We consider the dimerized spin-1 XXZ chain with single-ion anisotropy D. In absence of an explicit dimerization there are three phases: a large-
D, an antiferromagnetically ordered and a Haldane phase. This phase structure persists up to a critical dimerization, above which the Haldane phase disappears. We show that for weak dimerization the phases are separated by Gaussian and Ising quantum phase transitions. One of the Ising transitions terminates in a critical point in the universality class of the dilute Ising model. We comment on the relevance of our results to experiments on quasi-one-dimensional anisotropic spin-1 quantum magnets.
We study the spin-1/2 chain with nearest-neighbor (${\mathrm{\ensuremath{\kappa}}}_{1}$) and next-nearest-neighbor (${\mathrm{\ensuremath{\kappa}}}_{2}$) interactions in the regime ${\mathrm{\ensuremath{\kappa}}}_{2}$\ensuremath{\gg}:${\mathrm{\ensuremath{\kappa}}}_{1}$, which is equivalent to two chains with a ``zigzag'' interaction. In the continuum limit, this system is described in terms of two coupled level-1 Wess-Zumino-Witten field theories. We illustrate its equivalence with four off-critical Ising models (Majorana fermions). This description is used to investigate the opening of a gap as a function of ${\mathrm{\ensuremath{\kappa}}}_{1}$ and the associated spontaneous breakdown of parity. We calculate the dynamic spin structure factor near the wave vectors accessible to the continuum limit. We comment on the nonzero string order parameter and show the presence of a hidden
Z2\ifmmode\times\else\texttimes\fi{}
Z2 symmetry via a nonlocal transformation on the microscopic Hamiltonian. For a ferromagnetic interchain coupling, the model is conjectured to be critical, with different velocities for the spin-singlet and spin-triplet excitations.