Quantum lattice fluctuations in the exactly half-filled and nearly half-filled Su-Schrieffer-Heeger model.

Abstract

We study the effect of quantum lattice fluctuations in the exactly half-filled and nearly half-filled ground state of the Su-Schrieffer-Heeger model using the adiabatic quantum Monte Carlo method. In the exactly half-filled case, although the average magnitude of the quantum fluctuations is about the same as the magnitude of bond alternation, the lattice dimerization survives the fluctuations. The fluctuations produce those tails of density of states which extend deep into the classical gap. This agrees with the large subgap optical-absorption tail in undoped trans-polyacetylene. The tails are mainly caused by the anharmonic quantum lattice fluctuations of virtual soliton-antisoliton pairs. In the nearly half-filled case, the characteristic band structure of a charged soliton lattice survives the fluctuations. The quantum lattice fluctuations produce those tails of density of states that extend deep into the classical gaps also in the case. The tails are mainly caused by the virtual soliton-antisoliton pair fluctuations and local-density fluctuations of charged solitons. Since the extension of the density of states in the classical gap is estimated to be larger than the size of the gap in the heavily doped regime, we conclude that the Fermi surface is produced by the quantum lattice fluctuations in the regime. Although the Fermi surface is different from the conventional one, the state with the Fermi surface is metallic. This agrees with the experimentally observed metallic properties of heavily doped polyacetylene.