The ground state energy density 0.02 (上) 0.02 0.04 0.06 -0.08 n=1 -0.1 n=1 n=08 n=0.8 n=06 -0.12 =!=n=0.6 11n=0 -0.14 ……n=04 3 10 510 C1=100andc2=50 At the critical point, the second E=∑∑k-hM2 order derivative of the ground state i=1j=1 energy density is not continue, thus it is a second order phase transitionThe ground state energy density At the critical point, the second order derivative of the ground state energy density is not continue, thus it is a second order phase transition