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arXiv 0710.0463cond-mat.str-el
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自旋为1/2的四聚J_1-J_2模型的基态phase diagram和磁性:束缚磁振子的BEC和无横向磁化

Ground-state phase diagram and magnetic properties of a tetramerized spin-1/2 J_1-J_2 model: BEC of bound magnons and absence of the transverse magnetization

H. T. Ueda, K. Totsuka

摘要:

我们用不同的方法研究了自旋为1/2的具有元格结构的$J_1$ - $J_2$模型的基态和磁化过程.对于小的格间相互作用,该模型预计有一个自旋间隙,我们计算了第一和第二激发能。如果最低激发态的能隙闭合,则相应的粒子凝聚形成磁序。通过对五重态能隙和五重态激发间磁相互作用的分析,发现由于强阻挫和量子效应,在$J_1/J_2\sim -2$附近存在自旋-自旋相。当施加高磁矩时,不是自旋1激发而是自旋2激发软化并决定磁化过程。我们应用平均场近似的有效哈密顿量找到三种不同类型的阶段(传统的BEC阶段,“条纹”超固相和1/2平台)。与自旋二聚体系统中的BEC不同,该BEC相不伴有横向磁化。可能的连接到最近发现的自旋隙化合物(CuCl)LaNb_2O_7进行了讨论。

英文摘要:

We study the ground state and the magnetization process of a spin-1/2 $J_1$-$J_2$ model with a plaquette structure by using various methods. For small inter-plaquette interaction, this model is expected to have a spin-gap and we computed the first- and the second excitation energies. If the gap of the lowest excitation closes, the corresponding particle condenses to form magnetic orders. By analyzing the quintet gap and magnetic interactions among the quintet excitations, we find a spin-nematic phase around $J_1/J_2\sim -2$ due to the strong frustration and the quantum effect. When high magnetic moment is applied, not the spin-1 excitations but the spin-2 ones soften and dictate the magnetization process. We apply a mean-field approximation to the effective Hamiltonian to find three different types of phases (a conventional BEC phase, ``striped'' supersolid phases and a 1/2-plateau). Unlike the BEC in spin-dimer systems, this BEC phase is not accompanied by transverse magnetization. Possible connection to the recently discovered spin-gap compound (CuCl)LaNb2O7 is discussed.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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