Superconductivity at KTaO3 interfaces, and its possible origins
Dr. Anand Bhattacharya
Argonne National Laboratory
In recent years, there has been significant interest in the interplay between superconductivity, broken inversion symmetry, and spin-orbit coupling as a path to realizing novel pairing states, typically explored via proximity coupling of a superconductor to a spin-orbit coupled interfacial electron gas. In this seminar I will discuss a new superconducting electron gas formed at interfaces of 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. Unlike STO, no evidence of superconductivity has been found till date in electron-doped KTO in the bulk, though there was a report of superconductivity at the (001) interface of KTO with Tc ~ 50 mK. Recently, we discovered that electron gases formed at the (111) and (110) interfaces of KTO are robust two-dimensional superconductors 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. The in-plane critical magnetic fields are far in excess of the Pauli paramagnetic limit, likely due to strong spin-orbit coupling effects. 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 KTaO3 (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, allowing for reversible electrical switching of superconductivity at T = 2 K. Based on these findings, we propose a mechanism 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. The inter-orbital mechanism could apply to other interfaces with phonon modes that break inversion symmetry, and may provide insights into the pairing mechanism in doped quantum paraelectrics like SrTiO3, which has remained an open question for decades. Looking further, KTaO3 interfaces are also a promising platform for exploring novel devices for quantum science, and I will present some initial results in this direction.
References:
C. Liu et al., Science (2021). https://www.science.org/doi/abs/10.1126/science.aba5511
C. Liu et al., arXiv:2203.05867 (2022) https://doi.org/10.48550/arXiv.2203.05867
M. Yu et al., Nano Lett. (2022) https://doi.org/10.1021/acs.nanolett.2c00673
Hosted by Prof. Jin