Bose-Einstein Condensation of Magnons in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>Cs</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>CuCl</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:math>

We report on results of specific heat measurements on single crystals of the frustrated quasi-2D spin-$1/2$ antiferromagnet ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ (${T}_{N}=0.595\text{ }\text{ }\mathrm{K}$) in external magnetic fields $B&lt;12\text{ }\text{ }\mathrm{T}$ and for temperatures $T&gt;30\text{ }\text{ }\mathrm{mK}$. Decreasing $B$ from high fields leads to the closure of the field-induced gap in the magnon spectrum at a critical field ${B}_{c}\ensuremath{\simeq}8.51\text{ }\text{ }\mathrm{T}$ 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 ${T}_{c}(B)\ensuremath{\propto}({B}_{c}\ensuremath{-}B{)}^{1/\ensuremath{\phi}}$, with the measured critical exponent $\ensuremath{\phi}\ensuremath{\simeq}1.5$. These findings are interpreted as a Bose-Einstein condensation of magnons.

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