Mechanisms of charge transfer and redistribution in LaAlO3/SrTiO3 revealed by high-energy optical conductivity

Abstract: In condensed matter physics the quasi two-dimensional electron gas at the interface of two different insulators, polar LaAlO3 on non-polar SrTiO3 (LaAlO3/SrTiO3) is a spectacular and surprising observation. This phenomenon is LaAlO3 film thickness-dependent and may be explained by the polarization catastrophe model, in which a charge transfer of 0.5 electron from the LaAlO3 film into the LaAlO3/SrTiO3 interface is expected. Here we show that in conducting samples (more than 4 unit cells of LaAlO3) there is indeed a 0.5 electron transfer from LaAlO3 into the LaAlO3/SrTiO3 interface by studying the optical conductivity in a broad energy range (0.5-35 eV). Surprisingly, in insulating samples (less than 4 unit cells of LaAlO3) a redistribution of charges within the polar LaAlO3 sub-layers (from AlO2 to LaO) as large as 0.5 electron is observed, with no charge transfer into the interface. Hence, our results reveal the different mechanisms for the polarization catastrophe compensation in insulating and conducting LaAlO3/SrTiO3 interfaces.