The ground-state properties of the spin-1/2 Heisenberg chain, which has two exchange couplings (J>J') alternatingly, are studied. This model interpolates the spin-1 antiferromagnetic Heisenberg chain (AFHC) (J'\ensuremath{\rightarrow}-\ensuremath{\infty}, J>0), the dimerized spin-1/2 AFHC (J'\ensuremath{\simeq}J>0), and the uniform spin-1/2 AFHC (J'=J>0) continuously. The string order introduced by den Nijs--Rommelse and Tasaki changes continuously through the Haldane phase and the dimer phase to vanish at J=J'. It is at a maximum for J'=0. This implies that the string order is related to the local singlet correlation in terms of the spin-1/2 model. The lowest excitation can be regarded as the renormalized triplet wave. The reversal of the anisotropy effect on the excitation spectrum of the spin-1 AFHC is also explained in a intuitive way based on this picture.
The isomorphous compounds
(CH3)2CHNH3CuCl3 and
(CH3)2CHNH3CuBr3 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
0.44<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 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.