  The perovskite Ba8CoNb6O24 comprises equilateral effective spin-1/2 Co2+
triangular layers separated by six non-magnetic layers. Susceptibility,
specific heat and neutron scattering measurements combined with
high-temperature series expansions and spin-wave calculations confirm that
Ba8CoNb6O24 is basically a twodimensional (2D) magnet with no detectable spin
anisotropy and no long-range magnetic ordering down to 0.06 K. In other words,
Ba8CoNb6O24 is very close to be a realization of the paradigmatic spin-1/2
triangular Heisenberg model, which is not expected to exhibit symmetry breaking
at finite temperature according to the Mermin and Wagner theorem.
