Experimental Realization of Spin-1/2 Triangular-Lattice Heisenberg Antiferromagnet

We report the results of magnetization and specific heat measurements on Ba${}_3$CoSb${}_2$O${}_9$, in which the magnetic Co${}^{2+}$ ion has a fictitious spin-1/2, and show evidence that a spin-1/2 Heisenberg antiferromagnet on a regular triangular lattice is actually realized in Ba${}_3$CoSb${}_2$O${}_9$. We found that the entire magnetization curve including the one-third quantum magnetization plateau is in excellent agreement with theoretical calculations at a quantitative level.