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, 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. For the case of layered, two-dimensional superconductors, such as the high-Tc copper oxides, the orbital effect is reduced when the applied magnetic field is parallel to the conducting layers. Here we report resistance and magnetic-torque experiments on single crystals of the quasi-two-dimensional organic conductor lambda-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. 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.

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Measurements of the Shubnikov-de Haas ͑SdH͒ and angular-dependent magnetoresistance oscillations were performed to study the Fermi surface for a magnetic-field-induced superconductor -(BETS) 2 FeCl 4 . We found two SdH oscillations ascribed to the minimum and maximum cross-sectional areas of corrugated cylindrical Fermi surface, which are estimated to be 14% and 17% of the first Brillouin zone, respectively. The mass of the conduction electrons is 4.1 m 0 , suggesting the enhancement due to many-body effects. Assuming a parabolic energy band, we obtain a Fermi energy of ϳ20 meV and an interlayer transfer integral of ϳ1 meV.

Crystal structure of 2-Carboxy-1,3-dibenzo[a,c]tropolone (4) is characterized by intramolecular hydrogen bonds with d(O···O) of 2.400(5) Å, which is one of the shortest distances reported for 3-hydroxyenones. An averaged structure of 4 suggested by solid state 13C (CP/MAS) NMR may be attributed to a genuine symmetry rather than a dynamically averaged one. Namely, the tautomer may have a symmetric structure resulting from the extremely short O···O distance.