Bose-Einstein Condensation of Magnons inCs2CuCl4

We report on results of specific heat measurements on single crystals of the frustrated quasi-2D spin-$1/2$ antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ ($T_N=0.595\mathrm{K}$) in external magnetic fields $B<12\mathrm{T}$ and for temperatures $T>30\mathrm{mK}$. Decreasing $B$ from high fields leads to the closure of the field-induced gap in the magnon spectrum at a critical field $B_c\simeq 8.51\mathrm{T}$ and a magnetic phase transition is clearly seen below $B_c$. In the vicinity of $B_c$, the phase transition boundary is well described by the power law $T_c(B)\propto (B_c-B{)}^{1/\varphi }$, with the measured critical exponent $\varphi \simeq 1.5$. These findings are interpreted as a Bose-Einstein condensation of magnons.