Many unconventional superconductors have been discovered since the 1980’s. In some of them, it is suspected that an unconventional spin-triplet pairing occurs, where the Cooper pairs have a spin S = 1. Such materials have attracted a lot of interest because they may host interesting topological properties. The order parameter of a spin-triplet superconductor is not described by a scalar as in the singlet case, but by a vector containing the 3 different triplet components, Sz = –1,0,1. The possible spin-triplet phases, i.e., the allowed such vectors, depend on the crystal symmetry and the strength of the spin-orbit (SO) coupling of the material. Theoretically, they are typically studied either in the absence of SO coupling or when SO coupling is dominant. However, oftentimes real materials possess intermediate strengths of SO coupling. To understand the properties of the triplet superconductivity in that situation is particularly important if one wants to study the spin dynamics.
Surprisingly little is known about the spin dynamics of superconductors. In contrast, collective spin modes have played a central role in the study of superfluid 3He, where they have been shown to be a powerful probe of the pairing state. Though measuring the spin dynamics in superconductors is challenging, new materials and improved measurement techniques bring it into experimental reach.
The aim of the internship is to develop a minimal multiband model of a spin-triplet superconductor that allows one to study the crossover from weak to strong SO coupling. The model will be inspired by CdRh2As3, a material in which a field-induced triplet phase was recently discovered [1,2]. The study will pave the way for computing the dynamic spin susceptibility and identifying possible resonances. In the longer term, the insights gained form the study will allow us to construct simpler phenomenological models in order to compute observables.
The project will be performed mainly by using the analytical tools of condensed matter field theory. Interested candidates should have a good basis in quantum mechanics, statistical physics, and solid-state physics. A PhD may follow.