Band gap narrowing in ferroelastic twin walls at CaTiO₃(001) surface

Domain walls are a trade-off between the energy cost of the wall and the energy gain for domain formation. They break translational symmetry and can exhibit fascination properties absent in the bulk parent structure. In the pure ferroelastic CaTiO3 the order parameter is the strain defined by the oxygen octahedral tilts in the unit cell. At the domain walls, one octahedral tilt goes to zero, allowing the central Ti cation to off-center, creating polarity. We first map the polar domain walls using low energy electron microscopy (LEEM). Then, using electron energy loss spectroscopy in the LEEM we reveal band gap narrowing of up to 0.5 eV in the domain walls. The narrowing may be due to the intrinsically different symmetry [1] and/or the presence of oxygen vacancies [2] in the walls. The intrinsic modification of the electronic structure within the wall is compared with density functional theory calculations and opens up perspectives for engineering of domain wall functionality.

[1] G.F. Nataf et al., Phys. Rev. Mater. 1, 074410 (2017)
[2] L. He, D. Vanderbilt, Phys. Rev. B 68, 1341031 (2003)