Evidence of a Bulk Interlayer Excitonic Insulator Phase in the Oxypnictide CeMnAsO1-xFx

Excitonic insulators are a remarkable class of insulators that can exhibit a condensate of electron-hole pairs below a critical temperature T _C and are predicted to give rise to exotic quantum phenomena. Thus far, the excitonic insulator phase has been predominantly explored in chalcogenide-based systems. However, the nature of the excitonic order parameter governing the EI transitions in these candidate systems is still controversial as there are few candidate materials that exhibit an EI phase without an accompanying symmetry-breaking lattice distortion. The discovery of new chemical systems that can host the EI phase is hence very important. Here we report evidence of a tuneable, bulk correlated interlayer excitonic insulator in the oxypnictide CeMnAsO _{1 −  x} F _x ( x ≥ 0.035) with a maximum T _C of 104 K. Crucially, no symmetry-breaking lattice distortion or charge density wave is observed across the transition which will enable further study of the EI phase diagram. The proposed excitonic insulator (EI) phase emerges when the electronic band gap is tuned below a critical threshold through chemical doping and is marked by a significant upturn in the resistivity. First-principles calculations reveal the formation of bound excitons between spatially separated electrons and holes in distinct layers within the crystal structure with a binding energy, E _b = 50 meV. A reversal in the Hall and Seebeck coefficients is observed below T _C as holes from the CeO/F layer bind with electrons in the Mott insulating As-Mn-As block. Neutron diffraction shows that T _C can be further controlled by reducing the interlayer distance and enhancing the electronic coupling between layers. This work identifies CeMnAsO _{1 −  x} F _x as a promising chemical platform for exploring novel quantum phases arising from excitons and expands the range of materials considered to host the EI phase.