Absence of localized electrons in interface superconductors

Recently, an exciting discovery of orientation-dependent superconductivity was made in two-dimensional electron gas (2DEG) at the interfaces of LaAlO3/KTaO3 (LAO/KTO) and EuO/KTaO3 (EuO/KTO). The superconducting transition temperature can reach a Tc of up to -2.2 K, which is significantly higher than its 3d counterpart LaAlO3/SrTiO3 (LAO/STO) with a Tc of -0.2 K. However, the underlying origin remains to be understood. To uncover the nature of electrons in KTO-based interfaces, we employ x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray spectroscopy (RIXS) to study LAO/KTO and EuO/KTO with different orientations. We reveal the absence of dd orbital excitations in all the measured samples. Our RIXS results are well reproduced by calculations that considered itinerant 5d electrons hybridized with O 2p electrons. This suggests that there is a lack of localized Ta 5d1 electrons in KTO interface superconductors, which is consistent with the absence of magnetic hysteresis observed in magnetoresistance (MR) measurements. These findings offer new insights into our understanding of superconductivity in Ta 5d interface superconductors and their potential applications.