Experimental Approach to the Thermodynamics of the Pure Two-Dimensional   Spin-1/2 Triangular Lattice Antiferromagnet in Ba8CoNb6O24
  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. 
Y. Cui, J. Dai, P. Zhou, P. S. Wang, T. R. Li, W. H. Song, L. Ma, Z.   Zhang, S. Y. Li, G. M. Luke, B. Normand, T. Xiang, an...
cond-mat.str-el cond-mat.supr-con