Seminarium Fizyki Materii Skondensowanej

Quantum spin liquids are states of matter with non-trivial patterns of quantum entanglement which feature remarkable properties, such as the emergence of particles with anyonic statistics. While the existence of these states has been theoretically anticipated for many decades since the visionary ideas of Phillip Anderson, a new wave of interest has been triggered by the exactly solvable honeycomb model of Alexei Kitaev which has a quantum spin liquid state with two kinds of emergent particles: an itinerant fermion (known as the spinon) and a vortex-like particle (known as the vison) which has non-abelian anyon exchange statistics. This model is approximately realized in alpha-RuCl_3, but understanding this material and its connection to the ideal Kitaev model has remained elusive partly because of our poor understanding of the impact of perturbations that destroy its exact solvability. In this talk, I will present our understanding of the impact of a Zeeman field on the Kitaev model. By analyzing in detail the behavior of its emergent anyon particles, we will see that there are very different fates for the ferromagnetic and anti-ferromagnetic Kitaev models. The ferromagnetic Kitaev spin liquid likely undergoes a single continuous phase transition directly into the ordinary spin-polarized state as the Zeeman field increases. We will see, however, that the anti-ferromagnetic Kitaev spin liquid likely goes through a series of phase transitions through other non-trivial intermediate quantum spin liquids before transitioning into the trivial spin-polarized state as the Zeeman field increases. We will critically discuss the possible connections of these results to experiments in alpha-RuCl_3.