Unconventional quantum oscillations and evidence of nonparabolic electronic states in quasi-two-dimensional electron system at complex oxide interfaces

The simultaneous occurrence of electric-field controlled superconductivity and spin-orbit interaction makes two-dimensional electron systems (2DES) constructed from perovskite transition metal oxides promising candidates for the next generation of spintronics and quantum computing. It is, however, essential to understand the electronic bands thoroughly and verify the predicted electronic states experimentally in these 2DES to advance technological applications. Here, we present novel insights into the electronic states of the 2DES at oxide interfaces through comprehensive investigations of Shubnikov-de Haas oscillations in two different systems: EuO/KTaO$_3$ (EuO/KTO) and LaAlO$_3$/SrTiO$_3$ (LAO/STO). To accurately resolve these oscillations, we conducted transport measurements in high magnetic fields up to 60 T and low temperatures down to 100 mK. For 2D confined electrons at both interfaces, we observed a progressive increase of oscillations frequency and cyclotron mass with the magnetic field. We interpret these intriguing findings by considering the existence of non-trivial electronic bands, for which the $E-k$ dispersion incorporates both linear and parabolic dispersion relations. In addition to providing experimental evidence for topological-like electronic states in KTO-2DES and STO-2DES, the unconventional oscillations presented in this study establish a new paradigm for quantum oscillations in 2DES based on perovskite transition metal oxides, where the oscillations frequency exhibits quadratic dependence on the magnetic field.