CommunicationMixed valence as a necessary criteria for quasi-two dimensional electron gas in oxide hetero-interfaces

- Quasi-two dimensional electron gas (2DEG) is not universal to all polar-nonpolar insulating oxide hetero-interfaces, as being commonly thought to date.
- LaAlO3-SrHfO3 hetero-structure system fails to exhibit 2DEG. This is because Hf ions does not undergo electronic reconstruction as like its isoelectronic counterpart, Ti.
- Our analysis shows significant compression of the inter-planar distance (d), which in the realms of parallel plate capacitor model is the distance between the charged plates.

The origin of quasi-two dimensional electron gas at the interface of polar-nonpolar oxide hetero-structure, such as LaAlO3/SrTiO3, is debated over electronic reconstruction and defects/disorder models. Common to these models is the partial valence transformation of substrate Ti ions from its equilibrium 4+ state to an itinerant 3+ state. Given that the Hf ions have a lower ionization potential than Ti due to the 4f orbital screening, one would expect a hetero-interface conductivity in the polar-nonpolar LaAlO3/SrHfO3 system as well. However, our first principles calculations show the converse. Unlike the Ti3+−Ti4+ valence transition which occur at a nominal energy cost, the barrier energy associated with its isoelectronic Hf3+−Hf4+ counterpart is very high, hence suppressing the formation of quasi-two dimensional electron gas at LaAlO3/SrHfO3 hetero-interface. These calculations, therefore, emphasize on the propensity of mixed valence at the interface as a necessary condition for an oxide hetero-structure to exihibit quasi two-dimensional electron gas.