Conformal invariance and the phase diagram of the anisotropic Heisenberg spin-1/2 ladder

The phase diagram of two coupled anisotropic spin-1/2 Heisenberg chains forming a ladder is studied via the finite-size machinery arising from conformal invariance. Our results show that this system has similar behaviour to the anisotropic spin-1 Heisenberg chain. Both models for low values of anisotropies have a massless phase of Gaussian type, governed by a c = 1 conformal field theory. The operator that destroys the massless phase in these models is the same as in the Gaussian picture. We also verify that the massive phase in the anisotropic Heisenberg ladder has the same type of nonlocal order as that in the spin-1 Heisenberg chain.