Competition and Interplay Between Magnetism and Superconductivity

Abstract

This dissertation is composed of nine publications and this introduction, which outlines the theories of superconductivity, magnetism and non-equilibrium physics that are applied in the publications. The publications can be divided into two bodies which share some common themes. The publications of the first body deal with the physics of flat electronic bands, and in particular the superconducting and magnetic phase transitions on them. In the related part of the introduction part I present the basic theories of superconductivity and magnetism, and discuss the properties which make the flat bands prone to spontaneous symmetry breaking. We formulate the theory of electronphonon superconductivity on a flat band model, and show how its predictions differ from those of a simpler BCS model. Of the materials which have flat bands, we are particularly interested in those based on graphene e.g., twisted bilayer graphene, for which we show that the conventional BCS theory of superconductivity based on the attractive electron-phonon interaction is compatible with the experimental observations. Flat bands are often enabled by some topological property. We classify the topological transitions on a system of rhombohedrally stacked honeycomb lattices. The publications of the second body are about superconducting spintronics. One central theme in these publications is the magnetic proximity effect and its various application. We propose that a magnetically proximitized superconductor can be used as the functional unit of a new kind of thermoelectric radiation detector. A second theme is the effect of superconductivity on the spin pumping effect. This is studied in four publications, in which we predict e.g. a cooling effect due to precessing magnetization, a giant spin battery effect, and an antiferromagnetic coupling between two magnets mediated by spin supercurrent. A third theme is the Higgs mode i.e. the amplitude mode of the superconducting order parameter. The magnetic proximity effect enables a new coupling between a charge degree of freedom and the Higgs mode, which can be utilized to measure it electrically. We also study the magnon-Higgs coupling mediated by spin-orbit interaction. In the introductory part related to this body, I outline the Keldysh theory of non-equilibrium states and the quasiclassical theory of superconductivity, which have been heavily utilized in the publications.