Mobile inter-site bipolarons in presence of long-range interactions

Abstract

We report an exact diagonalization study of strongly correlated electron–phonon (EP) systems like doped polar insulators within the framework of the polaronic t−Jp model considering realistic long-range Coulomb and EP (Fröhlich) interactions on linear and square clusters. For a small ratio of polaronic hopping amplitude (t) and exchange interaction (Jp), polarons predominantly occupy the nearest-neighbor (NN) position with NN spin–singlet pairing (d-wave type). With the increase in the t∕Jp ratio, bipolaron size increases and finally becomes large and constant depending on the lattice size. The crossover from small to large bipolaron is nearly independent of system size but depends on the cluster geometry. Bipolaron effective mass and kinetic energy calculation show that at t∕Jp∼0.3, light and small bipolarons are perfectly mobile. In linear chain, bipolaron composite is formed within a region of 0<t∕Jp<0.5. Results suggest the existence of the superconducting phase at the vicinity of t∕Jp∼ 0.3.