Unconventional sign-reversing superconductivity in LaFeAsO_(1-x)F_x

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Abstract

We argue that the newly discovered superconductivity in a nearly magnetic, Fe-based layered compound is unconventional and mediated by antiferromagnetic spin fluctuations, though different from the usual superexchange and specific to this compound. This resulting state is an example of extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets. The main role of doping in this scenario is to lower the density of states and suppress the pair-breaking ferromagnetic fluctuations.

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Neutron scattering and superconducting order parameter in YBa2Cu3O7

I. Mazin, Victor Yakovenko (1995)

We discuss the origin of the neutron scattering peak at 41 meV observed in YBa\(_2\)Cu\(_3\)O\(_7\) below \(T_c\). The peak may occur due to spin-flip electron excitations across the superconducting gap which are enhanced by the antiferromagnetic interaction between Cu spins. In this picture, the experiment is most naturally explained if the superconducting order parameter has \(s\)-wave symmetry and opposite signs in the bonding and antibonding electron bands formed within a Cu\(_2\)O\(_4\) bilayer.

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s-wave superconductivity from antiferromagnetic spin-fluctuation model for bilayer materials

A. Liechtenstein, O. K. Andersen, I. Mazin (1995)

It is usually believed that the spin-fluctuation mechanism for high-temperature superconductivity results in d-wave pairing, and that it is destructive for the conventional phonon-mediated pairing. We show that in bilayer materials, due to nearly perfect antiferromagnetic spin correlations between the planes, the stronger instability is with respect to a superconducting state whose order parameters in the even and odd plane-bands have opposite signs, while having both two-dimensional \(s\)-symmetry. The interaction of electrons with Raman- (infrared-) active phonons enhances (suppresses) the instability.

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"Chain scenario" for Josephson tunneling with pi-shift in YBa2Cu3O7

A. D. Zaikin, I. Mazin, A Golubov (1995)

We point out that all current Josephson-junction experiments probing directly the symmetry of the superconducting state in YBa2Cu3O7, can be interpreted in terms of the bilayer antiferromagnetic spin fluctuation model, which renders the superconducting state with the order parameters of extended \(s\) symmetry, but with the opposite signs in the bonding and antibonding Cu-O plane bands. The essential part of our interpretation includes the Cu-O chain band which would have the order parameter of the same sign as antibonding plane band. We show that in this case net Josephson currents along and perpendicular to the chains have the phase shift equal to pi.