Superconductivity in an Organic Conductor Stabilized by a High Magnetic Field

The application of a sufficiently strong magnetic field to a superconductor will in general destroy the superconducting state. However, for the quasi-two-dimensional organic conductor λ-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene, we find that superconductivity is induced under magnetic fields in the range between 18 T and 42 T when the magnetic fields are applied exactly parallel to the conducting layers of the crystals, although it has an insulating ground state below 10 T. This is the first material whose superconducting phase is stabilized only under a magnetic field. The resulting phase diagram indicates that the internal magnetic field due to the negative exchange interaction between the localized Fe moments and the conduction electron spins is crucial.