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

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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.