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Strong electron-lattice coupling as the mechanism behind charge densiy wave transformations in transition-metal-dichalkogenides

We consider single band of conduction electrons interacting with displacements of the transitional ions.In the classical regime strong coupling transforms the harmonic elastic energy for an ion to the one of the well with two deep minima,so that the system is described in terms of Ising spins. Inter-site interactions via the exchange by electrons order spins at lower temperatures. Extention to the quantum regime is discussed. Below the CDW-transition the energy spectrum of electrons remains metallic because the structural vector Q and the FS sizes are not related.Large values of the CDW gaps seen in the tunneling experiments find their natural explanation as due to the deep energy minima in the bound two-well electron-ion complex.