University of Minnesota
School of Physics & Astronomy

Condensed Matter Seminar

Thursday, October 15th 2009
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Prof. Jörg Schmalian, Iowa State University and Ames Laboratory
Subject: Inferring the Cooper pair wave function of the iron arsenides superconductors from their phase diagram.

We use simple coexistence rules for the phase diagram of the iron arsenides to deduce the pairing state in these new superconductors. Recent experimental evidence in the iron arsenite superconductor Ba(FeCo)2As2 demonstrates that antiferromagnetic long range order coexists with bulk superconductivity. We show that static antiferromagnetism can be used to probe the Cooper pair wave function in this coexistence region. Magnetism leads to a Josephson like coupling between Cooper pairs in different Fermi surface sheets. The relative phase of the gap function then determines whether superconductivity and antiferromagnetism are merely competing or are mutually exclusive. While conventional superconductivity yields a phase diagram with a first order transition, terminating at a bicritical poing, a tetracritical point is only possible for unconventional pairing states, where the order parameter has different sign on different pieces of the Fermi surface. We demonstrate that the observed phase coexistence is not compatible with conventionalsuperconductivity and find strong evidence for superconductivity where the order parameter on different Fermi surface sheets is out of phase. The robustness of our conclusion is caused by the fact that the system is in the vicinity of an SO(6)-symetric fixed poing.

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