Experimental and theoretical investigation of colossal permittivity in Nb-doped Ti O 2 at cryogenic temperatures

We propose a strategy to enhance dielectric permittivity by tuning local electronic structures, exemplified by Nb-doped rutile-type TiO₂. Substituting only 0.5% of Ti⁴⁺ with Nb⁵⁺ increases the relative permittivity by {~}1200---fivefold higher than that of pristine TiO₂---while maintaining low dielectric loss (tan {δ} {~} 0.02) at cryogenic temperatures. First-principles calculations reveal that excess electrons introduced by Nb substitution localize between Nb and adjacent Ti ions, forming molecular polarons with intrinsic electric dipole moments. These polarons undergo quantum tunneling--assisted flip-flop hopping between their first-nearest-neighbor Ti sites, allowing field-induced dipole reorientation even at 4.2 K. This mechanism explains the persistence of high permittivity at cryogenic temperatures, where classical hopping and Maxwell--Wagner--Sillars effects become negligible, and provides a microscopic explanation for the saturation observed at higher Nb concentrations because of Nb--Nb dimer formation and polaron trapping. Our findings establish a design framework for quantum-enhanced dielectrics based on local correlations and tunneling dynamics in simple oxides.