Structure Property Relationships of the Exotic Insulator-Insulator Transition in CeMnAsO_1-x F _x : A Potential...

A novel quantum insulator-insulator transition was recently reported upon F^- doping the Mott insulator CeMnAsO_1-x F _x . Below the transition, the resistivity increases by more than 2 orders of magnitude over a narrow temperature range, and a colossal Seebeck effect is observed alongside glassy dynamics. A combination of neutron diffraction, heat capacity, and transport measurements has enabled structure-property relationships of the exotic transition to be elucidated. Variable temperature high resolution neutron diffraction shows that there is no change in symmetry at the transition and that the transition can be controlled by increasing the electronic and magnetic coupling between the CeO/F and As-Mn-As blocks. Physical property measurements, combined with first-principles calculations, suggest that the transition is due to the formation of an unusual interlayer excitonic insulator state below the insulator-insulator transition, T _II.