Electric and magnetic properties and phase diagram of a series of organic superconductors λ−BETS2GaXzY4−z [BETS=bis(ethylenedithiotetraselenafulvalene; X,Y=F, Cl, Br; 0<z<2)]

Abstract

The physical properties of a series of organic superconductors with fourfold quasistacking structures, $\lambda -{\mathrm{BETS}}_2\mathrm{Ga}{X}_z{Y}_{4-z}$ [BETS=bis(ethylenedithiotetraselenafulvalene; $X,Y=\mathrm{F},$ Cl, Br; $0<z<\mathrm{}2\left)\right]$ are changed continuously by tuning the value of $z.\mathrm{}$ The compound $\lambda -{\mathrm{BETS}}_2{\mathrm{GaBr}}_z{\mathrm{Cl}}_{4-z},$ has a superconducting ground state for $0<z<\mathrm{}{z}_c$ (≈0.8) and a semiconducting state for $z_c<z<\mathrm{}\mathrm{2.0.}\mathrm{}$ The maximum $T_c$ is about 10 K, showing the existence of a 10-K class of non-ET organic superconductors [ET=bis(ethylenedithio)tetrathiafulvalene]. The temperature dependence of the resistivity closely resembles that for κ-type ET superconductors but the susceptibility behavior is different. The susceptibility of $\lambda -{\mathrm{BETS}}_2{\mathrm{GaBr}}_{1.5}{\mathrm{Cl}}_{2.5}$ is isotropic and decreases fairly sharply below 30 K. No indication of a spin-flop transition was observed at 10 K, suggesting that the insulating phase that neighbors the superconducting phase is not antiferromagnetic at least down to 10 K.