CROSSOVER FROM VORTEX STATES TO THE FULDE-FERRELL-LARKIN-OVCHINNIKOV STATE IN 2-DIMENSIONAL S-WAVE AND D-WAVE SUPERCONDUCTORS

The effects of a tilted magnetic field on two-dimensional type-II supe rconductors of s- and d-wave pairing are studied. This includes a stud y of the crossover from vortex states to the Fulde-Ferrell-Larkin-Ovch innikov (FFLO) state when the direction of the magnetic field is chang ed from perpendicular to the conducting plane to a parallel orientatio n. The FFLO state is obtained in the limit of a parallel magnetic fiel d, and vortex phases with a large quantum number n > 0 (a higher Landa u level) appear for tilted magnetic fields. At fixed tilt angle, the c ritical field exhibits a characteristic gtemperature dependence due to the change of the discrete quantum number. The relevance of the resul ts for the observation and the identification of the FFLO state in the organic and high-T-c superconductors is discussed.