1205.0977 (Mohammad Merdan et al.)

Longitudinal excitations in triangular lattice antiferromagnets    [PDF]

Mohammad Merdan, Y. Xian

We study the longitudinal excitations of quantum antiferromagnets on a triangular lattice by a recently proposed microscopic many-body approach based on magnon-density waves. We calculate the full longitudinal excitation spectra of the antiferromagnetic $XXZ$ Heisenberg model for a general spin quantum number in the isotropic limit. Similar to the square lattice model, we find that, at the center of the first hexagonal Brillouin zone $\Gamma(\mathbf q=0)$ and at the magnetic ordering wavevectors $\pm[\mathbf Q= (4\pi/3,0)]$, the excitation spectra become gapless in the thermodynamic limit, due to the slow, logarithmic divergence of the structure factor. However, these longitudinal modes on two-dimensional models may be considered as quasi-gapped, as any finite-size effect or small anisotropy will induce a large energy gap, when compared with the counterpart of the transverse spin-wave excitations. We also find that the triangular lattice longitudinal mode is in general softer, with smaller energy gap values, than that of the square lattice model due to the frustrations in the triangle lattice system.

View original: http://arxiv.org/abs/1205.0977