Hydrogen-Ti<sup>3+</sup> Complex as a Possible Origin of Localized Electron Behavior in Hydrogen-Irradiated SrTiO<sub>3</sub>

A recent muon spin rotation ($\mu^+$SR) study on a paramagnetic defect complex formed upon implantation of $\mu^+$ pseudo-proton into SrTiO$_3$ is reviewed with specific focus the relation experimental signatures coexisting delocalized and localized electrons in hydrogen-irradiated metallic films. The complex, composed interstitial Ti$^{3+}$ small polaron, characterized by dissociation energy about 30 meV. Density functional theory (DFT) calculations generalized gradient approximation (GGA) +$U$ scheme for corresponding hydrogen reveal that thermodynamic donor level associated electron transfer from an H$^+$-Ti$^{3+}$ to conduction band can form just below minimum realistic $U$ values. These findings suggest coexistence be realized electron-rich conditions.