Thermodynamics of a one-dimensional frustrated spin-1/2 Heisenberg   ferromagnet
  We calculate the thermodynamic quantities (correlation functions <S_0S_n>, correlation length xi, spin susceptibility chi, and specific heat C_V) of the frustrated one-dimensional spin-half J2-J2 Heisenberg ferromagnet, i.e. for J2< 0.25|J1|, using a rotation-invariant Green's-function formalism and full diagonalization of finite lattices. We find that the critical indices are not changed by J2, i.e., chi = y_0 T^{-2} and xi = x_0 T^{-1} at T \to 0. However, the coefficients y_0 and x_0 linearly decrease with increasing J2 according to the relations y_0=(1-4J_2/|J_1|)/24 and x_0 =(1-4J_2/|J_1|)/4, i.e., both coefficients vanish at J2=0.25|J1| indicating the zero-temperature phase transition that is accompanied by a change of the low-temperature behavior of chi (xi) from chi \propto T^{-2} (xi \propto T^{-1}) at J2 < 0.25|J1| to chi \propto T^{-3/2} (xi \propto T^{-1/2}) at J2 = 0.25|J1|. In addition, we detect the existence of an additional low-temperature maximum in the specific heat when approaching the critical point at J2=0.25|J1|. 
M. Haertel, J. Richter, D. Ihle, S.-L. Drechsler