Appearance of a gapless phase over the intermediate region between the Haldane phase and the singlet dimer phase in random alternating Heisenberg chains(CH3)2CHNH3Cu(ClxBr1−x)3

Abstract

The isomorphous compounds $({\mathrm{CH}}_{3}{)}_{2}{\mathrm{CHNH}}_{3}{\mathrm{CuCl}}_{3}$ and $({\mathrm{CH}}_{3}{)}_{2}{\mathrm{CHNH}}_{3}{\mathrm{CuBr}}_{3}$ consist of ferromagnetic-antiferromagnetic and antiferromagnetic-antiferromagnetic alternating Heisenberg chains with $S=1/2,$ respectively. The 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 states of the random alternating Heisenberg chains $({\mathrm{CH}}_{3}{)}_{2}{\mathrm{CHNH}}_{3}\mathrm{Cu}({\mathrm{Cl}}_{x}{\mathrm{Br}}_{1\ensuremath{-}x}{)}_{3},$ 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 phase appears over the intermediate region $0.44lxl0.87.$ 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 clarifies that bond randomness wipes out the energy gap in both the Haldane state and the singlet dimer state, when the strength of bond randomness exceeds a certain value.