Bose-Einstein condensation of quasi-two-dimensional frustrated quantum magnet (CuCl)LaNb2O7

A quasi-two-dimensional S = 1/2 Heisenberg square-lattice antiferromagnet (CuCl)LaNb2O7 is studied by specific heat and magnetic susceptibility measurements in external fields up to 14T. The experimental results in low fields verify the absence of the order-disorder transition, as reported previously. By further application of magnetic fields, we obtain direct evidence for the phase transition, which can be interpreted as the Bose-Einstein condensation (BEC) of magnons. However, the critical field of 10T significantly deviates from that estimated from the zero-field gap, indicating that, unlike known experimental examples, the one-magnon mode is not a primary source to drive the BEC.