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.
University of Minnesota Ph.D. dissertation.December 2016. Major: Physics. Advisor: Andrey Chubukov. 1 computer file (PDF); iv, 157 pages.
Hinojosa Alvarado, Alberto.
Sign-changing s-wave symmetry in iron-based superconductors: Manifestations and extensions.
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