  Frustrated quantum magnets pose well-defined questions concerning quantum
fluctuation effects and the nature of the many-body wavefunction, which
challenge theory, numerics, experiment and materials synthesis. The S = 1/2
triangular-lattice antiferromagnet (TLAF) presents a case where classical order
is strongly suppressed by quantum fluctuations, leading to extensive
renormalization of physical properties at all energy scales. However, purely
two-dimensional (2D) models are difficult to realise in the 3D world and their
physics is controlled by the Mermin-Wagner theorem, which describes the
dominant effects of additional thermal fluctuations. Here we report the
magnetic properties Ba8CoNb6O24, whose Co2+ions have an effective spin 1/2 and
construct a regular TLAF with very large interlayer spacing. We find no
magnetic ordering down to 0.028 K, strong low-energy spin fluctuations in
qualitative agreement with theoretical analysis and a diverging correlation
length, all indicating a Mermin-Wagner trend towards zero-temperature ordering
in this ideal 2D system.
