NMR study of metallic strontium titanate

Electron-doped strontium titanate (SrTiO₃) is a peculiar metal and superconductor due to extremely low densities of high-mobility carriers and the proximity to a ferroelectric quantum-critical point. In particular, the normal-state transport properties are not well understood. Transport measurements find a robust T²-dependent resistivity, as might be expected for a Fermi liquid. However, the Fermi energy is so low that Fermi-liquid theory cannot explain the observed behavior. Nuclear magnetic resonance (NMR) is a powerful local probe that has not yet been used to investigate this issue. Here we present a ^47,49Ti NMR study across the temperature-doping phase diagram of Nb- and oxygen-vacancy-doped SrTiO₃. From Knight shift measurements, we gain insight into the local electronic spin susceptibility. We discuss the results within Fermi-liquid and polaronic models of metallic SrTiO₃.