Semiclassical expansion of the ground state for a model of interacting   spins in QED
  In this article, we consider fixed spin 1/2 particles interacting through the quantized electromagnetic field in a constant magnetic field. We give some asymptotic expansions for the ground state and the ground state energy of the Hamiltonian operator $H(h)$ describing this system. The first terms of these expansions enable to recover elementary formulas for the energy and the magnetic field of the spins when considered as magnets. A first order radiative correction is computed for the energy. 
Laurent Amour, Jean Nourrigat
math-ph math.MP