Crossed spin-1/2 Heisenberg chains as a quantum impurity problem
  Using equivalencies between different models we reduce the model of two spin-1/2 Heisenberg chains crossed at one point to the model of free fermions. The spin-spin correlation function is calculated by summing the perturbation series in the interchain coupling. The result reveals a power law decay with a nonuniversal exponent. 
S. A. Reyes and A. M. Tsvelik