Magnetic Field-Induced Condensation of Triplons in Han Purple Pigment   BaCuSi$_2$O$_6$
  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. 
M. Jaime, V. F. Correa, N. Harrison, C. D. Batista, N. Kawashima, Y.   Kazuma, G.A. Jorge, R. Stern, I. Heinmaa, S.A. Zvyagi...