We report magnetotransport measurements on the quasi-two-dimensional charge-transfer salts ${\beta }^{″}\text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left[\left({\mathrm{H}}_3\mathrm{O}\right)M{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]Y$, with $Y={\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NO}}_2$ and ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CN}$ 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 $B_{c2\Vert }\approx 33\mathrm{T}$, comparable to the highest critical field of any BEDT-TTF superconductor, and critical temperature $T_{\mathrm{c}}\approx 7\mathrm{K}$ is observed when $M=\mathrm{Ga}$ and $Y={\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NO}}_2$. The presence of magnetic $M$ ions reduces the in-plane upper critical field to $\approx 18\mathrm{T}$ for $M=\mathrm{Cr}$ and $Y={\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NO}}_2$ and $M=\mathrm{Fe}$ and $Y={\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CN}$. 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 $\lesssim 6\%$ 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 ${\beta }^{″}\text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left[\left({\mathrm{H}}_3\mathrm{O}\right)M{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]Y$ 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 $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}X$ salts.

• Received 7 September 2004

DOI:https://doi.org/10.1103/PhysRevB.72.014543