Doping Dependence of the Effective Mass and Kadowaki-Woods Scaling Violation*

We discuss wide-doping-range (8 x 10¹⁷ to 4 x 10²⁰ cm^-3 Hall electron density) low temperature specific heat measurements on single crystal SrTiO₃:Nb, correlated with electronic transport data and tight-binding modeling [1]. Lattice dynamic contributions to specific heat are shown to be well understood, albeit with unusual sensitivity to doping, likely related to soft modes. Electronic contributions to specific heat provide effective masses that increase substantially, from 1.8 to 4.8m_e, across the two SrTiO₃ Lifshitz transitions. It is shown that this behavior can be quantitatively reconciled with quantum oscillation data and calculated band structure, establishing a doping-independent mass enhancement factor of 2.0. With the doping-dependent T² resistivity prefactor and Sommerfeld coefficient known, Kadowaki-Woods scaling has been tested over the entire doping range. Despite Fermi liquid behavior in electronic specific heat, standard Kadowaki-Woods scaling is dramatically violated, highlighting the need for new theoretical descriptions of T² resistivity in SrTiO₃.
[1] McCalla et al., Phys. Rev. Materials 3, 022001(R) (2019)