Anand Bhattacharya (Argonne National Lab)

Date: Mon. March 20th, 2023, 12:45 pm-1:45 pm
Location: Foldy & Zoom
Website: https://www.anl.gov/profile/anand-bhattacharya

Superconductivity at interfaces of KTaO₃ and its possible origin.

Anand Bhattacharya

Materials Science Division, Argonne National Laboratory

Abstract:

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 T_c 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 T_c on the crystalline facet of KTO at which the interfacial electron gas is formed – in our samples the maximum T_c 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 T_c on the areal carrier density (n_2D) is observed, over nearly an order of magnitude of n_2D. 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.

References:

1. Liu et al., Science (2021). https://www.science.org/doi/abs/10.1126/science.aba5511
2. Liu et al., Nature Communications (2023). https://doi.org/10.1038/s41467-023-36309-2
3. Yu et al., Nano Lett. (2022). https://doi.org/10.1021/acs.nanolett.2c00673

 

Host: Shulei Zhang