Magnon Bose-Einstein condensation and various phases of three-dimensonal quantum helimagnets under high magnetic field

We study high-field phase diagram and low-energy excitations of three-dimensional (3D) quantum helimagnets. Slightly below the saturation field, the emergence of magnetic order may be viewed as Bose-Einstein condensation (BEC) of magnons. The method of dilute-Bose-gas enables a quantitative analysis of quantum effects in these helimagnets and thereby three phases are found: cone, coplanar fan, and a phase-separated one. As an application, we map out the phase diagram of a 3D helimagnet which consists of frustrated J(1)-J(2) chains as a function of frustration and an interchain coupling. Moreover, we also calculate the stability of the two-magnon bound state to investigate the possibility of the bound-magnon BEC.