PhD defense – Hugo Witt

On December 17 at 1:30 pm (TRT Auditorium), Hugo Witt will defend his PhD thesis entitled “Multiband superconductivity and spin-orbit coupling in KTaO3 interfaces”, prepared under the supervision of Isabella Boventer, Nicolas Bergeal and  Manuel Bibes.

Abstract:

The relentless pursuit of higher computational performance and energy efficiency, coupled with the fundamental physical limits of semiconductor miniaturization, demands a radical rethinking of information processing paradigms. In this context, the field of beyond-CMOS technologies emerges as the next paradigm. Spintronics, by harnessing the electron’s spin rather than its charge, offers a compelling pathway to integrate memory and logic functions, enabling non-volatile, collective state switching and reducing the energy footprint of electronic devices, provided that efficient spin-charge interconversion can be achieved. The spin and orbital degrees of freedom could also be key in realizing topological superconductivity, which eludes researchers and could open a revolutionary avenue for quantum computation by encoding information in non-local, topologically protected states such as Majorana Zero modes. The realization of such systems, however, hinges on the discovery or precise engineering of materials that combine superconductivity with strong spin-momentum locking, such as Rashba systems.

This thesis investigates the electronic properties of a new material platform towards energyefficient spintronic devices and topological superconductivity. The focus is placed on KTaO3-based two-dimensional electron gases (2DEGs), which exhibit exceptionally strong Rashba spin-orbit coupling and superconductivity, that can be dynamically modulated via electric fields. Through photoemission spectroscopy, high-field magnetotransport, and absorption spectroscopy, the anisotropic band structure of 111- and 110-oriented KTaO3 2DEGs is mapped, revealing orientation-dependent electronic properties. Non-reciprocal transport measurements of the in-plane anisotropic magnetoresistance further uncover their influence, with quadratic and bilinear components attributed to multiband scattering and charge-to-spin conversion via the Edelstein effect. The estimated Rashba coefficients display directional dependence, directly linked to the spin and orbital textures of the system. In the superconducting regime, DC transport measurements demonstrate the two-dimensional nature of superconductivity in KTaO3 interfaces, with critical temperatures up to 2.5 K and a pronounced sensitivity to crystallographic orientation and disorder. We fabricate field-effect mesoscopic devices and report on the control of superconductivity via back- and top-gating, leading to the construction of dome-shaped phase diagrams that reveal distinct influences of the gate configuration on normal and superconducting states. In another experiment, tunneling spectroscopy of AlOx/KTaO3 junctions identifies two superconducting gaps, consistent with the multiband electronic structure observed experimentally. We investigate the symmetry of the gap, and show that it is compatible with unconventional pairing mechanisms enabled by the interactions at play at this interface.

These results collectively establish KTaO3-based interfaces as a versatile and tunable platform for next-generation spintronic and quantum devices. The ability to electrically control both spin-orbit coupling and superconductivity, combined with the compatibility of these systems with established semiconductor processes, positions oxide electronics at the forefront of the beyond-CMOS paradigm. The integration of these materials into functional devices could enable energy-efficient computing and robust quantum information processing, addressing the pressing challenges of modern electronics.