polar/non-polar heterojunctions

The deposition of polar perovskites on non-polar substrates has been predicted to lead to a increasing electric field in the film; after a critical thickness is reached, an electronic reconstruction alleviates this polar catastrophe by removing the built-in potential. In epitaxial heterojunctions of polar LaMnO₃(001) (LMO) on non-polar SrTiO₃(001) (STO), it has been observed that antiferromagnetic LMO films become ferromagnetic at a critical thickness of 5 - 6 u.c.; this is attributed to the electronic reconstruction. Recent DFT calculations (Chen et al., Phys. Rev. Lett. 119, 156801 (2017)) of LMO/STO heterojunctions predict the magnitude of the built-in potential to be 0.177 V/Å, but the presence of this potential has not yet been directly probed. With in situ XPS, we have directly probed both the band alignment and the presence of a built-in potential in epitaxial LMO/STO heterojunctions as a function of thickness. The valence state of Mn is evaluated with Mn L-edge XAS, and the local structural features, valence state, and extent of intermixing across the interface are evaluated with high resolution STEM imaging with EDS and EELS measurements. In light of our results, we propose a model for the electronic structure of stoichiometric LMO/STO heterojunctions.