Ambipolarity of hydrogen in matter revealed by muons

Despite being the simplest element, hydrogen (H) exhibits complex behavior in materials due to its unique ambipolar character. In particular, it is recognized as one of the most important impurities in semiconductor physics, because H is often unintentionally incorporated into materials and significantly influences the electrical properties of the host material. One of the few means that have been applied to obtain experimental information about the local electronic state of diluted H is the use of muon (Mu) as pseudo-H. Here, we present an overview on the “ambipolarity model” that provides a new paradigm for the microscopic understanding of Mu-related defects. Its essence lies in the fact that the information Mu yields is not about the equilibrium double-charge transition level ($E^{+/-}$) but about the donor/acceptor levels ($E^{0/-}$ and $E^{+/-0}$) associated with the relaxed-excited states of Mu. Most notably, the model resolves serious discrepancies between the implications from implanted-Mu studies and theoretical predictions on the electronic state of H from ab initio density functional theory calculations in oxide semiconductors that have hindered the coherent integration of both Mu and H knowledge. The model also suggests that hydride state (H${}^{-}$) plays important roles in oxide materials, as found in a variety of recent examples. Based on these successes, the model is currently serving as a reliable guide for the interpretation of various Mu states observed in other insulating materials, for which several recent examples are presented.