The effects of nonlinear couplings and external magnetic field on the   thermal entanglement in a two-spin-qutrit system
  We investigate the effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system by applying the concept of negativity. It is found that the nonlinear couplings favor the thermal entanglement creating. Only when the nonlinear couplings $|K|$ are larger than a certain critical value does the entanglement exist. The dependence of the thermal entanglement in this system on the magnetic field and temperature is also presented. The critical magnetic field increases with the increasing nonlinear couplings constant $|K|$. And for a fixed nonlinear couplings constant, the critical temperature is independent of the magnetic field $B$. 
Guo-feng Zhang, Shu-shen Li