Properties of ferroelectric ultrathin films from first principles

Advances in first-principles computational approaches have, over the past ten years, made possible the investigation of basic physical properties of simple ferroelectric systems. Recently, first-principles techniques also proved to be powerful methods for predicting finite-temperature properties of solid solutions in great details. Consequently, bulk perovskites are rather well understood nowadays. On the other hand, one task still remains to be accomplished by ab-initio methods, that is, an accurate description and a deep understanding of ferroelectric nanostructures. Despite the fact that nanometer scale ferroelectric materials have gained widespread interest both technologically and scientifically (partly because of novel effects arising in connection with the reduction of their spatial extension), first-principles-based calculations on ferroelectric nanostructures are rather scarce. For instance, the precise effects of the substrate, growth orientation, surface termination, boundary conditions and thickness on the finite-temperature ferroelectric properties of ultrathin films are not well established, since their full understandings require (i) microscopic insights on nanoscale behavior that are quite difficult to access and analyze via experimental probes, and (ii) the development of new computational schemes. One may also wonder how some striking features exhibited by some bulk materials evolve in the corresponding thin films. A typical example of such feature is the morphotropic phase boundary of various solid solutions, where unusual low-symmetry phases associated with a composition-induced rotation of the spontaneous polarization and an enhancement of dielectric and piezoelectric responses were recently discovered. In this paper, recent findings resulting from the development and use of numerical first-principles-based tools on ferroelectric ultrathin films are discussed.