Microscopic model of ferroelectricity in stress-free PbTiO3 ultrathin films

The ground-state polarization of PbTiO3 thin films is studied using a microscopic effective Hamiltonian with parameters obtained from first-principles calculations. Under short-circuit electrical boundary conditions, (001) films with thickness as low as three unit cells are found to have a perpendicularly polarized ferroelectric ground state with significant enhancement of the polarization at the surface. (C) 2000 American Institute of Physics. [S0003- 6951(00)02119-7].