Field-induced Bose-Einstein Condensation of triplons up to 8 K in Sr3Cr2O8*

Single crystals of the spin dimer system Sr$_3$Cr$_2$O$_8$ have been grown for the first time. Magnetization, heat capacity, and magnetocaloric effect data up to 65 T reveal magnetic order between applied fields of H$_{c1}$ $\sim$ 30.4 T and H$_{c2}$ $\sim$ 62 T. This field-induced order persists up to T$^{max}_c$ $\sim$ 8 K at H $\sim$ 44 T, the highest observed in any quantum magnet where H$_{c2}$ is experimentally-accessible. We fit the temperature-field phase diagram boundary close to H$_{c1}$ using the expression T$_c$ $=$ A(H-H$_{c1}$)$^\nu$. The exponent $\nu$ $=$ 0.65(2), obtained at temperatures much smaller than T$^{max}_c$, is that of the 3D Bose-Einstein condensate (BEC) universality class. This finding strongly suggests that Sr$_3$Cr$_2$O$_8$ is a new realization of a triplon BEC where the universal regimes corresponding to both H$_{c1}$ and H$_{c2}$ are accessible at $^4$He temperatures.