Stoichiometric control of 2D superconductivity and mobility at SrTiO₃-based interfaces

SrTiO₃-based quasi-two-dimensional electron gases (q-2DEGs), that exhibit coexistence of gate tunable multi-orbital 2D superconductivity and strong Rashba spin-orbit coupling (RSOC), have ingredients to generate topological superconducting electronics for quantum application. However, a comparison between the electron mean free path and the superconducting coherence length reveals that superconductivity in these systems is in the dirty limit, which tends to suppress nonconventional pairing and therefore challenges these expectations. Here, in LaAlO₃/SrTiO₃ interfaces, we show that precise control of the La/Al ratio during the epitaxial growth of LaAlO₃, monitored by XPS and STEM-EELS measurements, allows us to finely tune the electron mobility, carrier density and the superconducting properties. Remarkably, our findings reveal electronic mobilities that surpass previous reports by up to 15-fold, with mean free paths comparable to the superconducting coherence length, thereby approaching the clean limit. In addition, we also show that in our extremely clean q-2DEGs, superconductivity can develop at low electron density, where electrons only populate low-energy d_xy subbands. This is in sharp contrast to the conventional understanding of superconductivity in LaAlO₃/SrTiO₃ q-2DEGs, which relies solely on Cooper pairing in the high-energy d_xy/d_xz subbands. What makes this even more surprising is that the density of states in the d_xy subbands is very low, which, according to the BCS theory, is considered detrimental to superconductivity.