Broken symmetry states in quasi-one-dimensional molecular conductors -competitions, co-existences, and frustration -

Quasi-one-dimensional molecular conductors with a quarter-filled band exhibit a variety of phase transitions, into different electronic and electron-lattice coupled ordered states accompanying symmetry breaking. To investigate their physical properties especially at finite temperatures and provide a systematic understanding, we study extended Hubbard type models which take account of the effects of interchain Coulomb repulsion, with and without frustration, as well as the electron-lattice couplings. Different theoretical methods, such as mean-field theories, the bosonization and renormalization group scheme, and numerical techniques, are applied in order to capture different features of the experimentally observed ordering phenomena: charge order, lattice dimerization (dimer-Mott insulator), and spin-Peierls lattice tetramerization, showing competitions and different kinds of coexistent states among them.