"Universal" $\Delta_{max}/T_c$ in Fe Based Superconductors

Abstract

Iron-based superconductors display a large degree of variability in electronic structure at the Fermi surface, resulting in superconducting gap structures,$T_c$'s, and other properties that vary considerably from family to family. Recently it was noted that across many different families of Fe-based systems, the ratio $\Delta_{max}/T_c$ is found to be quasi-universal with a large value of ~3.5 compared to the the one-band BCS weak-coupling result of 1.76. Here $\Delta_{max}$ is the measured maximum gap across the Fermi surface. This remarkable fact was attributed to strong-coupling effects arising from Hund's metal physics. Here we perform a "high-throughput" scan across band masses, Fermi energies, and interaction parameters in a weak-coupling Suhl-Matthias-Walker model. We find that unexpectedly large values of $\Delta_{max}/T_c$ can be achieved within weak coupling, and that quasiuniversal behavior similar to experiment emerges for those systems where interband interactions dominate intraband ones. However, within the current framework, a large mass contrast between bands is required.