Superconductivity at interfaces of the quantum paraelectric KTaO3

Anand Bhattacharya, Argonne National Laboratory

In this talk I will discuss the recently discovered two-dimensional superconductivity found at interfaces of the incipient ferroelectric KTaO3 (KTO). In its pristine insulating state, KTO is believed to be a ‘quantum paraelectric’, where the onset of ferroelectricity at low temperatures is thwarted by quantum fluctuations. A metallic electron gas can be obtained at interfaces of KTO by depositing a variety of insulating metal-oxide overlayers. Electron microscopy studies reveal the presence of both oxygen vacancies near the interface of KTO and diffusion of cations into KTO from the oxide overlayers, which dope the interfacial region of KTO with electrons. These interfacial electron gases were found to be superconducting up to temperatures as high has 2.2 K. Remarkably, the superconducting state is orientation selective, where electron gases formed at the (111) and (110) crystalline interfaces of KTO are robust two-dimensional superconductors, with Tc as high as 2.2 K and 1 K respectively, while electron gases formed at the (001) interface of KTO and oxide overlayers remain normal down to 25 mK. In this light, I will present a proposed mechanism for superconductivity at KTO interfaces where pairing involves an inter-orbital coupling mechanism mediated by the same soft phonon that is responsible for the incipient ferroelectricity in KTO. This mechanism favors superconductivity in states with maximal orbital degeneracy, and the lifting of this degeneracy due to quantum confinement effects explains the orientation selective nature of superconductivity at KTO interfaces. The broken inversion symmetry and strong spin-orbit coupling in KTO interfacial electron gases also lead to a spin-textured Fermi surface. I will outline how orbital degeneracy gives rise to a uniaxial ‘in-plane Ising’ spin texture for electron gases formed at KTO (110) interfaces, evidenced by their interaction with an insulating magnetic overlayer in both their superconducting and normal states.

References:
1. C. Liu et al., Science 371, 716 (2021).
2. C. Liu et al., Nature Communications 14, 951 (2023).
3. J. Yang et al., arXiv 2502.19599.