Functional renormalization group for the anisotropic triangular antiferromagnet

We present a functional renormalization group scheme that allows us to calculate frustrated magnetic systems of arbitrary lattice geometry beyond $O(200)$ sites from first principles. We study the magnetic susceptibility of the antiferromagnetic (AFM) spin-$1/2$ Heisenberg model ground state on the spatially anisotropic triangular lattice, where J^' denotes the coupling strength of the intrachain bonds along one lattice direction and J the coupling strength of the interchain bonds. We identify three distinct phases of the Heisenberg model. Increasing ξ=J^'/J from the effective square lattice ξ=0, we find an AFM N\'eel order to spiral order transition at ξc1~0.6--0.7, with an indication that it is of second order. In addition, above the isotropic point at ξc2~1.1, we find a first-order transition to a magnetically disordered phase with collinear AFM stripe fluctuations.