Collective effects in low-dimensional systems with coupled quasiparticles

Abstract

Many of the most fascinating and challenging phenomena in condensed matter physics occur in systems with coupling between quasiparticles of different nature. This thesis is concerned with the study of collective effects which may occur due to coupling between electrons, magnons, and phonons in various two-dimensional systems, and is based on four research papers. In the first paper, we examine a spin model analog of the Haldane model which has a topologically non-trivial magnon band structure. We discuss the effect of coupling the topological magnons to phonons, and suggest signatures both in the transverse magnon spin Hall conductivity and through exotic magnon-polaron edge states. In the second paper, we use a tight binding approach to model electronphonon coupling in graphene, and study possible phonon-mediated superconductivity in doped graphene using a detailed model for the effective phononmediated electron-electron interaction. In the third paper, we provide a revealing physical picture for the eigenexcitations of the quantum antiferromagnet, and discuss the implications of this in various physical settings. Amongst others, we emphasize that coupling asymmetrically to the two sublattices of the antiferromagnet through an uncompensated interface may enhance the effective coupling strength to the antiferromagnetic magnons. In the fourth paper, we discuss superconductivity mediated by antiferromagnetic magnons in a heterostructure of a normal metal coupled to antiferromagnetic insulators. We find that sublattice coupling asymmetry plays an important role in determining the pairing symmetry of the superconducting phase. Using Eliashberg theory instead of BCS theory, we furthermore demonstrate the importance of a proper treatment of the frequency dependence of the effective pairing interaction for magnon-mediated superconductivity.