Keldysh pseudo-fermion renormalization group for quantum magnetism

The functional renormalization group (FRG) approach for spin models relying on a pseudo-fermionic description has proven to be a powerful technique in simulating ground state properties of strongly frustrated magnetic lattices. However, the FRG as well as many other theoretical models, suffer from the fact that they are formulated in the imaginary-time Matsubara formalism and thus are only able to predict static correlations directly. Nevertheless, describing the dynamical properties of magnetic systems is a theoretical challenge, as they are the key to bridging the gap to neutron scattering experiments. We remedy this shortcoming by establishing a methodical approach based on the Keldysh formalism, originally developed to handle non-equilibrium physics.

This approach allows for calculating the dynamic properties of spin systems on arbitrary lattices. We can identify the correct low-energy dynamic spin structure factors for examplary Heisenberg systems and the Kitaev Honeycomb model.