Van An Dinh, Yujiro Hashimoto, Koji Kimura, Taro Kuwano, Dung Ngoc Dinh, Ryoji Asahi, Kouichi Hayashi, Hiroki Taniguchi, Yoshitada Morikawa
We propose a strategy to enhance dielectric permittivity by tuning local electronic structures, exemplified by Nb-doped rutile-type $\mathrm{Ti}{\mathrm{O}}_{2}$. Substituting only 0.5% of ${\mathrm{Ti}}^{4+}$ with $\mathrm{N}{\mathrm{b}}^{5+}$ increases the relative permittivity by \ensuremath{\sim}1200---fivefold higher than that of pristine $\mathrm{Ti}{\mathrm{O}}_{2}$---while maintaining low dielectric loss (tan \ensuremath{\delta} \ensuremath{\sim} 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.