Correlation between upper critical field and dimensionality in an unconventional superconductor
upper critical field
β-(BDA-TTP)2SbF6 is a layered organic superconductor with relatively high transition temperature Tc of 7.5 K. In order to investigate the superconducting(SC) state of β-(BDA-TTP)2SbF6, its interlayer resistance has been measured at magnetic fields up to 15 T and temperatures down to 1.5 K. The upper critical fields(Hc2) parallel and perpendicular to the conduction layer are determined and dimensional crossover from anisotropic three-dimensional(3D) behavior to 2D behavior is found around 6 K. When a magnetic field is rotated within the conduction layer at 6 K, Hc2 for magnetic field along the a* + c* direction is slightly larger than that for along the a* - c* direction. Normal-state magnetoresistance has the same symmetry as Hc2 and shows maximum(minimum) when a magnetic field is applied parallel to the a* - c*(a* + c*) direction. This behavior can be explained in terms of a Fermi surface anisotropy. At 3.5 K, however, we find clear fourfold symmetry of Hc2 despite the normal-state magnetoresistance shows twofold symmetry arising from the Fermi surface anisotropy. Origin of fourfold symmetry of Hc2 is discussed in terms of SC gap structure in β-(BDA-TTP)2SbF6.