Anisotropic Superconductivity in -(BDA-TTP)2SbF6: STM SpectroscopyK. NomuraDepartment of Physics, Hokkaido University, JapanECRYS-2008, Cargese

CollaboratorsR. MuraokaHokkaido UniversityN. MatsunagaHokkaido UniversityK. IchimuraHokkaido UniversityJ. YamadaHyogo University

Outline1. Introduction -(BDA-TTP)2SbF62. STM Spectroscopyresults on conducting planeresults on lateral surfacesymmetry of the superconducting gap3. Summary

Crystal structure of b-(BDA-TTP)2SbF6Two-dimensional organic conductorFermi surface Triclinic a=0.8579 (nm) b=1.7636 c=0.6514 a=93.791 (deg) b=110.751 g=89.000Superconductingtransition temperatureTc=6.9KJ. Yamada et al. JACS 123, 4174 (2001)

Electronic specific heatCe/Tc=1.1Y. Shimojo et al.JPSJ 71, 717 (2002)specific heat jumpanisotropic superconduvtivitysymmetry of pair wave function ?non-activated behavior BCS Ce/Tc=1.43

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

STM spectroscopydI/dV is directly obtained by Lock-in detectiontip configuration

Tunneling differential conductance on the a-c surface (I // b axis)AAAB

Fitting (s-wave)BCSfinite conductance inside the gapis not reproduce by the s-wave Gap anisotropy D: gap amplitudeG: level broadening

Fitting (d-wave)d-wave symmetry0=1.6~2.8meV 20/kBTc=5.4~9.4 (Tc=6.9K)

20/kBTc=4.35(mean field approximation)

Tunneling differential conductance on the lateral surface (I b axis)aangle between a*-axis and tunneling direction (observed value)The gap is anisotropic in k-space.gap amplitude and functional form depend on the tunneling direction.

Line nodes model with k-dependence of tunneling probability: angle between electron wave vector and normal vector to the barrierq : angle between tunneling direction and gap maximumtransmission coefficient DWKB approximationb=20G=0.25mVD0=5mV

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

Relation between and a(k)= 0(coska-coskc)

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

gap symmetry in k-ET)2Cu(NCS)2Q~(0.5,0.6)Q~(0,0.25)K. Kuroki et al. PRB 65, 100516 (2002)dx2-y2 likedxy like

Superconductivity in b-(BDA-TTP)2SbF6nesting vectornodesnodes around a*c*nesting vector determines node direction. spin fluctuation mechanismattractive force between nearest neighbors (stacking direction)nodes around a*, c*spin fluctuation gap symmetry

SummarySTS on conducting surfaceAnisotropic superconductivity was confirmed from the functional form of tunneling differential conductance.01.6~2.8meV 20/kBTc5.4~9.4 (Tc=6.9K) STS on lateral surface observation of angle dependence of gapgap minimum (node) around a*c* direction(k)= 0(coska-coskc) (dx2-y2 like) consistent with spin fluctuation mechanism

ZBCP for k-(BEDT-TTF)2Cu[N(CN)2]Br

SummarySTS on conducting surfaceAnisotropic superconductivity was confirmed from the functional form of tunneling differential conductance.01.6~2.8meV 20/kBTc5.4~9.4 (Tc=6.9K) STS on lateral surface observation of angle dependence of gapgap minimum (node) around a*c* direction(k)= 0(coska-coskc) (dx2-y2 like)

ZBCP was not yet observed.

k-BEDT-TTF2Cu(NCS)2k-BEDT-TTF2Cu[N(CN)2]Brno state along /4 directionstates along/4 directionObservation of ZBCP is determined by states along /4 direction

Mechanism of ZBCPY. Tanaka and S. KashiwayaZBCP

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