Shadow gap in the over-doped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ compound

Abstract

The electron band around $M$ point in (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ compound -- completely lifted above the Fermi level for $x > 0.7$ and hence has no Fermi Surface (FS) -- can still form an isotropic s-wave gap ($\Delta_e$) and it is the main pairing resource generating an s-wave gap ($\Delta_h$) with an opposite sign on the hole pocket around $\Gamma$ point. The electron band developing the SC order parameter $\Delta_e$ but having no FS displays a {\it shadow gap} feature which will be easily detected by various experimental probes such as angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscope (STM). Finally, the formation of the nodal gap $\Delta_{nodal}$ with $A_{1g}$ symmetry on the other hole pocket with a larger FS is stabilized due to the balance of the interband pairing interactions from the main hole band gap $\Delta_h=+\Delta$ and the hidden electron band gap $\Delta_e = -\Delta$.