Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z25: Novel Superconductors II |
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Sponsoring Units: DCMP Chair: Chih-Kang Ken shih, University of Texas at Austin Room: D166 |
Friday, March 25, 2011 11:15AM - 11:27AM |
Z25.00001: Momentum-dependent multiple gaps of MgB$_{2}$ probed by electron tunneling spectroscopy on MgB$_{2}$/native oxide/Pb junctions Ke Chen, Wenqing Dai, Qi Li, X.X. Xi Distinct multi-band superconductivity is a unique feature that distinguishes MgB$_{2}$ from all other phonon-mediated Bardeen-Cooper-Schrieffer (BCS) superconductors. According to a first-principles calculation employing an anisotropic Eliashberg formalism [Choi \textit{et al.} Nature 418, 758 (2002)], there is a distribution of superconducting energy gap values on the Fermi surface of MgB$_{2}$ (two $\sigma $ bands and two $\pi $ bands). However, only two distinct gaps have been observed experimentally, leading to the suggestion that consideration of the fully anisotropic electron-phonon interaction may not be necessary for real MgB$_{2}$ samples. Here, we present an electron tunneling spectroscopy study on MgB$_{2}$/native oxide/Pb junctions that clearly shows the distribution of energy gaps. By deconvoluting the tunneling spectrum based on the density of state of Pb, we derive the momentum-dependent energy gaps of MgB$_{2}$, which are in good agreement with the anisotropic Eliashberg calculation. The result affirms the importance of the anisotropic electron-phonon interaction in MgB$_{2}$ as well as its important impact on device applications. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z25.00002: Non-Cuprate Superconductor with Cubic Structure and T$_{c}$ = 85 K J.M. Estrada, E. Chavira, I. Rosales, O. Novelo, E. Fregoso, E.E. Marinero, M. Nishioka, V. Garcia-Vazquez, M. Suchomel We have synthesized a new superconductor material, namely: \textbf{Ba(Yb}$_{0.38}$\textbf{ In}$_{0.10}$\textbf{Sn}$_{0.42}$\textbf{ Pb}$_{0.10}$\textbf{)O}$_{2.66}$, by solid-state reaction in air and ambient pressure. The new compound is determined to have a T$_{c}$ of 85 K, which is new record for a non-cuprate structure. Employing synchrotron XRD and Rietveld refinement, 5 structural phases are identified in the reaction products. The phases identified are: BaTb$_{0.5}$Sb$_{0.5}$ O$_{3}$ (41.3{\%}), Yb$_{2}$BaCuO$_{5}$ (26.1{\%}), CuO (22.6{\%}), Yb$_{2}$Cu$_{2}$O$_{5}$ (4.4{\%}) and Ba$_{1.99}$Y$_{1.01}$ Cu$_{3}$O$_{8}$ (5.6{\%}). The microstructure exhibits cubic morphology (SEM) and EDX analysis is utilized to determine the stoichiometry of the new superconducting material, \textbf{Ba(Yb}$_{0.38}$\textbf{ In}$_{0.10}$\textbf{Sn}$_{0.42}$\textbf{ Pb}$_{0.10}$\textbf{)O}$_{2.66}$, which is isostructural to the cubic BaTb$_{0.5}$Sb$_{0.5}$ O$_{3}$ phase. Magnetic and Resistance measurements vs T indicate a superconducting transition at T$_{c}$ at 85 K. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z25.00003: Suppression of time reversal symmetry breaking superconcutivity in Pr(Os,Ru)$_{4}$Sb$_{12}$ and (Pr,La)Os$_{4}$Sb$_{12}$ Lei Shu, W. Higemoto, Y. Aoki, A.D. Hillier, K. Ohishi, K. Ishida, R. Kadono, A. Koda, O.O. Bernal, D.E. MacLaughlin, Y. Tunashima, Y. Yonezawa, S. Sanada, D. Kikuchi, H. Sato, H. Sugawara, T.U. Ito, M.B. Maple Zero-field muon spin relaxation ($\mu$SR) experiments have been carried out in the Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ and Pr$_{1-y}$La$_y$Os$_4$Sb$_{12}$ alloy systems to investigate broken time-reversal symmetry (TRS) in the superconducting state, signaled by the onset of a spontaneous static local magnetic field $B_{\rm s}$. In both alloy series $B_{\rm s}$ initially decreases linearly with solute concentration. Ru doping is considerably more efficient than La doping ,with a $\sim$$50\%$ faster initial decrease. The data suggest that broken TRS is suppressed for Ru concentration larger than 0.6, but persists for essentially all La concentrations. Our data support the theory of TRS-breaking superconductivity via crystal-field excitonic Cooper pairing of Koga, Matsumoto, and Shiba. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z25.00004: Signatures of half-quantum vortices in magnetoresistance of perforated samples Victor Vakaryuk, Valerii Vinokur Recent cantilever magnetometry measurements of annular micron-size samples of $\rm Sr_2RuO_4$ [1] have revealed evidence for the existence of half-quantum vortices (HQVs) in this material. Here we suggest to look for HQVs at temperatures close to $T_c$ in magnetoresistance of ``punctured'' (perforated) $\rm Sr_2RuO_4$ samples which consist of an array of regularly spaced micron-size holes in an otherwise uniform superconducting matrix. Due to the dissipative nature of resistive measurements signatures of HQVs might be seen even if their thermodynamic stability is not expected. We analyze the dependence of magnetoresistance on the thermodynamic stability of HQVs and point out features which may help to identify them. \\[4pt] [1]~J.~Jang, D.G.~Ferguson, V.~Vakaryuk, R.~Budakian, S.B.~Chung, P.M.~Goldbart, Y.~Maeno (2010). [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z25.00005: Nonintegral flux penetration in superconductors with broken time-reversal symmetry via bent domain walls David Ferguson, Paul Goldbart $\textrm{Sr}_2\textrm{RuO}_4$ is a candidate material for realizing a superconducting state that spontaneously breaks time-reversal symmetry~[1]. In such a state, the spatial pattern of the superconductivity may be broken up into regions of differing chirality, separated by domain walls. Here, we show that, near to bends in such domain walls, nonintegral (and even nonquantized) multiples of the superconducting magnetic flux quantum would penetrate the system~[2]. We discuss the implications of this ``bend flux'' effect for various experimental probes that are sensitive to time-reversal symmetry breaking. For the example of scanned-probe magnetic imaging, the observation of localized-nonquantized flux penetrating a z-axis surface of the sample, could be interpreted in terms of the presence of bent walls between domains of opposing chirality, and hence would be suggestive of time-reversal symmetry-breaking superconductivity. Alternatively, if observations should reveal localized but only {\it quantized\/} flux, this would suggest either (i)~the absence of domain walls or (ii)~their presence, but as a parallel array of straight walls. \\[4pt] [1]~C.\ Kallin and A.\ J.\ Berlinsky, J.\ Phys.\ Cond.\ Mat.{\bf 21\/}, 164210 (2009). \hfil\break\noindent [2]~D.\ G.\ Ferguson and P.\ M.\ Goldbart, arXiv:1011.2765v1 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z25.00006: Scenarios for half-integer fluxoid behavior of annular rings of Sr$_2$RuO$_4$ Raffi Budakian, David Ferguson, Paul M. Goldbart, Joonho Jang, Victor Vakaryuk Recently, cantilever torque magnetometry experiments on annular rings of superconducting Sr$_2$RuO$_4$ have revealed half- height steps in the magnetization~[1]. These features are suggestive of the existence, in these annular samples, of half- quantum fluxoid states (i.e., the coreless analogs of half- quantum vortices). We consider the existence and energetic stability (for various forms of triplet superconductivity) of half-quantum fluxoid states in annular samples. We also consider alternative scenarios that could give rise to magnetization steps. One particular scenario requires the presence, in the bulk of the sample, of thermodynamically stable ``wall vortices'' By analyzing the equilibrium state of the superconductor, as a function of the applied magnetic field, we conclude that any wall-vortex scenario consistent with the observations of Ref.~[1] would require a (to date, unexplained) fine tuning of various material parameters.\\[4pt] [1]~J.\ Jang, D.\ Ferguson, V.\ Vakaryuk, R.\ Budakian, S.\ Chung, P.Goldbart, and Y.\ Maeno, (2010 unpublished) [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z25.00007: The Atomic-Scale Onset of Charge Density Waves in NbSe$_{2}$ Chockalingam S P, L. Zhao, C. Arguello, E. Rosenthal, C. Gutierrez, J. Kang, A. Pasupathy, S. Jia, R. Cava Many modern complex materials exhibit spatially ordered electronic states such as charge and spin density waves, and scanning tunneling microscopy (STM) has recently been used successfully to visualize some of these phases. Unfortunately such phases often occur simultaneously in a single sample making the interpretation of experimental data difficult. In order to gain insight into the nature of spatial order in a simple material, we study the transition-metal dichalcogenide NbSe$_{2}$, which displays a 2D charge density wave (CDW) phase at low temperature. How does this CDW phase manifest itself in real space at the atomic scale? In order to answer this question, we have performed variable-temperature STM experiments on NbSe$_{2 }$single crystals at various temperatures around T$_{c}$ (33.5 K). We discover that static, short-range CDW order exists around crystal defects and impurities at temperatures up to 3T$_{c}$. We will describe the temperature evolution as well as the energy dependence of the short-range CDW using spectroscopic mapping. Our experiments provide a high-resolution measurement of the changes in electronic structure caused by the formation of CDW in the dichalcogenides. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z25.00008: Multiband Superconductivity in $2H$-NbSe$_{2}$ Probed by Cryomagnetic STM Spectroscopy Igor Fridman, J.Y.T. Wei, C. Kloc, V. Lukic, Rongwei Hu, C. Petrovic Using a novel magnetic field geometry, we study multiband pairing in single crystals of superconducting $2H$-NbSe$_{2}$ under finite superfluid momentum. Spectroscopy and conductance imaging were performed with a scanning tunneling microscope (STM) at 300 mK and in a field of up to 9 T, applied in the $ab$-plane. We observed multiple spectral features that evolve systematically with field, and a two-sloped zero-bias conductance that dips anomalously at 0.7 T. Our analysis yields distinct evidence for multiple gaps coming from the various Fermi-surface sheets, and has possible implications on the origin of the coexisting charge density wave order. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z25.00009: Spin density in UCoGe Jonathan Taylor, Jonathan Duffy, Matthew Butchers, Chris Stock, E. Bauer Below T = 0.8K, superconductivity and ferromagnetism (T$_{C}$ = 3K) coexist in UCoGe. The total magnetic moment is 0.03 $\mu_{B}$ at low temperatures. UCoGe is considered to be a weak itinerant ferromagnet. Recent theoretical studies Predict UCoGe indicate that UCoGe is ferromagnetic, but that the small total magnetic moment arises from the near cancellation of large U $5f$ spin and orbital moments. Theory also predicts there to be a Co spin moment. However, the total moment is predicted to be considerably larger than observed experimentally. Using magnetic Compton scattering experiments together with KKR electronic structure calculations we have measured the spin density of the ferromagnetic superconductor UCoGe and determined that at 1.3K the U$5f$ and Co$3d$ spin moments are antiparallel, in agreement with theory, but disagreeing with polarized neutron diffraction results. The measured total spin moment is determined to be -0.35$\mu_{B}FU^{-1}$, and the orbital moment hence determined to be 0.5$\mu_{B}FU^{-1}$. These moments are reduced with respect to the LDA calculations. The small ratio of U orbital to spin moments indicates that the 5f moment is itinerant. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z25.00010: Interplay of charge density modulations and superconductivity Jason W. Sadowski, K. Tanaka Although charge density waves (CDW) in transition metal dichalcogenides (TMDs) have been studied for over three decades, there is yet no consensus on the mechanism for CDW in two-dimensional TMDs. Moreover, the layered 2H-type TMDs which exhibit coexistence of CDW order and superconductivity (SC) present an intriguing opportunity for studying the interplay of SC and charge density modulations. In particular, 2H-NbSe$_2$ is most suitable for such investigation as its transition temperatures for CDW and SC are well separated, and as it is easy to cleave is ideal for surface-sensitive measurements. The relation between CDW and SC in this material is still under debate, with one experiment indicating an enhancement of SC by CDW [1] and another implying a competition of the two kinds of orders [2]. Motivated by these experiments, we study the effects of Fermi surface structure on CDW and its interplay with SC in terms of the Bogoliubov-de Gennes formalism. \\[4pt] [1] Kiss {\it et al.}, Nat. Phys. {\bf 3}, 720 (2007). \\[0pt] [2] Borisenko {\it et al.}, Phys. Rev. Lett. {\bf 102}, 166402 (2009). [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z25.00011: First-principles study of the noncentrosymmetric superconductors Li$_2$Pt$_3$B and Li$_2$Pd$_3$B Tatsuya Shishidou, Tamio Oguchi Effect of spin-orbit coupling (SOC) associated with lack of space-inversion symmetry has been a central issue in condensed-matter physics. Li$_2$Pt$_3$B and Li$_2$Pd$_3$B are superconducting below 2.7 K and 7 K, respectively, and have the same crystal structure (cubic $P4_3 32$, No.~212), which is noncentrosymmetric and is characterized by highly distorted corner-sharing BPt(Pd)$_6$ octahedra. Despite the same valency and structure, they show quite different superconducting behavior. NMR measurements [1] indicate that Li$_2$Pt$_3$B would be a spin-triplet superconductor with line nodes in the gap function while Li$_2$Pd$_3$B is a conventional spin-singlet $s$-wave superconductor. SOC would be a key to understand this difference. To clarify the electronic band structure and Fermi surface of these compounds, we performed density-functional (GGA PBE) calculations with FLAPW method. Relativistic effects were fully taken into account. The band structures calculated are in good accordance with previous work[2] and the spin splitting due to SOC is quite significant in Li$_2$Pt$_3$B. This work was supported by a MEXT KAKENHI on Innovative Areas ``Topological Quantum Phenomen''. [1] M.~Nishiyama, Y.~Inada, and G.-q.~Zheng, PRL {\bf 98}, 047002 (2007); PRB {\bf 71}, 220505(R) (2005). [2] K.-W.~Lee and W.~E.~Pickett, PRB {\bf 72}, 174505 (2005). [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z25.00012: The electron-boson spectral density function of underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and YBa$_2$Cu$_3$O$_{6.50}$ Jungseek Hwang We investigate the electron-boson spectral density function, $I^2\chi(\omega,T)$, of CuO$_2$ plane in underdoped Bi$_2$Sr$_2 $CaCu$_2$O$_{8+\delta}$ (Bi-2212) and underdoped YBa$_2$Cu$_3 $O$_{6.50}$ (Y-123) systems using the Eliashberg formalism. We apply a new (in-plane) pseudogap model to extract the electron- boson spectral function. For extracting the spectral function we assume that the spectral density function consists of two components: a sharp mode and the broad Millis-Monien-Pines (MMP) mode. We observe that both the resulting spectral density function and the intensity of the pseudogap show strong temperature dependences: the sharp mode takes most spectral weight of the function and the peak position of the sharp mode shifts to lower frequency and the depth of pseudogap, $1-\tilde {N}(0,T)$, is getting deeper as temperature decreases. We estimate fictitious (maximum) superconducting transition temperatures, $T_c(T)$, from the extracted spectral functions at various measured temperatures using a generalized McMillan formula. The estimated (maximum) $T_c$ also shows a strong temperature dependence; it is higher than the actual $T_c$ at all measured temperatures and decreases with temperature lowering. Since as lowering temperature the pseudogap is getting stronger and the maximum $T_c$ is getting lower we propose that the pseudogap may suppress the superconductivity in cuprates. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z25.00013: Modulation of Spin-Orbit interaction and superconductivity in two-dimensional electron gas at the Mott-Insulator - Band-Insulator interface: LaTiO3 - SrTiO3 Johan Biscaras, N. Bergeal, A. Kushwaha, T. Wolf, A. Rastogi, Ramesh Chandra Budhani, Jerome Lesueur It has been shown recently that a two-dimensional electron gas could form at the interface of two insulators such as LaAlO$_{3}$ and SrTiO$_{3}$ [1], or LaTiO$_{3}$ (a Mott insulator) and SrTiO$_{3}$ [2]. We present low temperature transport and magneto-transport measurements on LaTiO$_{3}$/SrTiO$_{3}$ hetero-structures, whose properties can be modulated by field effect using a metallic gate on the back of the substrate. The corresponding phase diagram has been investigated, and superconductivity evidenced for the first time in this system which involves a Mott insulator [3]. We will discuss the role of the confinement potential and the SrTiO$_{3}$ band structure on the phase diagram, and show the specific role of the spin-orbit coupling measured by localization corrections to the magnetoconductivity. Finally, the superconducting to insulator transition will be analyzed. [1] N. Reyren et al, Science 317, 1196 (2007) [2] A. Ohtomo et al, Nature 419, 378 (2002) [3] J. Biscaras et al, Nature Commun, DOI: 10.1038/ncomms1084 (2010) [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z25.00014: Upper Critial Field Calculations in p-Wave Tripet Ferromagnetic Superconductors Christopher L{\"o}rscher, Richard Klemm We report $H_{c2}(T)$ calculations for novel triplet ferromagnetic superconductors using a uniaxially anisotropic pairing interaction, obtained by means of the linearized Gor'kov gap equation. In addition to the intrinsic anisotropy of the p-wave states, we also include effective mass anisotropies in our calculations. We investigate the $H_{c2}(T)$ phase diagrams for several combinations of anisotropy, leading to novel $H_{c2}(T)$ properties, including upward curvature. We discuss the relevance of our results to experiments on UCoGe. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z25.00015: Identifying d-vectors in spin-orbit coupled multi-orbital superconductors Christoph Puetter, Hae-Young Kee In multi-orbital systems, Hund's interaction has been recognized to play a significant role in spin-triplet pairing. On the other hand, spin-orbit coupling has been treated as a perturbation, which is not a good approximation in 4d or 5d transition metal compounds. We have treated both effects on an equal footing in t2g orbital systems and studied their combined effect on spin-triplet superconductivity. We also discuss the implications of our results for spin-triplet candidate materials. [Preview Abstract] |
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