EFFECT OF RANDOMNESS OF EXCHANGE INTERACTIONS IN QUANTUM SPIN SYSTEMS WITH ENERGY GAP

同构化合物（CH_3）CHNH_3CuCl_3和（CH_3）_2CHNH_3CuBr_3分别由S =1/2的铁磁-铁磁和非铁磁-反铁磁交替海森堡链组成，故前者的磁基态为Haldane态，后者的磁基态为单线态二聚体。为了阐明键的随机性对Haldane态和单线二聚体态的影响，研究了随机交替海森堡链（CH_3）CHNH_3Cu(Gl_xBr_1-x)_3即上述两种化合物的混合体系的磁态。通过磁化率和比热测量，证实了在中间区域0.44<x<0,87上存在无间隙磁相。在两个边界处，即Haldane相与无间隙相和单线态二聚体相之间的边界处，能量间隙突然变为零。本研究表明，当键随机性强度超过一定值时，键随机性会抹去霍尔丹态和单线态二聚体的能隙。磁化过程的测量也显示了在0.44<x<0,87以上的能隙的出现。

The isomorphous compouns (CH_3)CHNH_3CuCl_3 and (CH_3)_2CHNH_3CuBr_3 consists of ferromagnetic-ferromagnetic and aritiferromagnetic anatiferromagnetic alternating Heisenberg chains with S =1/2, respectively The magnetic ground state is therefore regarded as the Haldane state in the former and the singlet dimer state in the latter. To clarify the effect of bond randomness on the Haldane state as well as on the singlet dimer state, the magnetic state of the random alternating Heisenberg chains (CH_3)CHNH_3Cu(Gl_xBr_1-x)_3, I.e., mixed systems of The two compounds mentioned above, were investigated From magnetic susceptibility and specific heat measurements, it was confirmed that a gapless magnetic phase appeals over the intermediate region 0.44<x<0,87. At the two boundaries, I.e, the boundary between the Haldane phase and the gapless phase and that between the gapless phase and the singlet dimer phase, the energy gap abruptly becomes zero. This study clarified that bond randomness wipes but the energy gap in both the Haldane state and the singlet dimer state, when the strength of bond randomness exceeds a certain value. Measurements of magnetization process also showed the appearance of the energy gap over 0.44<x<0,87.