Ground-state instability of the Mott insulator Ca2RuO4: Impact of slight La doping on the metal-insulator transition and magnetic ordering

Ca2RuO4 is a Mott system with a structurally driven metal-insulator transit ion at T-MI = 357 K and Neel temperature at T-N = 110 K. Slight substitutio n of trivalent La for bivalent Ca in Ca2RuO4 drastically reduces T-MI and t he electrical resistivity, simultaneously precipitating robust ferromagneti sm in the antiferromagnetic host. T-C is conspicuously and consistently low er than T-MI, suggesting that the metal-insulator transition is not driven by the magnetic instability. The La substitution:also results in a rapid in crease in both the Pauli susceptibility and electronic specific heat coeffi cient, and:a crossover from hopping conductivity to disordered metallic beh avior at low temperatures. These dramatic changes;underline the subtlety of the competition between the antiferromagnetic and ferromagnetic coupling:a nd a near degeneracy of the ground state in Ca2RuO4.