Identification of large polarons in rutile and anatase polymorphs of titanium dioxide

Titanium dioxide (TiO₂) is a wide-gap semiconductor with numerous applications in photocatalysis, photovoltaics, and neuromorphic computing. The unique functional properties of this material critically depend on its ability to transport charge in the form of polarons, namely phonon-dressed electron wavepackets. It is currently well established that the most important polymorphs of TiO₂, the rutile and anatase phases, harbor small electron polarons and small hole polarons, respectively. However, whether additional polaronic species exist in TiO₂, and under which conditions, remain open questions. Here, we provide definitive answers to these questions by using recently developed ab initio techniques. We identify three novel species, namely a large hole polaron in rutile, a large quasi-two-dimensional electron polaron in anatase, and a large excitonic polaron in anatase. These findings complete the puzzle on the polaorn physics of TiO₂ and pave the way for systematically probing and manipulating polarons in a broad class of complex oxides and quantum materials.