Combining bond particles with the Gutzwiller approximation: A variational theory of the Kondo lattice

We investigate the two-dimensional Kondo lattice, the minimal model of a heavy fermion compound. Despite its apparent simplicity, its dynamics are both complex and controversial. In the moderate-to-strong coupling regime, the bond fermion theory provides an approximate mapping to an easily solvable fermionic system. This works surprisingly well from a qualitative standpoint, but there are some shortcomings as the method has no precise mathematical foundation. We here try to address this and improve on the method by making an explicit connection to wave function based variational methods and the Gutzwiller approximation. We apply the improved method to both the previously studied N\'eel magnetic phases, as well as incommensurately and ferromagnetically ordered phase.