Measurement of Helical Edge States in Ultra-thin LaTiO₃/SrTiO₃ Heterostructures

Emergent metallic behavior at the interface of the Mott insulator LaTiO₃ and the band insulator SrTiO₃ has been explained in terms of charge redistribution at the interface combined with strain-induced electronic structure modification. We have previously studied ultra-thin (3 unit cell thick) films of LaTiO₃ on SrTiO₃ substrates and found evidence for unexpectedly large spin-orbit coupling from quantum oscillations with an associated Berry phase, weak anti-localization in the magnetoresistance, and anisotropic in-plane magnetoresistance. In order to tune this material, we have electrically gated these films both on the back side of the SrTiO₃ substrate and on top of the LaTiO₃ film with an electrolytic gate. Most interestingly, we found a maximum in the resistance that is quantized to the quantum of conductance (2e²/h). This is independent of the dimensions of the sample. Additionally, by studying samples with four and six contact geometries, we measure nonlocal transport similar. These features are present up to a surprisingly high temperature of 230K. We attribute these observations to helical edge states at the LaTiO₃/SrTiO₃ interface.