Superconductivity at interfaces of KTaO3
*Anand Bhattacharya1

In this talk I will discuss our recent discovery of superconductivity in electron gases formed at interfaces between (111) oriented KTaO3 (KTO) and insulating overlayers of either EuO or LaAlO3. We find the superconducting Tc to be as high as 2.2 K, about an order of magnitude higher than in the LaAlO3/SrTiO3 system. Critical field and current-voltage measurements indicate that the superconductivity is two-dimensional. Using transmission electron microscopy and resonant x-ray scattering, we establish the presence of substitutional defects and oxygen vacancies at the KTO interface that can act as donors of electrons, and may lead to the formation of the interfacial electron gas. Furthermore, we have uncovered a number of unusual and intriguing properties of the superconducting state at KTO interfaces. Firstly, the superconductivity is orientation selective - similar electron gases at (001) KTO interfaces remain normal down to 25 mK.  Secondly, in higher mobility EuO/KTO (111) samples, we observe a large in-plane anisotropy in transport properties at low temperatures prior to onset of superconductivity, where the nominally six-fold symmetry of the KTO (111) surface electronic structure is spontaneously broken. Measurements of magnetoresistance suggest that the transport anisotropy is a due to the emergence of a nematic phase of ‘stripe’ like superconductivity, which is nearly homogeneous in one direction, but strongly modulated in the other, giving rise to a resistance anisotropy. Being that EuO is magnetic, the interaction between magnetism and superconductivity may play an important role in the emergence of this nematicity. These findings have provoked a number of theoretical ideas that I will briefly review, and also point towards possible future directions in this fast-developing area of research.

Ref.: C. Liu et al., Science 2021. DOI: 10.1126/science.aba5511