Bulk Kosterlitz-Thouless Type Molecular Superconductor β-(BEDT-TTF)2[(H2O)(NH4)2Cr(C2O4)3]•18-crown-6

A new molecular superconductor, beta ''-(BEDT-TTF)(2)[(H2O)(NH4)(2)Cr(C2O4)(3)]center dot 18-crown-6, has been synthesized from the organic donor molecule BEDT-TTF with the anion Cr(C2O4)(3)(3-). The crystal structure consists of conducting organic layers of BEDT-TTF molecules which adopt the beta '' packing motif (layer A), layers of NH4+ and A-Cr(C2O4)(3)(3-) (layer B), layers of (H2O)(NH4)18-crown-6 (layer C), and layers of NH4+ and Delta-Cr(C2O4)(3)(3-) (layer D) which produce a superstructure with a repeating pattern of ABCDABCDA. As a result of this packing arrangement, this is the 2D superconductor with the widest gap, between conducting layers where only a single donor packing motif is present (beta ''). Superconducting critical temperatures at ambient pressure observed by electrical transport and magnetic measurements are 4.0-4.9 and 2.5 K, respectively. The strong 2D nature of this system, the broad transition to T-zero at 1.8K, and the transition of alpha of V proportional to i(alpha) from 1 to 3 on I-V curves strongly suggest that the superconducting transition is very close to a Kosterlitz-Thouless transition. The magnetic field dependence of the superconducting critical temperature parallel to the conducting plane gives an upper critical field mu H-0(c2 parallel to) > 8 T, which is over the calculated Pauli-Clogston limit for this material.