Magnetic-field-induced superconductivity in a two-dimensional organic conductor

The application of a sufficiently strong magnetic field to a superconductor will, in general, destroy the superconducting state. Two mechanisms are responsible for this. The first is the Zeeman effect(1,2), which breaks apart the paired electrons if they are in a spin-singlet (but not a spin-triplet) state. The second is the so-called 'orbital' effect, whereby the vortices penetrate into the superconductors and the energy gain due to the formation of the paired electrons is lost(3). For the case of layered, two-dimensional superconductors, such as the high-T-c copper oxides, the orbital effect is reduced when the applied magnetic field is parallel to the conducting layers(4). Here we report resistance and magnetic-torque experiments on single crystals of the quasi-two-dimensional organic conductor lambda-(BETS)(2) FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene(5-8). We find that for magnetic fields applied exactly parallel to the conducting layers of the crystals, superconductivity is induced for fields above 17 T at a temperature of 0.1 K. The resulting phase diagram indicates that the transition temperature increases with magnetic field, that is, the superconducting state is further stabilized with magnetic field.