Magnetic Properties ofS=1/2 Heisenberg Chains with Bond Alternation and Randomness on the Strong Bonds

The $S=1/2$ Heisenberg chains with bond alternation and randomness on thestrong bonds are studied by the density matrix renormalization group method. Itis assumed that the odd-th bond is antiferromagnetic with strength $J$ andeven-th bond can take the values $\JA$ and $\JF$ $ (\JA > J > 0 > \JF)$randomly. The ground state of this model interpolates between the Haldane anddimer phases via a randomness dominated intermediate phase. Based on thescaling of the low energy spectrum and mean field treatment of the interchaincoupling, it is found that the magnetic long range order is induced byrandomness in the intermediate regime. In the magnetization curves, thereappears a plateau at the fractional value of the saturated magnetization. Thefine structures of the magnetization curves and low energy spectrum areunderstood based on the cluster picture. The relation with the recentexperiment for (CH$_3)_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x})_3$ is discussed.Comment: 6 pages, 7 figures, Final version to appear in J. Phys. Soc. Jp