Ab initio description of multicomponent superconductivity in bulk to atomically thin materials
7 May 2018
Campus Groenenborger, U.024 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger
Organization / co-organization:
Department of Physics
Milorad Milošević, Bart Partoens
PhD defence Jonas Bekaert - Faculty of Science- Department of Physics
Doctoraatsverdediging Jonas Bekaert - Faculteit Wetenschappen - Departement Fysica
In this thesis, an ab initio description of superconducting condensates consisting of multiple components is developed. Such multicomponent superconductivity can originate from a multiband electronic structure, from spin degrees of freedom, additional interactions, and so on. The description starts from a full characterization of the structural, electronic and vibrational properties of a material, obtained from density functional theory.
This is coupled here to a quantum-field theory of the superconducting state mediated by the electron-phonon interaction, called Eliashberg theory. This methodology enables the discovery of new properties of multicomponent superconductors of sizes ranging from bulk to atomically thin.
First, we explore how band condensates stemming from distinct electronic bands of compound superconductors are coupled, and how this affects the temperature evolution of the superconducting state, and its interaction with an applied magnetic field. Subsequently, a novel approach is elaborated to describe systems hosting not only lattice vibrations but also spin fluctuations, as a result of a competing magnetic state. This provides new insights into multicomponent superconductivity in recently discovered iron-based superconductors, where superconductivity is proven to be conventional yet strongly depleted due to ferromagnetic spin fluctuations.
In the second part of this thesis multicomponent superconductivity in atomically thin materials is investigated. Here, the superconducting spectrum is found to be enriched by emergent surface states, leading to the discovery of a three-gap superconducting state in a monolayer material, which can be profoundly changed by the addition of extra layers. The critical temperature of this new monolayer three-gap superconductor is relatively high, owing to multigap effects, and is shown to be enhanced further by means of strain and adatoms.
Finally, atomically thin materials are investigated where the superconducting state coexists with other novel quantum states, leading to new physics emerging from the interplay between the states. This thesis thus contributes to a better understanding of the role of atomic-scale interactions in emergent multicomponent superconductivity, and their evolution with dimensionality.