High Upper Critical Field Anisotropy in an Organic Superconductor with a Wide Anion Layer

The salt β”-(BEDT-TTF)₂[(H₂O)(NH₄)₂Cr(C₂O₄)₃]_­•18-crown-6 is a layered, quasi-2D organic superconductor that has an extraordinarily wide anion layer compared to other organic crystals, and was predicted to be a possible FFLO state candidate. We will present rf-penetration depth measurements using a tunnel diode oscillator (TDO) taken up to B=28T (DC) and T<100mk at the NHMFL in Tallahassee, FL. When the applied external magnetic field is oriented perpendicular to the conduction planes of the crystal, single frequency Shubnikov-de Haas (SdH) oscillations are observed above 6T. When oriented parallel to the layers, despite prior predictions, no superconductivity is observed above the calculated Clogston-Chandrasekhar paramagnetic limit, H_p, and no FFLO state is observed. We further support this with calculations of the mean free path & superconducting coherence length, among other Fermi surface parameters such as the Dingle temperature, showing the crystal is in the dirty limit. In addition, we will show evidence of bulk-anistropic to Josephson coupled (3D to 2D) dimensional crossover just below T_c. One remarkable takeaway from our data is that the anisotropy ratio of the upper critical fields, H_c2, in the parallel & perpendicular directions is incredibly high, due to a surprisingly small perpendicular H­_c2=0.2T. The mechanism causing this abnormally low upper critical field is not yet understood, and future work is needed to understand the impacts of the novel geometry on this material.