  We investigate the neutral-to-ionic insulator-insulator transition in
one-dimensional materials by treating a strong-coupling effective model based
on the ionic Hubbard model using the density-matrix renormalization group and
finite-size scaling. The effective model, formulated in a spin-one
representation, contains a single parameter. We carry out an extensive
finite-size scaling analysis of the relevant gaps and susceptibilities to
characterize the two zero-temperature transitions. We find that the transition
from the ionic band-insulating phase to an intermediate spontaneously dimerized
phase is Ising, and the transition from the dimerized phase to the
Mott-insulating phase is Kosterlitz-Thouless, in agreement with the
field-theory-based predictions.
