Non-Fermi liquid due to orbital fluctuations in iron pnictide superconductors

We study the influence of quantum fluctuations on the electron self-energy in the normal state of iron pnictide superconductors using a five-orbital tight-binding model with generalized Hubbard on-site interactions. Within a one-loop treatment, we find that an overdamped collective mode develops at low frequency in channels associated with quasi-one-dimensional d_xz and d_yz bands. When the critical point for the C₄-symmetry- broken phase (structural phase transition) is approached, the overdamped collective modes soften, and acquire increased spectral weight, resulting in non-Fermi-liquid behavior at the Fermi surface characterized by a frequency dependence of the imaginary part of the electron self-energy of the form ωλ, 0<λ<1. We argue that this non-Fermi-liquid behavior is responsible for the recently observed zero-bias enhancement in the tunneling signal in point-contact spectroscopy. A key experimental test of this proposal is the absence of non-Fermi-liquid behavior in the hole-doped materials. Our result suggests that quantum criticality plays an important role in understanding the normal-state properties of iron pnictide superconductors.