Pseudogap formation above the superconducting dome in iron pnictides

The nature of the pseudogap (PG) in high transition temperature (high Tc) superconducting cuprates has been a major issue in condensed matter physics. It is still unclear whether the high Tc superconductivity can be universally associated with the PG formation. Here we provide direct evidence of the existence of the PG phase via angle-resolved photoemission spectroscopy in another family of high Tc superconductor, iron pnictides. Our results reveal a composition-dependent PG formation in the multiband electronic structure of BaFe2(As1-xPx)2. The PG develops well above the magnetostructural transition for low x, persists above the nonmagnetic superconducting dome for optimal x, and is destroyed for x ∼ 0.6, thus showing a notable similarity with cuprates. In addition, the PG formation is accompanied by inequivalent energy shifts in the zx/yz orbitals of iron atoms, indicative of a peculiar iron orbital ordering which breaks the fourfold rotational symmetry.