Resistance measurements have been performed in two- dimensional organic alloys $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Cl}}_{4-x}{\mathrm{Br}}_x$ ($x=0.4$, 0.5, and 0.7), where BETS is bis(ethylenedithio)tetraselenafulvalene, to investigate the effect of the strong correlation between the $\mathrm{Fe}3d$ and BETS $\pi$ electrons on the electronic states. The ground state at low fields is an insulating phase, which becomes more stable as $x$ increases. In high magnetic fields along the $c$ axis, superconductivity is induced for all the samples. The critical temperature $T_c$ has a maximum at $31–33\mathrm{T}$, showing that the internal field created by the $\mathrm{Fe}3d$ moments is almost independent of $x$. The analysis of the phase diagrams based on Fischer theory shows that both the orbital critical field and the spin-orbit scattering rate are enhanced by the Br substitution. All of these features are consistently understood in terms of the anisotropic negative chemical pressure and disorder induced by the Br substitution.

• Received 16 June 2005

DOI:https://doi.org/10.1103/PhysRevB.72.184505