講師：Yasuyuki Kato (Los Alamos National Laboratory)
タイトル：Spin-1 antiferromagnets with single-ion anisotropy
I will discuss the zero-temperature phase diagrams and low-energy excitations of spin-1 antiferromagnets with a single-ion anisotropy on square and simple cubic lattices [1,2]. For easyplane anisotropy, we combine a generalized spin wave approach and large scale QMC simulations to study the nature of the different phases and quantum phase transitions. We consider two alternative approaches for describing the quantum paramagnetic state: the standard Holstein-Primakoff approximation and a modified treatment in which the local Hilbert space constraint is enforced by introducing a Lagrange multiplier. While both approximations produce qualitatively similar results, the latter approach is the only one that is in good quantitative agreement with the phase diagram and the quasiparticle dispersions obtained with QMC. For easy-axis anisotropy, we find a transition between XY-antiferromagnetic and ferronematic phases that spontaneously break the U(1) symmetry of the model. In the language of bosonic gases, this is a transition between a Bose-Einstein condensate (BEC) of single bosons and a BEC of pairs. Furthermore, we find three-magnon bound states that satisfy the Efimov scaling at the point where the two-magnon s-wave scattering length becomes infinite [3].
[1] Zhang, Yap, Wierschem, Kato, Batista, & Sengupta (in preparation).
[2] Wierschem, Kato, Nishida, Batista, & Sengupta, arXiv:1209.0688 (Phys. Rev. B).
[3] Nishida, Kato, & Batista, arXiv:1208.6214 (Nature Physics, in press).