Sign reversal of the order parameter in s-wave superconductors

TL;DR

This paper demonstrates that in multiband s-wave superconductors, the superconducting gap can have different signs on different Fermi surface sheets, challenging the conventional understanding and affecting experimental interpretations.

Contribution

It reveals conditions under which sign reversal of the s-wave order parameter occurs in multiband superconductors within BCS theory, highlighting implications for experimental identification.

Findings

Sign reversal of the superconducting gap can occur in multiband s-wave superconductors.

Conditions involve weaker interband pairing or strong magnetic impurity scattering.

Implications for interpreting experiments on materials like YBa2Cu3O7.

Abstract

We show that in a superconductor where two or more bands cross the Fermi level it is possible, in the framework of the conventional (s-wave) BCS theory, that the sign of the superconducting gap is different on the different sheets of the Fermi surface. At least one of two conditions has to be satisfied: (1) The interband pairing interaction is weaker than the Coulomb pseudopotential, while the intraband one is stronger, or (2) there is strong interband scattering by magnetic impurities. In the case of YBa$_2$% Cu$_3$O$_7$ we shall argue that the first condition is possibly satisfied, and the second one very likely satisfied. In many aspects such a sign-reversal $s-$wave superconductor is similar to a d-wave superconductor, and thus demands revising recent experiments aimed to distinguish between the $s-$ and $d-$wave superconductivity in this compound.