Thermodynamic Properties of Spin Ladders with Cyclic Exchange
  By high temperature series expansion and exact complete diagonalization the magnetic susceptibility chi(T) and the specific heat C(T) of a two-leg S=1/2 ladder with cyclic (4-spin) exchange are computed. Both methods yield convincing results for not too small temperatures. We find that a small amount of cyclic exchange influences the thermodynamical properties significantly. Our results can serve as reliable basis for an efficient analysis of experimental data 
Alexander Buehler, Ute Loew, Kai P. Schmidt, Goetz S. Uhrig