Heavy-fermion formation at the metal-to-insulator transition in Gd 1 − x Sr x TiO 3

The perovskitelike transition-metal oxide ${\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{TiO}}_{3}$ is investigated by measurements of resistivity, specific-heat, and electron paramagnetic resonance (EPR). Approaching the metal-to-insulator transition from the metallic regime $(xg~0.2),$ the Sommerfeld coefficient $\ensuremath{\gamma}$ of the specific heat becomes strongly enhanced and the resistivity increases quadratically at low temperatures, which both are fingerprints of strong electronic correlations. The temperature dependence of the dynamic susceptibility, as determined from the ${\mathrm{Gd}}^{3+}\ensuremath{-}\mathrm{EPR}$ linewidth, signals the importance of strong spin fluctuations, as observed in heavy-fermion compounds.