Non-degenerate heavy electrons and Planckian limit to scattering in doped strontium titanate

The metallicity of lightly doped SrTiO₃ is strange. The combination of the effective mass extracted from quantum oscillations and the amplitude of room-temperature mobility implies a mean-free-path (mfp) below the Mott Ioffe Regel (MIR) limit and a scattering time shorter than the Planckian time (τ_P=h/2πk_BT). Here, we present a study of electric resistivity and thermoelectric power above room temperature pointing to a possible exit out of this maze. According to our data, resistivity is metallic and does not saturate up to 900 K. In the non-degenerate regime, the Seebeck coefficient is set by the ratio of the de Broglie wavelength, λ_dB, to the interelectron distance, n^-1/3. We demonstrate that the temperature dependence of the shrinking λ_dB (before its saturation to a length twice the lattice parameter) points to a continuous change in the effective mass of charge carriers. Combining the latter with mobility keeps the mfp above the MIR limit and the scattering time longer than τ_P. Our results imply the existence of a hitherto unknown case of non-degenerate metallicity driven by temperature-induced mass amplification.