Large critical field of superconductivity at EuO/KTaO₃ (111) interface

The recent discoveries of superconducting oxide interfaces have provided a fertile ground for tuning the superconducting properties by engineering the heterostructure. Here, we report the large upper critical field (B_C2) observed at the superconducting EuO/KTaO₃(111) interface with a maximal B_C2 ~ 1.7 T. As a comparison, the superconducting interface is constructed in MgO/KTaO₃(111) heterostructure with a non-magnetic MgO top layer. Interestingly, the B_C2 of MgO/KTaO₃(111) is observed to be ~ 0.18 T which is nearly an order smaller than that of EuO/KTaO₃(111). Moreover, as back gate voltage changes, the upper critical field and critical temperature of EuO/KTaO₃(111) show opposite trends in the underdoped regime while those of MgO/KTaO₃(111) show similar tendencies. By further analyzing the upper critical field observed at various KTaO₃-based superconducting interfaces, B_C2 of EuO/KTaO₃(111) is extremely larger than any others with non-magnetic top layers. We attribute these unusual phenomena to the ferromagnetic insulator EuO, suggesting that the spin fluctuations induced by EuO enhance the pairing potential at the interface.