Quantitative Functional RG Approach to Spin Hamiltonians at Finite Temperatures*

The pseudo-fermion functional renormalization group (pf-fRG) has been successfully applied to study frustrated magnets and identify several spin liquid candidates. It can be used to calculate momentum-resolved spin susceptibilities that can be directly compared to neutron scattering experiments. The method offers a straightforward way to implement infinite lattices with long-range interactions and complicated lattice geometries. However, its applicability has been limited to zero temperature. We develop a generalized pf-fRG method based on the Popov-Fedotov trick that produces quantitatively accurate results for spin Hamiltonians at finite temperature. We show that we can accurately detect finite-temperature phase-transitions by a finite size scaling approach and recover the correct critical exponents.