  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.
