Resonant X-Ray Reflectometry Study of Orbital Polarization in Quantum Critical SmTiO₃ Heterostructures

Strongly correlated electron systems continue to shed new light on the fundamental interactions of electrons in solids. With hallmark behaviors like metal-to-insulator transitions and high temperature superconductivity, Mott insulating transition metal oxides are a canonical example of strong electron-electron correlations. Here we investigate epitaxial heterostructures of the Mott insulating rare earth titanate SmTiO₃ (SmTO) embedded with quantum wells of the band insulator SrTiO₃ (STO). This system exhibits both quantum critical transport and pseudogap behavior suggesting strong parallels with bulk Mott materials. In this work, we use resonant X-ray reflectometry at the Ti L-edge to probe the orbital polarization depth profiles of a SmTO film and SmTO-STO superlattices with varying STO quantum well thickness. Our results suggest that the d¹ electron in SmTO preferentially occupies the in-plane d_xy orbital despite nearly 1% compressive strain from the underlying substrate. Similar in-plane polarization is observed for both SmTO and STO layers in the superlattices. These results are analyzed in terms of competition between compressive strain, quantum confinement, and Jahn-Teller distortions to the crystal field.