ORBITAL ORDERING AND VIBRONIC STATES IN MANGANESE PEROVSKITES

摘要

The structural, magnetic, and transport properties of LaMnO_3 reveal a progressive change from a static orbital ordering into (001) planes at temperatures T < T~* ≈ 600 K to a 2D orbital disorder in the interval T~* < T < T_(JT) ≈ 740 K and a 3D orbital disorder with partial disproportionation into Mn(Ⅱ) + Mn(Ⅳ) for T > T_(JT). The thermal conductivity, measured below 300 K, shows a strong phonon contribution contrary to previous reports made on oxidized samples. Ferromagnetic order in the insulator LaMn_(0.5)Ga_(0.5)O_3 confirms the existence of 3D ferromagnetic Mn(Ⅲ)-O-Mn(Ⅲ) vibronic superexchange interactions. Pressure on single-crystal samples was used as a variable to clarify the complex phase diagram of the La_(1-x)Sr_xMnO_3 system in the compositional range 0.12 ≤ x ≤ 0.17. Three thermodynamically distinguishable ferromagnetic phases could be identified. Dynamic orbital disorder in vibronic phases and dynamic two-phase fluctuations are shown to suppress dramatically the thermal conductivity.