Sign-changing s-wave symmetry in iron-based superconductors: Manifestations and extensions

Abstract

I perform theoretical studies of the family of iron-based superconductors, which are a group of materials that can achieve a relatively high critical temperature Tc. In most of these multi-band compounds the superconducting gap parameter has s-wave symmetry along the Fermi surfaces, but the sign of the gap can change between Fermi surfaces yielding the so-called s+- symmetry. In this dissertation I focus on the experimental consequences of this gap structure and later on two of its possible extensions. In the first part, I review how the resonance in inelastic neutron scattering can be explained as a pole in the spin susceptibility in an s+- superconductor, computed using the random phase approximation. Then I extend the analysis to include the effect of pairing fluctuations and show that except in special cases these fluctuations merely shift the frequency of the resonance by a few percentage points. I also consider Raman spectroscopy experiments that measured the susceptibility in the B1g symmetry channel and found a strong temperature dependence in the static part and a resonance below Tc in the dynamic part. I show how both of these can be explained through the coupling of fermions to spin fluctuations via the Aslamazov-Larkin process. In the second part, I study the gap structure when superconductivity develops from a preexisting antiferromagnetic state. I show that magnetism induces an additional spin-triplet pairing component in addition to the standard singlet pairing. This additional pairing state can coexist with the standard one and leads to superconductivity that breaks time-reversal symmetry. I also consider the case of materials whose gap structure has accidental nodes on the electron pockets. I analyze how two competing types of hybridization effects between the electron pockets shift the nodes in different directions and the consequences for the gap structure.