The quantum physical reality of polar-nonpolar oxide heterostructures

Conducting interfaces between polar and nonpolar insulating oxides, e.g., LaAlO₃/SrTiO₃ [1], have generated interest for both fundamental physics and oxide-electronics applications. Current understanding is based on an amalgamation of a classical electrostatic model (polar catastrophe model) that was originally derived for semi-infinite solids and quantum density-functional-theory (DFT) results on ultrathin films. Here we report comprehensive DFT calculations that unveil a very different purely quantum physical reality. We first reassess the PCM’s foundations and utility. We show that, for ultrathin polar films, the interfacial dipole does not control the electrostatic potential in the polar film -- the surface and interface play equal roles, and the absence or presence of centrosymmetry in the physical LAO film result in different, purely quantum mechanisms for the generation of a conducting interface, neither involving physical-charge transfer. Our conclusion regarding the origin of the conducting 2DEG is supported by SHG data [2]. Predictions are made that can be tested and can guide technology development.
[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004).
[2] A. Savoia et al. Phys. Rev. B 80, 075110 (2009).