Anomalous Zeeman response of the coexisting superconducting and spin-density-wave phases as a probe of extended s-wave pairing in ferropnictide superconductors

In several members of the ferropnictides, spin-density-wave (SDW) order coexists with superconductivity over a range of dopings. In this paper, we study the anomalous magnetic Zeeman response of this coexistence state and show that it can be used to confirm the extended s-wave gap structure as well as the structure of the superconducting (SC) gap in the coexisting phase. On increasing the field, a strongly anisotropic reduction of the SC gap is found. The anisotropy is directly connected to the gap structure of the superconducting phase. The signature of this effect in quasiparticle interference measured by scanning tunneling microscopy, as well as heat transport in magnetic field, is discussed. For the compounds with the nodal SC gap, we show that the nodes are removed upon formation of the SDW. Interestingly, the size of the generated gap in the originally nodal areas is anisotropic in the position of the nodes over the Fermi surface, in direct connection with the form of SC pairing.