Large polaron evolution with temperature and carrier density in anatase TiO₂

Electronic and magneto transport properties of reduced anatase TiO₂ epitaxial thin films are analyzed as a function of temperature and carrier concentration. Semiconductor and metallic behaviors are observed at low and high temperature respectively, in agreement with previous studies. Unexpectedly, the mobility was found to increase with increasing carrier concentration, which rarely happens in a metallic system. Through our analysis, the increased mobility was attributed to a screening effect on the e-ph coupling. We also found that the magnetoresistance (MR) could be decomposed in to a linear and a quadratic component, separately characterizing the transport and trap behavior of carriers. Since the different polaron states have varying dependence on the temperature and carrier concentration, a phase diagram could be depicted by the analysis of the electronic and magneto transport data.