Spin-Peierls model

The two types of chains existing in the a-phases, the conducting perylene chains and the M(mnt)2 chains that for some metal can have localised spins, make the discussion on the nature of the low temperature transitions more complicated than in other simpler molecular conductors. In fact each of these chains is a priori prone to Peierls or spin—Peierls instabilities of the one-dimensional conducting and magnetic systems respectively and these a priori could be either coupled or uncoupled in a complex and interesting situation for which at present there are no detailed theoretical models. 

The previous treatment deals with a one-component order parameter (such as for a commensurate Peierls distortion) but does not apply to situations where the order parameter is complex with an amplitude and a phase (superconductivity, incommensurate Peierls, or spin density wave transitions). The latter situation is analogous to classical moments which can rotate freely in an XY plane. The coherence length of the XY model is less strongly divergent at low temperature than for the Ising model,... 

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