Spin-Orbit Coupling and Superconducting Stripes in an Oxide Heterostructure EuO/KTO(110)

Unconventional quantum states have been realized at the interfaces of oxide heterostructures, where they can be effectively tuned by the gate voltage. Recent studies reveal that the conductive interfaces in the SrTiO₃(STO)-based and KTaO₃(KTO)-based heterojunctions host a surprisingly enriched cascade of intriguing physical phenomena, most notably the emergence of two-dimensional (2D) superconductivity. Such 2D superconductivity is characterized by a Berezinskii-Kosterlitz-Thouless (BKT) transition; its unusual behavior in external magnetic fields and large tunability under varying electric fields render the superconducting oxide interfaces a promising platform for exploring the mechanism of unconventional superconductivity.<br/><br/>In this talk I will introduce our recent progress on the study of the interface between high-quality EuO (111) thin film and KTO (110) substrate. Both oxides are insulating, yet the interface is metallic and shows superconductivity with onset transition temperature <i>T</i>onset c = 0.6-1.4 K depending on the carrier density. The 2D nature of superconductivity is verified by the large anisotropy of the upper critical field and the characteristics of a BKT transition. By applying gate voltages, <i>T</i>onset c can be largely tuned with an enhancement of ~70%; such an enhancement can be possibly associated with a boosted spin-orbit coupling (SOC) energy. Further analysis based on the upper critical field (<i>H</i>_c2) and magnetoconductance reveals complex nature of SOC at the EuO/KTO (110) interface with different dominant scattering mechanisms in the superconducting and normal states. Our results demonstrate that the SOC should be considered an important factor in determining the 2D superconductivity at oxide interfaces.<br/><br/>More interestingly, we discovered a peculiar band-filling-controlled dimension reduction at the superconducting interface between EuO and (110)-oriented KTO. In devices with low carrier densities, electrical transport measurements reveal different <i>T</i>_c and <i>H</i>_c2 with current applied along the two orthogonal in-plane directions. Theoretical analysis suggests that strong coupling between Ta 5<i>d</i> and Eu 4<i>f</i> electrons occurs in the low-carrier-density samples, whereas in the high-carrier-density samples (wherein <i>T</i>_c becomes isotropic) such coupling is weakened. Complemented by experiments of local magnetic susceptibility imaging, our observations imply an unprecedented emergence of unidirectional stripe-like superconducting texture, presumably induced by the ferromagnetic proximity effect; we suggest that the superconducting phase coherence is first established within these “stripes”, leading to the peculiar directional dependence of <i>T</i>_c. The realization of such exotic superconducting states provides impetus for the study of novel physics in heterostructures possessing both magnetism and superconductivity.