Crossover from the Haldane state to the singlet dimer state in random alternating Heisenberg chains formed in (CH3)2CHNH3Cu(Cl Br1−)3

Abstract The ground states of the ferromagnetic–antiferromagnetic alternating chains (CH 3 ) 2 CHNH 3 CuCl 3 (abbreviated as IPACuCl 3 ) and the antiferromagnetic–antiferromagnetic alternating chains IPACuBr 3 are recognized as the Haldane state and the singlet dimer state, respectively. We are interested in how the ground state changes in IPACu(Cl x Br 1− x ) 3 when x is varied. From the magnetic susceptibility and specific heat measurements, the mixed systems for 0.83⩾ x ⩾0.56 were found to lose the energy gap even at H =0. We think that the gapless state is caused by the randomness of the intrachain exchange interactions.