Macroscopic Polarization from Antiferrodistortive Cycloids in Ferroelastic SrTiO3

Abstract

Based on a first-principles based multiscale approach, we study the polarity P of ferroelastic twin walls in SrTiO_{3}. In addition to flexoelectricity, which was pointed out before, we identify two new mechanisms that crucially contribute to P: a direct "rotopolar" coupling to the gradients of the antiferrodistortive oxygen tilts, and a trilinear coupling that is mediated by the antiferroelectric displacement of the Ti atoms. Remarkably, the rotopolar coupling presents a strong analogy to the mechanism that generates a spontaneous polarization in cycloidal magnets. We show how this similarity allows for a breakdown of macroscopic inversion symmetry (and therefore a macroscopic polarization) in a periodic sequence of parallel twins. These results open new avenues towards engineering pyroelectricity or piezoelectricity in nominally nonpolar ferroic materials.

First-Principles Theory of Spatial Dispersion: Dynamical Quadrupoles and Flexoelectricity

“…First-principles calculations of natu-ral gyrotropy are starting to appear; [42] we expect that, by bringing it within the scopes of DFPT, the formalism presented here will greatly simplify the calculation of this interesting quantity as well. In the context of ferroic materials, we also expect our method to facilitate the development of first-principles based continuum models [43] and effective Hamiltonians [44], where gradient-mediated couplings often play an important role.…”

Low-Temperature Dielectric Anisotropy Driven by an Antiferroelectric Mode in SrTiO3

“…In our case, however, we can safely rule out such a mechanism as the main driving force. Indeed, the domain wall polarity (P) is oriented, by symmetry, towards the apex of the twin [37]; this implies [ Fig. 1(a)] that only fa; cg twin walls can have a nonzero out-of-plane component of P and, hence, couple to a (001) field.…”

“…Clearly, our combined experimental and theoretical analysis allows us to unambiguously identify such an R mode with the antiferroelectric u Ti j phonon described in our work. Note that this mode plays a significant role in the physics of SrTiO 3 at low temperatures, as it also mediates an important contribution to the polarity of the twin walls [37].…”

Creating multiferroic and conductive domain walls in common ferroelastic compounds

“…While the focus of this work is on the effect of the dopants at the ferroelastic walls, we should point out that our first-principles results have interesting connections with (and may allow to benchmark) previous theories of this type of boundaries. Especially interesting are the works based on Ginzburg-Landau approaches, 23,[27][28][29] which discuss in detail the properties of the improper polarization naturally occurring at ferroelastic walls, and the couplings responsible for it. The phenomenological works also permit the investigation of situations that, because of the large supercells that would be required, are beyond the scope of current DFT simulations.…”