QUANTUM PHASE-TRANSITION AND LONG-RANGE ORDER IN THE GROUND-STATE OF A LATTICE OF PSEUDOSPINS COUPLED WITH OPTIC PHONONS

We study a pseudospin-phonon coupling system in its ground state and s how that, as a result of the competition between the intrasite pseudos pin tunneling and the phonon-induced intersite pseudospin correlation, a quantum phase transition to the pseudospin long-range ordering phas e may occur at some critical coupling constant. In the disordered phas e the short-range ferroelectric or antiferroelectric correlation gets stronger as the coupling constant increases. The calculated renormaliz ed tunneling matrix element, which is reduced by the pseudospin-phonon interaction, is not as small as the theory of previous authors predic ts, that is, the reduction is alleviated by taking into account the re tardation effect of the coupling.