Orbital Resonance Mode in Superconducting Iron Pnictides

TL;DR

This paper predicts a sharp orbital resonance mode in superconducting iron pnictides related to ferro orbital fluctuations, which can be observed in Raman spectra and depends on pairing symmetry and fluctuation strengths.

Contribution

It introduces the concept of an orbital resonance mode in iron-based superconductors and links it to pairing symmetry and orbital fluctuations, providing a new perspective on their superconducting state.

Findings

Resonance mode appears below the particle-hole continuum in superconducting state.

The mode is analogous to magnetic resonance in unconventional superconductors.

Application to Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ explains observed Raman peak.

Abstract

We show that the fluctuations associated with ferro orbital order in the $d_{xz}$ and $d_{yz}$ orbitals can develop a sharp resonance mode in the superconducting state with a nodeless gap on the Fermi surface. This orbital resonance mode appears below the particle-hole continuum and is analogous to the magnetic resonance mode found in various unconventional superconductors. If the pairing symmetry is $s_{\pm}$, a dynamical coupling between the orbital ordering and the d-wave subdominant pairing channels is present by symmetry. Therefore the nature of the resonance mode depends on the relative strengths of the fluctuations in these two channels, which could vary significantly for different families of the iron based superconductors. The application of our theory to a recent observation of a new $\delta$-function-like peak in the B$_{1g}$ Raman spectrum of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ is discussed.