We synthesized single crystals of composition Ba2CuSi2O6Cl2 and investigated its 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/kB=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. The magnetic-field-induced spin ordering in Ba2CuSi2O6Cl2 is described as the quasi-2D Bose-Einstein condensation of triplets.
@article{arxiv.1609.00438,
title = {Quasi-two-dimensional Bose-Einstein condensation of spin triplets in dimerized quantum magnet Ba$_2$CuSi$_2$O$_6$Cl$_2$},
author = {Makiko Okada and Hidekazu Tanaka and Nobuyuki Kurita and Kohei Johmoto and Hidehiro Uekusa and Atsushi Miyake and Masashi Tokunaga and Satoshi Nishimoto and Masaaki Nakamura and Marcelo Jaime and Guillaume Radtke and Andrés Saúl},
journal= {arXiv preprint arXiv:1609.00438},
year = {2016}
}