Bose–Einstein Condensation of Quasi-Two-Dimensional Frustrated Quantum Magnet (CuCl)LaNb2O7

Abstract

A quasi-two-dimensional S = 1/2 Heisenberg square-lattice antiferromagnet (CuCl)LaNb 2 O 7 is studied by specific heat and magnetic susceptibility measurements in external fields up to 14 T. 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 10 T 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.