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We have studied systematically the competition of particle-particle and particle-hole states that may be realized on a square lattice model, characterized by one nesting vector. The set of the possible order parameters includes ferromagnetism, charge and spin density waves, as well as conventional and unconventional zone-center and zone-boundary (staggered) superconductivity. Having studied 16 cases of combinations of the above ordered states we have managed to derive general results concerning the generation of an induced phase and the conditions under which several states may coexist. In fact, we have found that there are certain quartets of order parameters that always coexist if three of them coexist and also satisfy a system of self-consistence equations quite distinct from the typical BCS type. We have grouped these 16 cases according to the system of self-consistence equations they obey and have observed that there are two different kinds of mechanisms leading to coexistence of phases. (See e.g. arXiv:0804.2450) |
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