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The quantum magnet BaCuSi2O6, consisting of stacked spin dimer bilayers, undergoes an anomalous dimensional reduction from 3D to 2D close to the quantum critical point [1]. Mechanisms for this dimensional reduction were proposed based on inter-bilayer frustration resulting from an antiferromagnetic intra-bilayer exchange. Ab initio calculations propose a ferromagnetic intra-bilayer exchange rendering such a frustration impossible [2]. We have performed neutron spectroscopy on BaCuSi2O6 and Ba0.9Sr0.1CuSi2O6. Our results suggest ferromagnetic intra-bilayer exchanges with at least three different dimer types in BaCuSi2O6 and only one dimer type in Ba0.9Sr0.1CuSi2O2. We conclude that the existence of different dimer types in BaCuSi2O6 might lead to the observed 2D behavior.
[1]C.E.Sebastian et al., Nature 441, 617 (2006).
[2]V.V.Mazurenko et al., PRL 112, 107202 (2014).
