Superconducting KTaO₃ Rashba Two-Dimensional Electron Gases as a Possible Monolithic Platform for Topological Quantum Computing

The two-dimensional electron gas (2DEG) at the interface between SrTiO₃ (STO) and LaAlO₃¹, displays a wide array of functionalities such as high electronic mobility, low temperature superconductivity² and tunable Rashba spin-orbit coupling (SOC)³. Understanding the physics of STO 2DEGs has challenged our community since that original discovery but also led to exciting properties for device applications into fields as diverse as power electronics, photocatalysis, spin-orbitronics⁴ or topological quantum computing⁵. These latter two directions aim to specifically exploit the most unique properties of STO 2DEGs, namely Rashba SOC and 2D superconductivity. Yet, the Rashba coefficient in STO 2DEGs remains relatively small (a_R<50 meV.Å) and their superconducting T_C is low (~250 mK), hampering development towards these exciting goals.
Just like STO, KTaO₃ (KTO) is a quantum paraelectric material that in the bulk can be turned into a metal by minute electron doping, leading to high-mobility transport⁶. Because Ta is a 5d element, much heavier than Ti, KTO is also expected to possess stronger SOC and a high interest for spin-orbitronics.
In this talk, we will show how 2DEGs can be defined in KTO by deposition a few angstroms of Al by sputtering or molecular beam epitaxy. We will first focus on KTO(001) and discuss its electronic measured by angle-resolved photoemission spectroscopy (ARPES) and fitted by a tight-binding Hamiltonian. Our data provide the first direct visualization of Rashba-split bands in an oxide 2DEG⁷ with a Rashba coefficient a_R»300 meV.Å, much higher than in STO 2DEGs. We will report charge-spin and spin-charge conversion from the direct and inverse Edelstein effects using bilinear magnetoresistance and spin-pumping experiments⁸. The deduced Rashba coefficient agrees well with values deduced from ARPES.
In a second part we will present the generation of 2DEGs on KTO(111) and KTO(110). Consistent with recent results on LAO/KTO^9–11 and EuO/KTO interfaces¹⁰, the 2DEGs are superconducting with critical temperatures in the 1-2 K range, one order of magnitude higher than STO 2DEGs. This is unexpected since, unlike in STO, superconductivity is absent in bulk KTO. We will discuss the electronic structure of these KTO 2DEGs based on ARPES data and the nature of superconductivity from superfluid stiffness measurements¹². We will also show how electrostatic gating can be used to tune it and out of the superconducting state, and present tunneling spectroscopy measurements suggesting that KTO 2DEGs are unconventional superconductors.

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