Magnetization process and collective excitations in the s=1/2 triangular-lattice heisenberg antiferromagnet Ba₃CoSb ₂O₉

We have performed high-field magnetization and electronic spin resonance (ESR) measurements on Ba₃CoSb₂O₉ single crystals, which approximates the two-dimensional (2D) S=1/2 triangular-lattice Heisenberg antiferromagnet. For an applied magnetic field H parallel to the ab plane, the entire magnetization curve including the plateau at one-third of the saturation magnetization (M_s) is in excellent agreement with the results of theoretical calculations except a small step anomaly near (3/5)M _s, indicative of a theoretically undiscovered quantum phase transition. However, for Hâ̂¥c, the magnetization curve exhibits a cusp near M_s/3 owing to the weak easy-plane anisotropy and the 2D quantum fluctuation. From a detailed analysis of the collective ESR modes observed in the ordered state, combined with the magnetization process, we have determined all the magnetic parameters including the interlayer and anisotropic exchange interactions.