Description:

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 in KTO and diffusion of cations from the oxide overlayers, which dope the interfacial region of KTO with electrons.  These interfacial electron gases were recently discovered 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 in this manner remain normal down to 25 mK. I will connect the properties of this two-dimensional superconductor, including its gate-tunability, its superfluid stiffness and its response to external in-plane magnetic fields to its electronic structure. In particular, I will present a proposed mechanism for superconductivity that involves inter-orbital coupling, which favors superconductivity in states with maximal orbital degeneracy, and explains the orientation selective superconductivity at KTO interfaces. I will also outline how this degeneracy may give rise to a uniaxial ‘in-plane Ising’ spin texture at KTO (110) interfaces, evidenced by its interactions with a magnetic overlayer.

Anand Bhattacharya, Physicist, Argonne National Laboratory

Host: Venkat Chandrasekhar