Quasi-two-dimensional Bose-Einstein condensation of spin triplets in the dimerized quantum magnet Ba2CuSi2O6Cl2

We synthesized single crystals of composition Ba₂CuSi₂O₆Cl₂ and investigated their quantum magnetic properties. The crystal structure is closely related to that of the quasi-two-dimensional (2D) dimerized magnet BaCuSi₂O₆ also known as Han purple. Ba₂CuSi₂O₆Cl₂ has a singlet ground state with an excitation gap of Δ/k_B = 20.8 K. The magnetization curves for two different field directions almost perfectly coincide when normalized by the g factor except for a small jump anomaly for a magnetic field perpendicular to the c axis. The magnetization curve with a nonlinear slope above the critical field is in excellent agreement with exact-diagonalization calculations based on a 2D coupled spin-dimer model. Individual exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate a 2D exchange network and weak frustration between interdimer exchange interactions, supported by weak spin-lattice coupling implied from our magnetostriction data. Lastly, the magnetic-field-induced spin ordering in Ba₂CuSi₂O₆Cl₂ is described as the quasi-2D Bose-Einstein condensation of triplets.