UC Santa Barbara

In strongly correlated electron systems, the independent electron approximation fails and electron-electron correlations must be taken into consideration. Such systems display an abundance of technologically useful behaviors including metal-insulator transitions, superconductivity, and colossal magnetoresistance. Within the broad class of correlated materials, Mott insulators are a canonical example. These compounds have repulsive Coulomb interactions between on-site electrons large enough to open an energy gap between two portions of a valence band, thereby turning what conventional band theory would predict to be a metal into a (Mott) insulator. Despite many years of investigation, Mott insulators remain an exciting area of materials research owing in part to their proximity to quantum critical points and spin liquid ground states.

Here we have studied thin films of the Mott insulating rare earth titanates RTiO3 (R = Gd, Sm), and heterostructures of these compounds with the band insulator SrTiO3. At the RTiO3/SrTiO3 interface, electrostatic doping creates a two-dimensional electron liquid (2DEL) that resides within the SrTiO3 layers near the interface. In the case of thin SrTiO3 quantum wells between magnetic RTiO3 barriers, we used polarized neutron reflectometry and muon spin rotation to show that there is a critical well thickness below which the 2DEL electrons exhibit magnetic correlations. This critical thickness was found to be independent of the sign of the magnetic exchange interactions in the neighboring RTiO3 barriers. A follow up study on thin GdTiO3 layers embedded within SrTiO3 revealed magnetic dead layers at the interface of the two materials, but bulk-like ferrimagnetism within the center of the thin GdTiO3 layers. The independence of magnetism from the notable structural distortions observed within the thin GdTiO3 layers highlights the weak coupling between the magnetic exchange interactions and electronic bandwidth in this material. Finally, element-specific resonant X-ray measurements were used to probe electronic symmetry breaking within SmTiO3 films and found evidence of in-plane orbital polarization at room temperature. Together, these studies add to the collective understanding of electron-electron correlations within Mott insulating thin films, particularly those in proximity to a high-density 2D electron system.