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Phase diagram of the<i>XXZ</i>chain with next-nearest-neighbor interactions
We calculate the quantum phase diagram of the {\it XXZ} chain with nearest-neighbor (NN) $J_{1}$ and next-NN exchange $J_{2}$ with anisotropies $\Delta_{1}$ and $\Delta_{2}$ respectively. In particular we consider the case $\Delta_{1}=-\Delta_{2}$ to interpolate between the {\it XX} chain ($% \Delta_{i}=0$) and the isotropic model with ferromagnetic $J_{2}$. For $% …