Relativistic interaction Hamiltonian coupling the angular momentum of light and the electron spin

Abstract

The coupling of the electric and magnetic degrees of freedom in condensed matter systems has long been an area of basic and applied research. In a 2013 PRL (PRL 110, 137205), a coupling energy term between the electromagnetic angular momentum density and the magnetization of a material was proposed. In this PRB, the authors provide a direct link between the proposed coupling term and the Dirac equation. They demonstrate that the Dirac equation leads to an interaction Hamiltonian (which they refer to as ``angular magnetoelectric'', or AME, coupling Hamiltonian) that couples linearly the electron spin and the electromagnetic field's angular momentum. They show that the AME coupling Hamiltonian has an expectation value that for some materials (e.g., ferromagnets) is equal to the recently proposed energy coupling, and that it contributes to the Inverse Faraday Effect. The AME coupling Hamiltonian may lead to the prediction and understanding of novel magnetoelectric and magneto-optical phenomena.