  We suggest theoretical explanation of the high upper critical magnetic field,
perpendicular to conducting chains, Hc2, experimentally observed in the
superconductor (TMTSF)2ClO4, in terms of singlet superconducting pairing. In
particular, we compare the results of d-wave-like nodal, d-wave-like node-less,
and s-wave scenarios of superconductivity. We show that, in d-wave-like nodal
scenario, superconductivity can naturally exceed both the orbital upper
critical magnetic field and Clogston-Shandrasekhar paramagnetic limit as well
as reach experimental value, Hc2 = 6T, in contrast to d-wave-like node-less and
s-wave scenarios. In our opinion, the obtained results are strongly in favor of
d-wave-like nodal superconductivity in (TMTSF)2ClO4, whereas, in a sister
compound, (TMTSF)2PF6, we expect either the existence of triplet order
parameter or the coexistence of triplet and singlet order parameters.

Superconductivity