Landau theory of the phase transitions in half-doped manganites: Interplayof magnetic, charge, and structural order

The order parameters of the magnetic, charge, and structural orders at half -doped manganites are identified. A corresponding Landau theory of the phas e transitions is formulated. Many structural and thermodynamical behaviors are accounted for and clarified within the framework. In particular, the th eory provides a unified picture for the scenario of the phase transitions a nd their nature with respect to the variation of the tolerance factor of th e manganites. It also accounts for the origin of the incommensurate nature of the orbital order and its subsequently accompanying antiferromagnetic or der.