Macroscopic polarization in ferroelastic SrTiO3 via gradient-mediated couplings*

We study the microscopic mechanisms that are responsible for the
the polarity of ferroelastic twin boundaries (TBs) in tetragonal SrTiO3.
We carry out our investigation by using a new multiscale approach that starts from
the first-principles results, and uses them in order to build an effective
continuum Hamiltonian by taking the long-wavelength limit of the
relevant lattice-dynamical data.
Such a methodology has allowed us to identify three main gradient-mediated
mechanisms for polarity:
(i) flexoelectricity, which originates from the strain gradient at the TB;
(ii) a "rotopolar" coupling, which comes directly from the gradients of the
octahedral tilts;
(iii) a previously overlooked tri-linear coupling, which is mediated
by an antiferroelectric displacement of the Ti atoms.
Remarkably, the rotopolar coupling bears a strong analogy with the
mechanism that generates a polarization in cycloidal magnets, thereby
allowing for a macroscopic breakdown of inversion symmetry even in a
periodic sequence of parallel twins.