Negatively Charged Muonium and Related Centers in Solids

Muonium (Mu) centers formed upon implantation of ${\mu }^{+}$ in a solid have long been investigated as an experimentally accessible model of isolated hydrogen impurities. Recent discoveries of hydridic centers formed at oxygen vacancies have stimulated a renewed interest in H${}^{-}$ and corresponding Mu${}^{-}$ centers in oxides. However, the two diamagnetic centers, Mu${}^{+}$ and Mu${}^{-}$, are difficult to separate spectroscopically. In this review article, we summarize established and developing methodologies for identifying Mu${}^{-}$ centers in solids, and review recent Mu studies on said centers supposedly formed in mayenite, oxygen-deficient SrTiO${}_{3-x}$, and BaTiO${}_{3-x}$H${}_x$ oxyhydride.