Ground-state instability of the Mott insulatorCa2RuO4:Impact of slight La doping on the metal-insulator transition and magnetic ordering

Abstract

${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$ is a Mott system with a structurally driven metal-insulator transition at ${T}_{\mathrm{MI}}=357$ K and N\'eel temperature at ${T}_{N}=110$ K. Slight substitution of trivalent La for divalent Ca in ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$ drastically reduces ${T}_{\mathrm{MI}}$ and the electrical resistivity, simultaneously precipitating robust ferromagnetism in the antiferromagnetic host. ${T}_{C}$ is conspicuously and consistently lower than ${T}_{\mathrm{MI}},$ suggesting that the metal-insulator transition is not driven by the magnetic instability. The La substitution also results in a rapid increase in both the Pauli susceptibility and electronic specific heat coefficient, and a crossover from hopping conductivity to disordered metallic behavior at low temperatures. These dramatic changes underline the subtlety of the competition between the antiferromagnetic and ferromagnetic coupling and a near degeneracy of the ground state in ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}.$