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arXiv 1111.0361cond-mat.stat-mech
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Phase diagrams和量子自旋1/2 XXZ模型在钻石型等级晶格上的临界行为

Phase diagrams and critical behavior of the quantum spin-1/2 XXZ model on diamond-type hierarchical lattices

Xiu-Xing Zhang, Xiang-Mu Kong, Zhong-Yang Gao, Xiao-Song Chen

摘要:

利用实空间重整化群方法,研究了自旋为1/2的各向异性XXZ铁磁模型(各向异性参数Δ \in(- \infty,1])在分维分别为d_{f} =2.58和3的两种金刚石型分级(DH)晶格上的相图和临界行为.发现在各向同性Heisenberg极限(Δ=0)下,上述两种DH晶格都存在有限温度的相变.在- \infty <Δ<0的范围内研究了该系统,发现它们具有XY型不动点.同时,还计算了上述两个体系的临界指数。结果表明,对于d_{f} =2.58的晶格,Ising临界指数ν {I}与经典伊辛模型的值相同,各向同性Heisenberg临界指数ν {H}为有限值;对于d_{f} =3的晶格,ν {I}和ν {H}的值与简单立方晶格的结果一致.我们还讨论了各温度下的量子涨落,发现在低温区类XY模型的量子涨落比各向异性海森堡模型的量子涨落强。通过对涨落的分析,我们得出忽略项对类XY模型的最终结果有显著影响的结论。然而,我们可以在较大的温度下得到近似的结果,并给出定性正确的相图。

英文摘要:

In this paper, the phase diagrams and the critical behavior of the spin-1/2 anisotropic XXZ ferromagnetic model (the anisotropic parameter Δ\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 (Δ=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<Δ<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 ν_{I} is the same as that of classical Ising model and the isotropic Heisenberg critical exponent ν_{H} is a finite value, and for the lattice with d_{f}=3, the values of ν_{I} and ν_{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.

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