Two-dimensional electron gas in NaMnF$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ thin films: an \textit{ab}-initio study.

Control, by surface techniques of the two-dimensional electron gas (2DEG) in oxide/oxide thin-films and heterostructures [1,2], has been one of the most exciting topics recently studied in surface physics. On the other hand, oxide/fluoride heterostructures are expected to be as interesting as the mentioned counterparts. Moreover, our recent findings on A-site geometrically driven ferroelectricity on NaMnF$_{\mathrm{3}}$ (NMF) under epitaxial strain [3], have motivated experimental efforts to synthesize NMF thin films on SrTiO$_{\mathrm{3}}$ (STO) substrates. We present ab-initio calculations of the electronic and structural properties of these thin-films. We observed the appearance of a 2DEG at the NMF/STO when oxygen or fluorine vacancies are close to the interface. Thin-films without vacancies and intermixing exhibit the appearance of an electric field in STO, in absence of polar-charged layers. This can be explained in terms of the electrostatic potential along the heterostructure induced, and possibly controlled, by the NMF's spontaneous polarization. [1] S. Adhikari, et. al, Adv. Funct. Mater. 26, 5453--5459 (2016) [2] W. Dai, et. al, Nano Lett 16 (4), 2739--2743 (2016) [3] A. C. Garcia-Castro, et. al, Phys. Rev. Lett. 116, 117202 (2016)