Magnon BEC and various phases of 3D quantum helimagnets under high magnetic field

We study high-field phase diagram and low-energy excitations of three-dimensional 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 J1-J2 chains as a function of frustration and an interchain coupling. Moreover, we also calculate the stability of the 2-magnon bound state to investigate the possibility of the bound-magnon BEC.