Roles of spin fluctuation and frustration in the superconductivity of β-(BDA-TTP)2X (X=SbF6,AsF6) under uniaxial compression

beta-type BDA-TTP [BDA-TTP=2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene] salts possess high transition temperatures T(C) reaching 7 K among organic superconductors. T(C) of beta-(BDA-TTP)(2)X (X=SbF(6),AsF(6)) is studied by resistive measurements under uniaxial compression. T(C) once increases and takes a maximum under compression parallel to the donor stack while it decreases under compression perpendicular to the donor stack. These results are in agreement with the half-filled Hubbard model on the triangular lattice in which the compression controls the spin fluctuation and frustration in the weak pressure region.