Robust superconducting state in the low-quasiparticle-density organic metals β′-(BEDT-TTF)4[(H3O)M(C2O4)3]•Y:: Superconductivity due to proximity to a charge-ordered state -: art. no. 014543

We report magnetotransport measurements on the quasi-two-dimensional charge-transfer salts beta(')-(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 B-c2 parallel to approximate to 33 T, comparable to the highest critical field of any BEDT-TTF superconductor, and critical temperature T-c approximate to 7 K is observed when M=Ga and Y=C6H5NO2. The presence of magnetic M ions reduces the in-plane upper critical field to approximate 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 less than or similar to 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 beta(')-(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 kappa-(BEDT-TTF)(2)X salts.