Unconventional superconductivity studies on various superconducting devices

Abstract

This project is an experimental study of novel unconventional superconductivity observed in different devices. In the first part, we demonstrate that an unconventional superconducting state can be observed in monolayer NbSe₂ of presumably topological nature. Ising superconductivity in transition metal dichalcogenides (TMD) such as 2H-NbSe₂ allows them to sustain superconductivity under high magnetic field above the Pauli limit with quasiparticle spins pinned firmly to the direction perpendicular to the basal plane. We use field-angle-resolved magnetoresistance experiments for fields applied in different directions strictly parallel to the basal plane to measure the symmetry of the superconducting order parameter. The results agrees with theoretical predictions of the combination of a nodal topological superconducting phase and a nematic superconducting phase near the upper critical field, a six-fold symmetry superposed on a two-fold symmetry. We also measured six-layer NbSe₂ grown on hexagonal boron nitride, for which there is no anisotropy, suggesting that the anisotropy occurs solely in monolayer samples. Another measurement of NbSe₂ was done on a bulk device. It shows a 4-fold symmetry which does not agree with our expectations and therefore stimulates further experiments under more controlled conditions. In the second part, TaNb_0.6ZrHfTi_0.4 has been measured. The experimental results show that this high-entropy alloy superconductor is an isotropic s-wave superconductor

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Rolf Walter LORTZ (Supervisor)