Fluctuon effects in ferroelectric polarization switching

Abstract

The perplexing problem of the extremely large activation energy for nucleation Uac∼106 eV, which, as was shown by Landauer, appears in the Merz thermal-fluctuational theory of ferroelectric switching, is examined. Such enormous values of Uac are a result of the combined contribution of the energy of the depolarization field and the surface energy of the nucleus. While the depolarizing field term can be reduced by a redistribution of free charges, the surface energy term alone is too large (greater than 104 eV) for thermal-fluctuation-aided nucleation to be possible. This conclusion contradicts the numerous observations of a rapid increase in the number of repolarization nuclei with temperature, which suggest that the actual activation energy is on the order of an electron volt. This article is devoted to the problem of how the surface energy of a nucleus can be compensated. It is assumed that repolarization nuclei form by a fluctuon mechanism, and it is shown how this assumption can help in resolving the conflict between theory and experiment. It is suggested that upon application of an external electric field, electrons tunnel into the region where the spontaneous polarization vector fluctuates. This fluctuation region maintains its stability owing to the field of the electrons localized within the region. This leads to the appearance of a multielectron fluctuon, which is a bound state of electrons and a fluctuation and can be regarded as a repolarization nucleus. The electronic structure of these nuclei is calculated with density-functional theory. The calculations show that fluctuon effects can fully compensate the surface energy term. The assumption that repolarization nuclei have a fluctuon nature is shown to account for the principal laws governing the switching process, including low activation energy values. The results of possible experiments that could verify the proposed mechanism are predicted.