Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V38: Novel Superconductors II |
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Sponsoring Units: DCMP Chair: Ilya Vekhter, Louisiana State University Room: 385 |
Thursday, March 16, 2017 2:30PM - 2:42PM |
V38.00001: Spin-orbit driven Peierls transition and possible Agterberg-Barzykin-Gorkov superconductivity in CsW2O6 Igor Mazin, Sergey Streltsov, Rolf Heid, Klaus-Peter Bohnen We study \textit{ab initio} a pyrochlore compound, CsW$_{2}$O$_{6}$, which exhibits a yet unexplained metal-insulator transition. We find that (1) the reported low-$T$ structure is likely inaccurate and the correct structure has a twice larger cell; (2) the insulating phase is not of a Mott or dimer-singlet nature, but a rare example of a 3D Peierls transition, with a simultaneous condensation of three charge density waves; (3) spin-orbit interaction plays a crucial role, forming well-nested bands. The high-$T$ (HT) phase, if stabilized, could harbor a unique $e_{g}+ie_{g}$ superconducting state that breaks the time reversal symmetry, but is not chiral. This state was predicted in 1999 by Agterberg et al, but never observed. We speculate about possible ways to stabilize the HT phase while keeping the conditions for superconductivity. [Preview Abstract] |
Thursday, March 16, 2017 2:42PM - 2:54PM |
V38.00002: Evidence for an impurity band in the anomalous low carrier density superconductor Pb$_{1-x}$Tl$_x$Te Philip Walmsley, Paula Giraldo-Gallo, Boris Sangiorgio, Deanna Abrams, Michael Fechner, Lisa Buchauer, Benoit Fauque, Scott Riggs, Ross McDonald, Theodore Geballe, Nicola Spaldin, Kamran Behnia, Ian Fisher The narrow-band-gap semiconductor PbTe superconducts with a Tc an order of magnitude greater than comparable low-density metals, but only when it is doped with the specific element thallium. Here we present a comprehensive study of the evolution of the Fermi surface of hole doped PbTe as derived from Shubnikov de Haas quantum oscillations, combined with measurements of the evolution of the electrical transport and specific heat. We compare cases for dopants that cause superconductivity (thallium) and those that don’t (sodium), and identify the presence of a Tl-impurity band as the key feature associated with the presence of superconductivity. These results give the clearest description of the fermiology of PbTe to date, relevant for understanding both to its superconducting ground state and its high thermoelectric figure of merit. [Preview Abstract] |
Thursday, March 16, 2017 2:54PM - 3:06PM |
V38.00003: Searching for New Intermetallic Superconductors from the Viewpoint of Chemistry Weiwei Xie, Robert Cava The conventional pictures of superconductivity arising from the Bardeen-Cooper-Schrieffer (BCS) theory focus on the electron-phonon coupling that induces a superconducting state. Until now, BCS theory is still in a predominant position and is applied to determine whether a superconductor could be classified as BCS-like or not. Recently, with more high-temperature superconductors being discovered, more `universal' mechanisms were sought for to explain both conventional and unconventional superconductors. However, neither BCS nor other modified mechanisms are able to connect physical properties with chemical structures. Thus, the primary questions are whether the general statements of BCS theory can be associated with distinct chemical meanings, such as crystal structural features and specific chemical bonding, and whether {\&} how the physical phenomenon of superconductivity can be interpreted from the standpoint of chemistry. Moreover, the approach based on the chemistry interpretation has to be taken to explore the new superconductors. In this talk I will describe some empirical design rules and examples that have yielded new intermetallic superconductors. [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:18PM |
V38.00004: Exploration of possible superconductivity in chemically doped CrSb Ashutosh Dahal, Jagath Gunasekera, Seng Huat Lee, Yew San Hor, Deepak K. Singh Our efforts of finding a new superconductor in 11-system is based on a simple proposition that antiferromagnets (AFM) with stripe magnetic configuration indirectly plays the key role in the Cooper pair formation, essential for superconductivity. As CrAs exhibits superconductivity under pressure, CrSb based compounds with antiferromagnetic order and Neel temperature in excess of 700K would be a natural venue for the exploration of superconductivity. In this talk, I will discuss experimental results on chemically doped CrSb (on both Cr and Sb site), which reveals interesting new properties that are reminiscent of a superconductor. The transition to the possible superconducting state occurs at T \textasciitilde 8.5 K with a critical field of more than 2.5 T. However, the transition to the possible superconducting state is not unambiguous. We have tried various chemical doping and synthesis methods to understand this behavior. [Preview Abstract] |
Thursday, March 16, 2017 3:18PM - 3:30PM |
V38.00005: Quasilinear quantum magnetoresistance in pressure-induced superconductor CrAs Qun Niu, Wing Chi Yu, King Yau Yip, Zi Li Lim, Hisashi Kotegawa, Eiichi Matsuoka, Hitoshi Sugawara, Hideki Tou, Youichi Yanase, Swee Kuan Goh We have measured the transverse magnetoresistance of CrAs under pressure. Pressure-induced superconductivity was recently observed in CrAs in the vicinity of the magnetic order. The low-temperature magnetoresistance shows non-saturating quasilinear magnetic field dependence up to 14 T in the pressure range close to where the superconducting transition temperature is maximised. Our bandstructure calculations reveal a subtle band crossing near the Y-point of the Brillouin zone, which we argue is responsible for the observed quasilinear magnetoresistance. Comparison with CrP, which exhibits conventional quadratic magnetoresistance will be made and discussed in this presentation. [Preview Abstract] |
Thursday, March 16, 2017 3:30PM - 3:42PM |
V38.00006: The effect of pressure on extreme magnetoresistance and superconductivity in LaBi Fazel Tafti, Milton Torikachvili, Ryan Stillwell, Bruce Baer, Elissaios Stavrou, Sam Weir, Yogesh Vohra, Hung-Yu Yang, Evan McDonnell, Satya Kushwaha, Quinn Gibson, Robert Cava, Jason Jeffries Extreme magnetoresistance (XMR) in topological semimetals is a recent discovery which attracts attention due to its robust appearance in a growing number of materials. To search for a relation between XMR and superconductivity, we study the effect of pressure on LaBi taking advantage of its simple structure and simple composition. By increasing pressure, we observe the disappearance of XMR followed by the appearance of superconductivity at P $=$ 3.5 GPa. We find a region of coexistence between superconductivity and XMR in LaBi in contrast to other superconducting XMR materials. The suppression of XMR is correlated with increasing zero-field resistance instead of decreasing in-field resistance. At higher pressures, P $=$ 11 GPa, we find a structural transition from the face center cubic lattice to a primitive tetragonal lattice, in agreement with theoretical predictions. The relationship between extreme magnetoresistance, superconductivity, and structural transition in LaBi is discussed. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 3:54PM |
V38.00007: Quantum Oscillation in noncentrosymmetric superconductor BiPd Mojammel Alam Khan, David Graf, John DiTusa, David Young In a noncentrosymmetric (NCS) superconductor (lacking a center of inversion), the order parameter can have a mixture of spin singlet and triplet pairing components, often leading to superconducting properties that deviate from the BCS predictions. BiPd, which crystallizes in monoclinic structure, is NCS and recent experiments on BiPd show complex properties with the possibility of mixed pairing as well as evidence of Dirac surface states. Here we present the measurement of quantum oscillations (dHvA) using torque magnetometry on single crystals of BiPd under applied fields up to 35 T and at temperatures as low as 300 mK. Oscillations were observed at temperatures as high as 20 K, with a clear angle dependence of the oscillations with respect to the crystallographic b-axis.From the Fourier transform analysis of the oscillation data, several pieces of Fermi surface have been identified, which agrees with the theoretical calculations.The total Fermi surface is complex, being composed of multiple sheets of both hole- and electron-like orbits. A small piece of Fermi surface, with frequency 40 T, can be followed in the entire angle sweep. From the temperature dependence of the frequency amplitude the cyclotron effective mass and the corresponding Berry phase have been calculated. [Preview Abstract] |
Thursday, March 16, 2017 3:54PM - 4:06PM |
V38.00008: Phonon mechanism for superconductivity in YPtBi Jonathan Ruhman, Lucile Savary, Jorn W. F. Venderbos, Liang Fu, Patrick A. Lee YPtBi is a semi metal with low charge-carrier density, $n = 2\times 10^{18} \,\mathrm{cm}^{-3}$, influenced by strong spin-orbit coupling. Despite, the low density, this material becomes superconducting with a transition temperature of $T_c = 0.8$ K. According to recent ab-initio calculations, phonons can not be responsible for the superconducting instability, due to the low density of states at the Femri level. Notwithstanding, we obtain a sufficiently large transition temperature by considering the coupling to longitudinal-optical phonon modes through Coulomb forces. Here the long range nature of the Coulomb interaction makes it effective at low density, where the density of states is small, as apposed to local phonon couplings. We derive the appropriate Elaishberg theory for a semi-metal with a quadratic band touching point. We also discuss the possibility of triplet pairing generated by these polar phonons. [Preview Abstract] |
Thursday, March 16, 2017 4:06PM - 4:18PM |
V38.00009: Weak electron-phonon pairing in BiS2 superconductor from first principles Ryosuke Akashi, Corentin Morice, Takashi Koretsune, Siddharth Saxena, Ryotaro Arita The discovery of superconductivity in Bi$_4$O$_4$S$_3$, quickly followed by the one in La(O,F)BiS$_2$, opened up a new research field: novel BiS$_2$ superconductors. Many thorough experimental studies have been conducted but consensus on the nature of superconductivity in these materials has not been reached yet. One of the strongest pieces of evidence until now was the calculation of strong electron-phonon coupling in La(O,F)BiS$_2$ using ab-initio techniques, indicating that superconductivity in these materials is conventional and strongly coupled. Using density functional theory and the recently developed density functional theory for superconductors, we studied the possibility of phonon-mediated superconductivity. We first confirmed the arising of a commensurate charge density wave instability, in accordance with previous studies. Using a novel integration scheme for the electron-phonon coupling, we found that its strength is much lower than previously calculated, due to our improved convergence of density of state calculations. We finally conclude that the conventional phonon-mediated electron pairing cannot explain the high $T_c$s observed in this material. [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:30PM |
V38.00010: Upper critical field of \textit{Ln}O$_{\mathrm{0.5}}$F$_{\mathrm{0.5}}$BiS$_{\mathrm{2}}$ (\textit{Ln} $=$ La, Nd) superconductors at extreme conditions Yuankan Fang, C. T. Wolowiec, A. J. Breindel, D. Yazici, M. B. Maple, P.-C. Ho One of the most interesting phenomena displayed by many BiS2-based superconductors including \textit{Ln}O$_{\mathrm{1-}}_{x}$F$_{x}$BiS$_{\mathrm{2}}$ (\textit{Ln} $=$ La-Nd) is the rather abrupt enhancement of superconducting transition temperature ($T_{c})$ under high pressure (HP). In this study, we investigated the upper critical field $H_{c}_{\mathrm{2\thinspace }}$of polycrystalline samples of \textit{Ln}O$_{\mathrm{0.5}}$F$_{\mathrm{0.5}}$BiS$_{\mathrm{2}}$ (\textit{Ln} $=$ La, Nd) at ambient pressure and high pressure at various magnetic fields up to 8.5 T. For the LaO$_{\mathrm{0.5}}$F$_{\mathrm{0.5}}$BiS$_{\mathrm{2}}$ sample under HP $H_{c}_{\mathrm{2}}$ shows an anomalous behavior between 5 and 6 T. However, such anomalous behavior is very subtle for NdO$_{\mathrm{0.5}}$F$_{\mathrm{0.5}}$BiS$_{\mathrm{2}}$ under HP. The results explain why the pressure-induced enhancement of $T_{c}$ for NdO$_{\mathrm{0.5}}$F$_{\mathrm{0.5}}$BiS$_{\mathrm{2}}$ is not as large as that for LaO$_{\mathrm{0.5}}$F$_{\mathrm{0.5}}$BiS$_{\mathrm{2}}$ and also support the idea that local atomic environment, is possibly more essential to the enhancement of $T_{c}$ for BiS$_{\mathrm{2}}$-based superconductors than the structural phase transition. [Preview Abstract] |
Thursday, March 16, 2017 4:30PM - 4:42PM |
V38.00011: Determining the energy gaps of MgB$_2$ electrodes in all-MgB$_2$ Josephson junctions using tunneling spectroscopy Joseph Lambert, Masahito Sakoda, Michio Naito, Roberto Ramos Magnesium diboride (MgB$_2$) is a novel BCS superconductor, possessing two distinct momentum-dependent gaps. Substructure within the gaps has previously been characterized using tunneling spectroscopy of 1-gap/2-gap heterojunctions, in which the counter electrode is a conventional single-gap superconductor, such as Pb or Sn. Here, we report tunneling spectroscopy measurements of 2-gap/2-gap all-MgB$_2$ Josephson junctions, with different barrier materials including MgO. The crystal orientations of the two MgB$_2$ films are mostly c-axis parallel to the tunneling direction, resulting in very small contribution from the larger $\sigma$ gap. Additionally, due to differences in growth conditions, the two MgB$_2$ electrodes have different critical temperatures and gap values. We present our analysis of differential conductance measurements using a modified tunneling model in which each electrode is represented as a weighted sum of two BCS densities of states. We observe (1) a transition from SIS to NIS behavior as the temperature increases past the lower T$_c$ electrode, and (2) the presence of multiple quasiparticle peaks due to the sums and differences in various pairwise combinations of disparate $\pi$ and $\sigma$ gap values within each electrode. [Preview Abstract] |
Thursday, March 16, 2017 4:42PM - 4:54PM |
V38.00012: Tuning the Phase Diagram of MgB$_2$ through Magnetic and Non-magnetic Doping E.R. De Waard, S. Manni, P.C. Canfield, J. Barker, C.D. Dewhurst, M.R. Eskildsen Small-angle neutron scattering (SANS) studies of the vortex lattice (VL) in MgB$_2$ have revealed a complex VL phase diagram as well as an unprecedented degree of metastability that is demonstrably not due to vortex pinning, [C. Rastovski $et$ $al.$, Phys. Rev. Lett. {\bf 111}, 107002 (2013)]. The VL phase diagram is governed by non-local vortex-vortex interactions, which depend sensitively on Fermi surface anisotropies and the mean free path of the host superconductor. We have investigated the effects of modifying the vortex-vortex interaction by non-magnetic (C) as well as magnetic (Mn) doping. SANS studies revealed substantial modifications of the VL phase diagram in single crystals of Mg(B$_x$C$_{1-x}$)$_2$ and (Mg$_x$Mn$_{1-x}$)B$_2$. In the Mn-doped case, a large increase in the longitudinal correlation length ($\xi_L$) was observed. However, VL metastability was still present despite this increase of vortex pinning. [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:06PM |
V38.00013: An effective 2-band $e_g$ model of sulfur hydride H$_3$S for high-$T_{\textrm{c}}$ superconductivity Kazutaka Nishiguchi, Shingo Teranishi, Satoaki Miyao, Goh Matsushita, Koichi Kusakabe To understand high transition temperature ($T_{\textrm{c}}$) superconductivity in sulfur hydride H$_3$S, we propose an effective 2-band model having the $e_g$ symmetry as the minimal model for H$_3$S. Two $e_g$ orbitals centered on a sulfur S atom are chosen for the smallest representation of relevant bands with the van-Hove singularity around the Fermi levels except for the $\Gamma$-centered small hole pockets by the sulfur 3$p$ orbitals. By using the maximally localized Wannier functions, we derive the minimal effective model preserving the body-centered cubic (bcc) crystal symmetry of the H$_3$S phase having the highest $T_{\textrm{c}}$ ($\sim$ 203 K under pressures) among the other polymorphs of H$_3$S. [Preview Abstract] |
Thursday, March 16, 2017 5:06PM - 5:18PM |
V38.00014: Ultrasensitive detection of superconducting transitions within doped Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ James Wampler, Juan Trastoy, Gang Cao, Ivan Schuller Previous studies have shown that the electronic structure of electron-doped Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ highly resembles that of hole-doped cuprates. This fact and the similarity of their crystal structures make doped Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ a promising candidate for high-temperature superconductivity. [1] We doped polycrystalline Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ in both Sr and Ir sites using powder metallurgy and characterized them with X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). By mixing powders, it is possible to synthesize interesting off-stoichiometry minority phases, which XRD and VSM sometimes cannot detect. However, these phases may still be superconducting. In order to search for superconductivity in the bulk and minority phases, we used magnetic field modulated microwave spectroscopy (MFMMS). MFMMS has been shown to be an ultrasensitive technique, able to detect superconducting volumes as small as 10$^{\mathrm{-12}}$ cc. In some samples, we found strong MFMMS evidence for a superconducting transition that was beyond the sensitivity limit of either VSM and XRD. We will discuss the possible implications of these results. 1. T. F. Qi, G. Cao, et al., J. Appl. Phys., \textbf{109} 07D906 (2011) [Preview Abstract] |
Thursday, March 16, 2017 5:18PM - 5:30PM |
V38.00015: Robust Resistive Critical Field in B20 AuBe and other Noncentrosymmetric Superconductors DJ Rebar, M Khan, J Ball, P Adams, D Browne, D Young, J Prestigiacomo, JY Chan, JF DiTusa AuBe is a superconductor with a chiral B20 structure which allows the possibility of spin-triplet superconductivity (SC) due to the lack of inversion symmetry. This structure type figures prominently in the formation of helimagnetism and the skyrmion lattice in magnetic B20 systems so that it is imperative to investigate the properties of a rare superconductor with this structure. Specific heat measurement revealed bulk SC with an exponential (BCS) form below Tc while magnetization indicated a Type I behavior near Tc$=$3.2 K and a crossover to Type II behavior below 1.2 K. Resistance measurements of the critical field revealed a phase boundary that deviates from that determined from magnetization measurements at approximately 2.4 K and which rises with decreasing T to approximately 4.3x Hc2. The resistive critical field was also found to be robust against a Cr film deposited on the surface of AuBe indicating that a simple surface state is likely not responsible for the enhanced critical field. Other noncentrosymmetric compounds reported in literature exhibit similar behavior with resistance measurements displaying a significantly larger critical field than expected from BCS theory. We compare AuBe in context with this group and suggest further experiments. [Preview Abstract] |
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