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

Collaborators R. MuraokaHokkaido University N. MatsunagaHokkaido University K. IchimuraHokkaido University J. YamadaHyogo University

Outline 1. Introduction  -(BDA-TTP) 2 SbF 6 2. STM Spectroscopy results on conducting plane results on lateral surface symmetry of the superconducting gap 3. Summary

Crystal structure of  -(BDA-TTP) 2 SbF 6 BDA-TTP Two-dimensional organic conductor Fermi surface Triclinic a= (nm) b= c=  = (deg)  =  = Superconducting transition temperature T c =6.9K J. Yamada et al. JACS 123, 4174 (2001)

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

 - (BDA-TTP) 2 I 3 Triclinic a= (nm) b= c=  = (deg)  =  = J. Yamada et al. Chem. Comm (2006) →strong electron correlation K. Kanoda

STM spectroscopy tunneling current I is given by bias voltage V at low temperature dI/dV is directly obtained by Lock-in detection X Y Z Y e-e- sample piezo scanner controller feed back w w tunneling current PC gold paste gold wire （  m ） tip configuration

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

Fitting (s-wave) BCS finite conductance inside the gap is not reproduce by the s-wave Gap anisotropy  gap amplitude  : level broadening

Fitting (d-wave) d-wave symmetry Δ 0 =1.6~2.8meV 2Δ 0 /k B T c =5.4~9.4 (T c =6.9K) 2Δ 0 /kBT c =4.35 (mean field approximation)

Tunneling differential conductance on the lateral surface (I  b axis)  a ： angle 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 barrier  angle between tunneling direction and gap maximum transmission coefficient D WKB approximation   =0.25mV    mV

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 node  (k)=  0 (cosk a -cosk c )

a* c* Anisotropic superconducting gap  (k)=  0 (cosk a -cosk c ) a*>c*a*>c* a*=c*a*=c* node//stacking direction

gap symmetry in  （ ET ) 2 Cu(NCS) 2 Q~(±0.5π,±0.6π)Q~(0,±0.25π) K. Kuroki et al. PRB 65, (2002) d x 2 -y 2 liked xy like gap max. STS d x 2 -y 2 Arai et al. node.

Superconductivity in  -(BDA-TTP) 2 SbF 6 nesting vector ＝ nodes nodes around a*±c* nesting vector determines node direction.  spin fluctuation mechanism attractive force between nearest neighbors (stacking direction) nodes around a*, c* spin fluctuation  gap symmetry

Summary STS on conducting surface Anisotropic superconductivity was confirmed from the functional form of tunneling differential conductance. Δ 0 ＝ 1.6~2.8meV 2Δ 0 /k B T c ＝ 5.4~9.4 (T c =6.9K) STS on lateral surface observation of angle dependence of gap gap minimum (node) around a*  c* direction ➡  (k)=  0 (cosk a -cosk c ) (d x 2 -y 2 like) consistent with spin fluctuation mechanism

ZBCP  for  (BEDT-TTF) 2 Cu[N(CN) 2 ]Br

Summary STS on conducting surface Anisotropic superconductivity was confirmed from the functional form of tunneling differential conductance. Δ 0 ＝ 1.6~2.8meV 2Δ 0 /k B T c ＝ 5.4~9.4 (T c =6.9K) STS on lateral surface observation of angle dependence of gap gap minimum (node) around a*  c* direction ➡  (k)=  0 (cosk a -cosk c ) (d x 2 -y 2 like) ZBCP was not yet observed.

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

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