Non-Fermi-liquid scattering rates and anomalous band dispersion in ferropnictides

Angle-resolved photoemission spectroscopy is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictide systems. We find the scattering rate for any given band to depend linearly on energy but to be independent of the control parameter. We demonstrate that the linear energy dependence gives rise to a weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. The resulting small effective Fermi energy favors a BCS [J. Bardeen et al., Phys. Rev. 108, 1175 (1957)] -Bose-Einstein [S. N. Bose, Z. Phys. 26, 178 (1924)] crossover state in the superconducting phase.