Gap Symmetry and a Revival of Superconductivity in High Parallel Magnetic Fields in Q2D and Q1D Organic, High-Tc, and Sr2RuO4 Compounds

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From the BCS theory, we derive an equation determining the upper critical field, Hc2(T), parallel to conducting planes of both layered Q2D and Q1D superconductors. It extends the quasiclassical Ginzburg–Landau–Abrikosov–Gor'kov (GLAG) and Lawrence–Doniach descriptions of Hc2(T) to the case of high magnetic fields where the quantum nature of an electron motion along open Fermi surfaces is important. This equation demonstrates two new phenomena: (1) a complete restoration of superconductivity at H ≃ Hc5 > Hc2(0) in the case of p-wave pairing of electrons; (2) a surviving of superconductivity at Hc2(0) < H < H*p in the case of s(d)-wave pairing, where H*p ≠ Hp [Here, Hc2(0) and Hp are the quasiclassical GLAG upper critical field and the paramagnetic Clogston–Chandrasekhar limiting field, correspondingly; Hc5 and H*p are defined in the text]. Our analysis of recent experimental data on (TMTSF)2PF6 by I. J. Lee et al. and M. J. Naughton et al. shows that superconductivity significantly exceeds H*p. This demonstrates a strong suppression of the Pauli pair-breaking effects and may reflect a p-wave pairing of electrons. We propose to measure Hc2(T) in Sr2RuO4 (which is believed to be a p-wave superconductor) to prove a symmetry of a superconducting order parameter.