Unconventional Neel and dimer orders in a spin-1/2 frustrated ferromagnetic chain with easy-plane anisotropy

We study the ground-state phase diagram of a one-dimensional spin-1/2 easy-plane XXZ model with a ferromagnetic nearest-neighbor (NN) coupling $J_1$ and a competing next-nearest-neighbor (NNN) antiferromagnetic coupling $J_2$ in the parameter range $0<J_2/|J_1|<0.4$. When $J_2/|J_1|\lesssim1/4$, the model is in a Tomonaga-Luttinger liquid phase which is adiabatically connected to the critical phase of the XXZ model of $J_2=0$. On the basis of the effective (sine-Gordon) theory and numerical analyses of low-lying energy levels of finite-size systems, we show that the NNN coupling induces phase transitions from the Tomonaga-Luttinger liquid to gapped phases with either Neel or dimer order. Interestingly, these two types of ordered phases appear alternately as the easy-plane anisotropy is changed towards the isotropic limit. The appearance of the antiferromagnetic (Neel) order in this model is remarkable, as it is strongly unfavored by both the easy-plane ferromagnetic NN coupling and antiferromagnetic NNN coupling in the classical-spin picture. We argue that emergent trimer degrees of freedom play a crucial role in the formation of the Neel order.