Pseudospin symmetry, Peierls instability, and charge-density wave

We show that there is pseudospin SU(2) symmetry in the one-dimensional tight-binding model with the inter-molecular electron-phonon interaction. For a finite lattice at half-filling, the Peierls instability drives the lattice to dimerization at low temperature. However, the system remains SU(2) symmetry invariant and is not in the charge-density wave (CDW) state. We show in a BCS-like variational approach that an attractive on-site electron-electron interaction makes the pseudospin symmetry spontaneously broken in the thermodynamic limit and the CDW state arises as well. Hence, it is clear that spontaneous pseudospin symmetry breaking produces the CDW state.link_to_subscribed_fulltex