Superconductivity in an Organic Insulator at Very High Magnetic Fields

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor $\lambda -(\mathrm{BETS}{)}_2{\mathrm{FeCl}}_4$. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field compensates the exchange field of aligned ${\mathrm{Fe}}^{3+}$ ions. We further argue that the ${\mathrm{Fe}}^{3+}$ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state.