Ambipolarity of diluted hydrogen in wide-gap oxides revealed by muon study

Muon spin rotation has long been recognized as one of the few methods for experimentally accessing the electronic state of dilute hydrogen (H) in semiconductors and dielectrics, where muon behaves as a pseudo-H (designated by the elemental symbol Mu). Meanwhile, predictions on the electronic state of H in these materials by density functional theory (DFT) do not always agree with the observed states of Mu. Most notably, Mu frequently occurs in wide-gap oxides simultaneously in a neutral ([Formula: see text]) and a diamagnetic state ([Formula: see text] or [Formula: see text]), which DFT calculations do not explain; they predict that H is stable only in a diamagnetic state with the polarity determined by the equilibrium charge-transition level ([Formula: see text]) vs the Fermi level. To address this issue, we developed a semi-quantitative model that allows a systematic understanding of the electronic states reported for Mu in the majority of oxides. Our model assumes that muons interact with self-induced excitons to produce relaxed-excited states corresponding to donor-like ([Formula: see text]) and/or acceptor-like ([Formula: see text]) states and that these states correspond to the non-equilibrium electronic level ([Formula: see text] or [Formula: see text]) predicted by DFT calculations for H. The known experimental results are then explained by the relative position of [Formula: see text] and [Formula: see text] in the host’s energy band structure. In addition, the model sheds new light on the polaron-like nature of the electronic states associated with shallow donor Mu complexes.

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