Phase diagrams and critical behavior of the quantum spin-1/2 XXZ model   on diamond-type hierarchical lattices
  In this paper, the phase diagrams and the critical behavior of the spin-1/2 anisotropic XXZ ferromagnetic model (the anisotropic parameter {\Delta}\in(-\infty,1]) on two kinds of diamond-type hierarchical (DH) lattices with fractal dimensions d_{f}=2.58 and 3, respectively, are studied via the real-space renormalization group method. It is found that in the isotropic Heisenberg limit ({\Delta}=0), there exist finite temperature phase transitions for the two kinds of DH lattices above. The systems are also investigated in the range of -\infty<{\Delta}<0 and it is found that they exhibit XY-like fixed points. Meanwhile, the critical exponents of the above two systems are also calculated. The results show that for the lattice with d_{f}=2.58, the value of the Ising critical exponent {\nu}_{I} is the same as that of classical Ising model and the isotropic Heisenberg critical exponent {\nu}_{H} is a finite value, and for the lattice with d_{f}=3, the values of {\nu}_{I} and {\nu}_{H} agree well with those obtained on the simple cubic lattice. We also discuss the quantum fluctuation at all temperatures and find the fluctuation of XY-like model is stronger than the anistropic Heisenberg model at the low-temperature region. By analyzing the fluctuation, we conclude that there will be remarkable effect of neglecting terms on the final results of the XY-like model. However, we can obtain approximate result at bigger temperatures and give qualitatively correct picture of the phase diagram. 
Xiu-Xing Zhang, Xiang-Mu Kong, Zhong-Yang Gao, Xiao-Song Chen
cond-mat.stat-mech