Dimensional Crossover and Its Interplay with In-Plane Anisotropy of Upper Critical Field in beta-(BDA-TTP)(2)SbF6

摘要

Resistance measurements have been performed to investigate the dimensionality and the in-plane anisotropy of the upper critical field (H-c2) for beta-(BDA-TTP)(2)SbF6 in fields H up to 15 T and at temperatures T from 1.5 to 7.5 K, where BDA-TTP stands for 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene. The upper critical fields parallel and perpendicular to the conduction layer are determined and dimensional crossover from anisotropic three-dimensional behavior to two-dimensional behavior is found at around 6 K. When the direction of H is varied within the conducting layer at 6.0 K, H-c2 shows twofold symmetry: H-c2 along the minimum Fermi wave vector (maximum Fermi velocity) is larger than that along the maximum Fermi wave vector (minimum Fermi velocity). The normal-state magnetoresistance has twofold symmetry similar to H-c2 and shows a maximum when the magnetic field is nearly parallel to the maximum Fermi wave vector. This tendency is consistent with the Fermi surface anisotropy. At 3.5 K, we found clear fourfold symmetry of H-c2 despite the fact that the normal-state magnetoresistance shows twofold symmetry arising from the Fermi surface anisotropy. The origin of the fourfold symmetry of H-c2 is discussed in terms of the superconducting gap structure in beta-(BDA-TTP)(2)SbF6.