4f - 4f transitions and interactions in Er-doped CeO2: an ab initio study

Strong spin-photon interfaces with excited-state to ground-state transitions in the telecommunication band are essential for quantum information processing. The near zero nuclear spin environment 4f - 4f transition in Er-doped CeO2 is ideal. Here, we report an ab initio understanding of the electronic quantum states, interactions, and excitations in Er:CeO2 that forms a foundation for its quantum advancement. The stability of the dopant host configuration, and valence state and partially quenched orbital 4f magnetic moment of Er are confirmed by defect formation and electronic structure calculations. The defect formation energy reveals that the Er-doped system favors the incorporation of two Er dopants accompanied by the creation of an oxygen vacancy near Er, stabilizing the system in Er3+ and Ce4+ oxidation states. These two Er dopants prefer a ferromagnetic configuration, as confirmed by the lower total energy compared to the antiferromagnetic state. The hybrid functional theory corrected band gap of Er:CeO2, confirms the insulating characteristic. The strong crystal field environment forces the Er atom to have a partially quenched 4f orbital magnetic moment due to the splitting of energy levels and the shielding effect of the outer electrons. The crystal field parameters (CFPs) responsible for the 4f splitting are extracted from the crystal field potential calculated using density functional theory (DFT). For the Oh symmetry in Er-doped CeO2, four non-zero CFPs are identified, giving rise to five multiplet levels in both the ground and first excited states. The presence of oxygen vacancies lowers the local symmetry, increasing the number of non-zero CFPs and resulting in eight and seven energy levels in the ground and first excited states, respectively.