Superconductivity at interfaces of KTaO3 and its possible origin.
Materials Science Division, Argonne National Laboratory
Superconductivity in materials with broken inversion symmetry and strong spin-orbit coupling can lead to unconventional pairing states that may be of interest in quantum science and technology. In this seminar I will discuss a recently discovered superconducting electron gas formed at interfacesof a 5d transition metal oxide KTaO3 (KTO) that combines these attributes intrinsically, and whose unique properties provide strong clues about the origin of its superconductivity. KTO, like its widely studied 3d cousin SrTiO3 (STO), is a ‘quantum paraelectric’, where the onset of ferroelectricity at low temperatures is believed to be thwarted by quantum fluctuations, giving rise to a very large dielectric constant. However, unlike STO, no evidence of superconductivity has been found till date in electron-doped KTO in the bulk. Recently, we discovered that electron gases formed interfaces of KTO are robust two-dimensional superconductors1 over a wide range of carrier densities, with Tc as high as 2.2 K, about an order of magnitude higher than those found at STO interfaces. Furthermore, there is a striking dependence of Tc on the crystalline facet of KTO at which the interfacial electron gas is formed – in our samples the maximum Tc values at the KTO (111) and (110) interfaces are 2.2 K and ~ 1 K respectively, while the KTO (001) interface remains normal down to 25 mK. For the KTO (111) interface, a remarkable non-saturating linear dependence of Tc on the areal carrier density (n2D) is observed, over nearly an order of magnitude of n2D. The superconductivity can also be tuned by gate electric fields, which elucidates the role of the interface in mediating pairing and allows for reversible switching of superconductivity at T = 2 K. Based on these findings, we propose a mechanism2 for pairing via inter-orbital interactions induced by inversion-breaking transverse optical (TO1) phonons, the same mode that softens in the quantum paraelectric phase, that explains several key aspects of superconductivity at KTO interfaces. Our results may provide insights into the pairing mechanism in other doped quantum paraelectrics, which has remained an open question for decades. Looking further, KTO interfaces are also a promising platform for exploring novel devices3 for quantum science, and I will present some initial results in this direction.
Host: Shulei Zhang