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Anisotropic Superconductivity in  -(BDA-TTP) 2 SbF 6 : STM Spectroscopy

ECRYS-2008, Cargese. Anisotropic Superconductivity in  -(BDA-TTP) 2 SbF 6 : STM Spectroscopy. K. Nomura Department of Physics, Hokkaido University, Japan. Collaborators. R. Muraoka Hokkaido University N. Matsunaga Hokkaido University K. Ichimura Hokkaido University

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Anisotropic Superconductivity in  -(BDA-TTP) 2 SbF 6 : STM Spectroscopy

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  1. ECRYS-2008, Cargese Anisotropic Superconductivity in -(BDA-TTP)2SbF6: STM Spectroscopy K. Nomura Department of Physics, Hokkaido University, Japan

  2. Collaborators R. Muraoka Hokkaido University N. Matsunaga Hokkaido University K. Ichimura Hokkaido University J. Yamada Hyogo University

  3. Outline 1. Introduction -(BDA-TTP)2SbF6 2. STM Spectroscopy results on conducting plane results on lateral surface symmetry of the superconducting gap 3. Summary

  4. BDA-TTP Crystal structure of b-(BDA-TTP)2SbF6 Triclinic a=0.8579 (nm) b=1.7636 c=0.6514 a=93.791 (deg) b=110.751 g=89.000 Superconducting transition temperature Tc=6.9K Fermi surface Two-dimensional organic conductor J. Yamada et al. JACS 123, 4174 (2001)

  5. Electronic specific heat ・non-activated behavior ・specific heat jump Ce/γTc=1.1 （BCS Ce/γTc=1.43） anisotropic superconduvtivity symmetry of pair wave function ? Y. Shimojo et al.JPSJ 71, 717 (2002)

  6. K. Kanoda b-(BDA-TTP)2I3 Triclinic a=0.9246 (nm) b=1.6792 c=0.6495 a=95.263 (deg) b=106.576 g=95.766 →strong electron correlation J. Yamada et al. Chem. Comm. 1331 (2006)

  7. Y Y X piezo scanner controller Z feed back PC e- w w sample tunneling current tunneling current I is given by bias voltage V gold paste gold wire（f=50mm） at low temperature STM spectroscopy tip configuration dI/dV is directly obtained by Lock-in detection

  8. Tunneling differential conductance on the a-c surface (I // b axis) A A A B

  9. Fitting (s-wave) D: gap amplitude G: level broadening finite conductance inside the gap is not reproduce by the s-wave Gap anisotropy BCS

  10. Fitting (d-wave) d-wave symmetry Δ0=1.6~2.8meV 2Δ0/kBTc=5.4~9.4 (Tc=6.9K) 2Δ0/kBTc=4.35 (mean field approximation)

  11. Tunneling differential conductance on the lateralsurface (I b axis) a：angle between a*-axis and tunneling direction (observed value) gap amplitude and functional form depend on the tunneling direction. The gap is anisotropic in k-space.

  12. Line nodes model with k-dependence of tunneling probability • : angle between electron wave vector and normal vector to the barrier • q : angle between tunneling direction and gap maximum WKB approximation transmission coefficient D b=20 G=0.25mV D0=5mV

  13. Fitting (line nodes model with wave vector dependence of tunneling) a: angle between a*-axis and tunneling direction (observed value) • : angle between tunneling direction • and gap maximum

  14. node Relation between  and a (k)= 0(coska-coskc)

  15. c* a* Anisotropic superconducting gap (k)= 0(coska-coskc) a*=c* node//stacking direction a*>c*

  16. gap max. STSdx2-y2 Arai et al. node. gap symmetry in k-（ET)2Cu(NCS)2 Q~(±0.5π,±0.6π) Q~(0,±0.25π) dx2-y2like dxylike K. Kuroki et al. PRB 65, 100516 (2002)

  17. Superconductivity in b-(BDA-TTP)2SbF6 spin fluctuation  gap symmetry nesting vector ＝ nodes nodes around a*±c* attractive force between nearest neighbors (stacking direction) nodes around a*, c* nesting vector determines node direction.  spin fluctuation mechanism

  18. Summary • STS on conducting surface Anisotropic superconductivity was confirmed from the functional form of tunneling differential conductance. Δ0＝1.6~2.8meV 2Δ0/kBTc＝5.4~9.4 (Tc=6.9K) • STS on lateral surface observation of angle dependence of gap gap minimum (node)around a*c* direction 　　　　　　　　➡　(k)= 0(coska-coskc) (dx2-y2 like) consistent with spin fluctuation mechanism

  19. ZBCPfor k-(BEDT-TTF)2Cu[N(CN)2]Br

  20. Summary STS on conducting surface Anisotropic superconductivity was confirmed from the functional form of tunneling differential conductance. Δ0＝1.6~2.8meV 2Δ0/kBTc＝5.4~9.4 (Tc=6.9K) STS on lateral surface observation of angle dependence of gap gap minimum (node)around a*c* direction 　　　　　　　　➡　(k)= 0(coska-coskc) (dx2-y2 like) ZBCP was not yet observed.

  21. k-（BEDT-TTF）2Cu(NCS)2 k-（BEDT-TTF）2Cu[N(CN)2]Br states along /4 direction no state along /4 direction Observation of ZBCP is determined by states along /4 direction

  22. Mechanism of ZBCP ZBCP アンドレーエフ反射 Y. Tanaka and S. Kashiwaya

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