Crossover from the Haldane state to the singlet dimer state in random alternating Heisenberg chains formed in (CH3)(2)CHNH3 Cu(ClxBr1-x)(3)

The ground states of the ferromagnetic-antiferromagnetic alternating chains (CH3)(2)CHNH3CUCl3 (abbreviated as IPACuCl(3)) and the antiferromagnetic-a ntiferromagnetic alternating chains IPACuBr(3) are recognized as the Haldan e state and the singlet dimer state. respectively. We are interested in how the ground state changes in IPACu(ClxBr1-x)(3) when x is varied. From the magnetic susceptibility and specific heat measurements, the mixed systems f or 0.83 greater than or equal to x greater than or equal to 0.56 were found to lose the energy gap even at H = 0. We think that the gapless state is c aused by the randomness of the intrachain exchange interactions. (C) 2001 E lsevier Science B.V. All rights reserved.