Robust superconducting state in the low-quasiparticle-density organic metals -„BEDT-TTF...4†„H3O...M„C2O4...3‡ ·Y: Superconductivity due to proximity to a charge-ordered state

We report magnetotransport measurements on the quasi-two-dimensional charge-transfer salts BEDT-TTF 4 H3O M C2O4 3 Y, with Y =C6H5NO2 and C6H5CN using magnetic fields of up to 45 T and temperatures down to 0.5 K. A surprisingly robust superconducting state with an in-plane upper critical field Bc2 33 T, comparable to the highest critical field of any BEDT-TTF superconductor, and critical temperature Tc 7 K is observed when M =Ga and Y =C6H5NO2. The presence of magnetic M ions reduces the in-plane upper critical field to 18 T for M =Cr and Y =C6H5NO2 and M =Fe and Y =C6H5CN. Prominent Shubnikov–de Haas oscillations are observed at low temperatures and high magnetic fields, showing that the superconducting salts possess Fermi surfaces with one or two small quasi-two-dimensional pockets, their total area comprising 6% of the room-temperature Brillouin zone; the quasiparticle effective masses were found to be enhanced when the ion M was magnetic Fe or Cr . The low effective masses and quasiparticle densities, and the systematic variation of the properties of the BEDT-TTF 4 H3O M C2O4 3 Y salts with unit-cell volume points to the possibility of a superconducting groundstate with a charge-fluctuation-mediated superconductivity mechanism such as that proposed by Merino and McKenzie Phys. Rev. Lett. 87, 237002 2001 , rather than the spin-fluctuation mechanism appropriate for the BEDT-TTF 2X salts.