POLARON FORMATION AND LOCAL MAGNETIC-MOMENTS IN CUPRATE SUPERCONDUCTORS

Exact diagonalization calculations show a continuous transition from delocalized to polaron behavior as a function of intersite electron-lattice coupling. A transition, found previously at the Hartree-Fock level between a magnetic and a nonmagnetic state, does not subsist when fluctuations are included. Local phonon modes become softer close to the polaron and by comparison with optical measurements of doped cuprates we conclude that they are close to the transition region between polaronic and nonpolaronic behavior. The barrier to adiabatically move a hole vanishes in that region suggesting large mobilities.