RESTORATION OF SUPERCONDUCTIVITY AT HIGH MAGNETIC-FIELDS IN LAYERED HIGH-T-C AND ORGANIC SUPERCONDUCTORS

An integral equation determining the upper critical field parallel to the conducting plane of a layered superconductor is derived from micro scopic BCS theory. From this equation, it follows that the superconduc tivity-destruction mechanism due to the orbital effect becomes ineffec tive at magnetic fields higher than a critical field H-c4 due to a qua ntum nature of an electron motion in a magnetic field. This leads to t he appearance of reentrant superconducting phase with d(Tc)/dH > 0 in the case of p-pairing and under certain conditions in the case of s(d) -pairing. We also show that the orbital effect cannot destroy supercon ductivity at T = 0 at arbitrary magnetic fields. Therefore qualitative deviations from the description of GLAG (WHH) theory have to appear f or arbitrary symmetry of the order parameter at low enough temperature s. We argue that p(s)-wave superconductor Sr2RuO4 as well as s(d)-wave superconductors TlBa2CaCu2O7, Tl2Ba2Ca3Cu4Ox, beta-(ET)(2)AuI2, and k appa-(ET)(2)I-3 are good candidates for the observation of the above m entioned phenomena.