Dynamical spin structure factors of quantum spin nematic states

Details Dynamical spin structure factors of quantum spin nematic states Dynamical spin structure factors of quantum spin nematic states

2011-09-29

arxiv.org/abs/1109.6464v1

Physics

Condensed Matter

Strongly Correlated Electrons

14 pages, 9 figures

Scientific paper

Dynamical spin structure factors of quantum spin nematic phase are studied using the large-N loop expansion in the spin-1/2 square-lattice J1-J2 model with ferromagnetic J1 and competing antiferromagnetic J2. As the starting mean-field state, we employ a spin-triplet pairing state of spinon fields, called Z2 planar state, which was found as a large-N saddle point solution of the fermionic mean-field analysis [R. Shindou and T. Momoi, Phys. Rev. B 80, 064410 (2009)]. Using the standard large-N expansion, we take into account the fluctuation within 1-loop level, which is equivalent to the random phase approximation. The spin structure factors thus obtained signifies the existence of the gapless k-linear spin-wave modes at q=(0,0) and that at q=(0,pi). The spectral weight of the former gapless modes vanish as linear functions of the momentum, while the latter mode has essentially no spectral weight in the dynamical spin structure factors. We also observed that the first gapped mode at q=(pi,pi), which can be observed in the longitudinal dynamical spin-structure factor, is a `Higgs boson' associated with the Z2 state. Namely, with decreasing J2, the mass of the `Higgs boson' vanishes at a critical value J2/J1=Jc2, so that the state reduces to the U(1) planar state. We found that, when this happens, other gapped magnetic modes at q=(pi,pi) simultaneously exhibit instabilities. As a result, the U(1) planar phase not only breaks the translational symmetries of the square lattice, but also it breaks a staggered U(1) spin-rotational symmetry which is possessed by the Z2 planar state.

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