Hi all, As you may know, the interface of LaAlO3 and SrTi03 has received a lot of attention because of the presence of conducting electrons, superconductivity, and ferromagnetism. Because LaAl03 is a polar crystal, the polar catastrophe is often used as a first explanation for the presence of excess surface electrons. However, I think the carrier density at the interface, as measured by hall resistance, is much lower than the naive value from the polar catastrophe. Apparently, one possible explanation in this material is that Mott physics may localize the electrons. My interest is more general The polar catastrophe appears very generic: for example, it seems to apply to the free surface of LaAlO3 as well, yet apparently this free surface does not conduct. Does anyone know if there is a common mechanism or set of mechanisms that prevents one from seeing this surface conductivity in polar crystals in general e.g. disorder or mott physics? Or am I misinformed when I assume that such conducting behavior is uncommon (this is the impression I have from a few talks)?