgur Ozturk, Elif Asikuzun, Seydanur Kaya, Serap Safran, Ahmet Kilic, Cabir Terzioglu The effects of Ar ambient pressure (vacuum, 0B, 10B and 20B) and annealing times (0.5 h and 1 h) on microstructural, superconducting and mechanical properties of bulk superconducting MgB$_{2}$ are investigated. The samples are produced using the solid state reaction method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements were performed for determination of the crystal structure, and surface morphology of MgB$_{2}$ samples, respectively. The superconducting properties were studied by~AC magnetic susceptibility~and DC resistivity measurements. Increasing the Ar pressure decreased the lattice parameters and hence the average grain size. Increasing the annealing time results in larger lattice parameters and larger grain formation. The susceptibility measurements revealed two step transition which is reminiscent of granular superconductors. The intra-grain transition temperature is determined to be 38.4 K for all samples. The inter-grain transition temperatures of 37.2 K is obtained for samples produced under Ar ambient. The samples produced under Ar ambient have better superconducting properties than the ones produced in vacuum. Increasing the annealing time under vacuum further decreases the superconducting properties probably due to Mg loss. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S11.00009: Isotropic superconducting gap and electron-boson coupling in MgB2 multiband superconductor Daixiang Mou, Rui Jiang, Valenetin Taufour, Rebecca Flint, Serguei Bud'ko, Paul Canfield, Adam Kaminski MgB$_2$ is a prototype multiband BCS/Elishberg superconductor with high transition temperature T$_C$ $\sim$39K. In this talk, we will present electronic properties of this compounds measured by tunable laser ARPES. Momentum dependent gap structure on two $\sigma$ Fermi surface around $\Gamma$ is nearly isotropic with $\Delta_0$$\sim$7meV, directly proving its s-wave pairing symmetry. Our data reveals a strong renormalization of the dispersion (kink) at $\sim$65 meV, which is caused by coupling of electrons to E$_{2g}$ phonon mode. In contrast to cuprates, the 65 meV kink in MgB$_2$ does not change significantly across T$_C$. More interestingly, we observe strong coupling to a second, low energy collective mode at 10 meV. This excitation vanishes above T$_C$ and is likely a signature of the elusive Leggett mode. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S11.00010: Measurement of the Penetration Depth and Coherence Length of MgB2 in All Directions Using Transmission Electron Microscopy James Loudon, S. Yazdi, T. Kasama, N.D. Zhigadlo, J. Karpinski We demonstrate that images of flux vortices in a superconductor taken with a transmission electron microscope can be used to measure simultaneously the penetration depth and coherence length in all directions at the same temperature and magnetic field. This is particularly useful for MgB$_2$ where these quantities vary with the applied magnetic field and values are difficult to obtain at low field or in the $c$ direction. We obtained images of flux vortices from a sample cut in the $ac$ plane by focussed ion beam milling and compared these with simulations which accounted for flux vortices with a non-zero core in a thin, anisotropic superconductor. This gave penetration depths $\Lambda_{ab}=100\pm 35$ nm, $\Lambda_c=120\pm 15$ nm and coherence lengths $\xi_{ab}=41\pm 13$ nm and $\xi_c=34\pm 10$ nm at 10.8 K in a field of 4.8 mT. The implications of these values for type-1.5 superconductivity will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S11.00011: Superconductivity beyond the dimer model in 2D organic charge transfer salts Michaela Altmeyer, Daniel Guterding, Harald O. Jeschke, Roser Valenti We present a theoretical investigation of BEDT-TTF charge transfer salts containing $\kappa$-type layers. Using ab-initio density functional theory we construct realistic models with unprecedented accuracy for a broad variety of materials. We analyze the pairing symmetry and strength within random phase approximation spin fluctuation theory and interpret our findings microscopically in a tight-binding analysis. In particular we show that the minimal model for this class of materials needs to describe all BEDT-TTF molecules independently and give an example where the customary dimer model breaks down. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S11.00012: Inhomogeneous superconducting state in $\beta ''$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$ Georgios Koutroulakis, H. K\"{u}hne, R. Kato, J. A. Schlueter, J. Wosnitza, S.E. Brown We present $^{13}$C nuclear magnetic resonance (NMR) measurements on the quasi-2D organic superconductor $\beta ''$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$, at ultra-low temperature ($T\sim$100mK). For a magnetic field applied precisely within the conducting layers, the field evolution of the NMR spectrum reveals a phase transition within the superconducting state ($H_{c2}>$12T) near to the Pauli limit $H_P\sim$9.5T. The transition is identified by the significant line-broadening of the spectrum, associated with the electronic spin polarization distribution due to the emergence of spatially inhomogeneous superconductivity, the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The character of this novel SC state is studied via static and dynamic (i.e. spin-lattice relaxation) NMR measurements. Moreover, the stability of the putative FFLO phase upon rotation of the field away from the in-plane condition is investigated. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S11.00013: Electronic structure of a dual-layered organic charge transfer salt Harald Jeschke, Michaela Altmeyer, Roser Valenti We examine the electronic properties of polymorphs of (BEDT-TTF)$_2$Ag(CF$_3$)$_4$(TCE) (1,1,2-trichloroethane) within density functional theory (DFT). While a phase with low superconducting transition temperature $T_c=2.6$~K exhibits a $\kappa$ packing motif, two high $T_c$ phases are layered structures consisting of $\alpha'$ and $\kappa$ packed layers. We determine the electronic structures and discuss the influence of the insulating $\alpha'$ layer on the conducting $\kappa$ layer. We find that the stripes of high and low charge in the $\alpha'$ layer correspond to a stripe pattern of hopping parameters in the $\kappa$ layer. This finding is the basis for studying the effect of the different underlying Hamiltonians on the superconducting properties. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S11.00014: Thermodynamics and existing phase of Ba-phenanthrene Satoshi Heguri, Quynh Thi Nhu Phan, Yoichi Tanabe, Katsumi Tanigaki The recent discovery of superconductivity in potassium doped picene suggested the possibility of a new class of superconductors. The problem is that no satisfactory guide to improve the superconducting shielding fraction had been provided until recently. However, a high superconducting shielding fraction of 65 {\%} was reported for Ba$_{\mathrm{1.5}}$(phenanthrene). Considering this situation, phenanthrene (PHN) appears to be a key material for confirming the existence of metallicity and superconductivity in the aromatic hydrocarbon (AHC) family, and also for clarifying the physical properties and superconducting mechanism of AHC superconductors. In the present work, the thermodynamics for intercalation of PHN with Ba is studied in comparison with its isomer of anthracene (AN). Contrarily to previous reports by other authors, the important observation that Ba is intercalated into neither PHN nor AN without affecting their molecular structures is unambiguously made by differential scanning calorimetry measurements and annealing time dependences observed by powder x-ray diffraction measurements. The reactions of Ba and PHN at elevated temperatures lead this system to molecular decomposition instead of intercalation. The phenomena of metallicity and superconductivity in PHN intercalated with alkaline earth metals (Ba or Sr) should be reconsidered. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S11.00015: Superconducting and Pauli Paramagnetic Anisotropies in TmNi$_2$B$_2$C M.R. Eskildsen, P. Das, Z. Weems, C. Rastovski, S.J. Kuhn, L. DeBeer-Schmitt, K. Littrell, S.L. Bud'ko, P.C. Canfield A direct measure of the intrinsic superconducting anisotropy ($\Gamma_{ac}$) is difficult to obtain in materials where multiple bands contribute to the superconductivity or where $H_{c2}$ is orbitally limited for one field direction and Pauli limited along another. We report on small-angle neutron scattering studies of the vortex lattice (VL) in TmNi$_2$B$_2$C with fields applied along the basal plane. This material orders antiferromagentically and the magnetic and superconducting states are known to strongly influence each other, even in the paramagnetic phase above $T_N$. Our measurements allowed an independent determination of $\Gamma_{ac}$ (through the VL anisotropy), and the magnetic-superconducting interaction anisotropy reflected in the Pauli paramagnetic effect (spin-polarization of unpaired quasiparticles in the vortex cores) on the VL scattered intensity (form factor). [Preview Abstract] |
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