  Besides being an ancient pigment, BaCuSi$_2$O$_6$ is a quasi-2D magnetic
insulator with a gapped spin dimer ground state. The application of strong
magnetic fields closes this gap creating a gas of bosonic spin triplet
excitations called triplons. The topology of the spin lattice makes
BaCuSi$_2$O$_6$ an ideal candidate for studying the Bose-Einstein condensation
of triplons as a function of the external magnetic field, which acts as a
chemical potential. In agreement with quantum Monte Carlo numerical
simulations, we observe a distinct lambda-anomaly in the specific heat together
with a maximum in the magnetic susceptibility upon cooling down to liquid
Helium temperatures.
