Clues to potential dipolar-Kondo and RKKY interactions in a polar metal

The coexistence of electric dipoles and itinerant electrons in a solid was postulated decades ago, before being experimentally established in several ‘polar metals’ during the last decade. Here, we report a concentration-driven polar-to-nonpolar phase transition in electron-doped BaTiO 3 . Comparing our case with other polar metals, we find a particular threshold concentration ( n * ) linked to the dipole density ( n d ). The universal ratio n_d/n^* ≈ 8.0(6) suggests a common mechanism across different polar systems, possibly explained by a dipolar Ruderman-Kittel-Kasuya-Yosida theory. Moreover, in BaTiO 3 , we observe enhanced thermopower and upturn on resistivity at low temperatures near n * , resembling the Kondo effect. We argue that local electric dipoles act as two-level-systems, whose fluctuations couple with surrounding electron clouds, giving rise to a potential dipolar-counterpart of the Kondo effect. Our findings unveil a mostly uncharted territory for exploring emerging physics associated with electron-dipole correlations, encouraging further theoretical work on dipolar-RKKY and Kondo interactions.