  Recently, an exciting discovery of orientation-dependent superconductivity
was made in two-dimensional electron gas (2DEG) at the interfaces of
LaAlO$_3$/KTaO$_3$ (LAO/KTO) or EuO/KTaO$_3$ (EuO/KTO). The superconducting
transition temperature can reach a $T_c$ of up to $\sim$ 2.2 K, which is
significantly higher than its 3$d$ counterpart LaAlO$_3$/SrTiO$_3$ (LAO/STO)
with a $T_c$ of $\sim$ 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 $5d^1$ electrons in KTO interface superconductors, which
is consistent with the absence of magnetic hysteresis observed in
magneto-resistance (MR) measurements. These findings offer new insights into
our understanding of superconductivity in Ta $5d$ interface superconductors and
their potential applications.
