Low-Temperature High-Resolution Solid-State (cryoMAS) NMR of Han Purple BaCuSi2O6

Abstract

We present low-temperature (low-T, sub-liquid-N2) high-speed high-resolution 29Si solid-state (cryoMAS) nuclear magnetic resonance studies on a model 2D-BEC quantum magnet BaCuSi2O6, known also as Han Purple. We observe broadened 29Si lines below the well-established 100 K structural phase transition confirming the existence of inhomogeneities at low temperatures. Interestingly, the low-T spectra of \({\mathrm{BaCuSi}}_{2}{\mathrm{O}}_{6}\) closely resemble those of the novel compound Ba2CoSi2O6Cl2 taken at room temperature. This suggests that the Co compound features structural modulations or inhomogeneities already at room temperature. The low-T crystal structure and magnetism of \({\mathrm{BaCuSi}}_{2}{\mathrm{O}}_{6}\) are more complex than previously believed, and deserve further investigation.