Random Bond $\mathit{XXZ}$ Chains with Modulated Couplings

The magnetization behavior of $q$-periodic antiferromagnetic spin- $1/2$ Heisenberg chains under uniform magnetic fields is investigated in a background of disorder exchange distributions. By means of both real space decimation procedures and numerical diagonalizations in $\mathrm{XX}$ chains, it is found that for binary disorder the magnetization exhibits wide plateaus at values of $1+2(p-1\mathrm{})/q$, where $p$ is the disorder strength. In contrast, no spin gaps are observed in the presence of continuous exchange distributions. We also study the magnetic susceptibility at low magnetic fields.