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 J2 in the parameter range 0< J2/ | J 1| <0.4. When J2 / | J1 | <1/4, the model is in a Tomonaga-Luttinger liquid phase which is adiabatically connected to the critical phase of the XXZ model of J2=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 Néel or dimer order. Interestingly, these two types of ordered phases appear alternately as the easy-plane anisotropy is changed toward the isotropic limit. The appearance of the antiferromagnetic (Néel) order in this model is remarkable, as it is strongly unfavored by both the easy-plane ferromagnetic NN coupling and the 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 Néel order.
|ジャーナル||Physical Review B - Condensed Matter and Materials Physics|
|出版ステータス||Published - 2010 3月 26|
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