Pseudo-proper two-dimensional electron gas formation

In spite of the interest in the two-dimensional electron gases (2DEGs) experimentally found at surfaces and interfaces, important uncertainties remain about the observed insulator-metal transitions (IMTs). Here we show how an explicit pseudo-proper coupling of carrier sources with a relevant soft mode significantly affects the transition. The analysis presented here for 2DEGs at polar interfaces is based on group theory, Landau-Ginzburg theory, and illustrated with first-principles calculations for the prototypical case of the LaAlO3/SrTiO3 interface, for which such a structural transition has recently been observed. This direct coupling implies that the appearance of the soft mode is always accompanied by carriers. For sufficiently strong coupling an avalanche-like first-order IMT is predicted.