Investigated by Neutron Scattering

Quasi low-dimensional systems tipically display a dimensional crossover into 3D behavior either because of a diverging correlation length close to a critical point or due to vanishing thermal fluctuations when approaching zero temperature. Contrary to this, the quantum magnet BaCuSi₂O₆, which consists of stacked 2D layers hosting spin dimers, undergoes a dimensional reduction from 3D to 2D in close vicinity to the quantum critical point to its Bose Einstein Condensate phase [1]. Mechanisms for this dimensional reduction were proposed based on frustration of the interlayer exchange resulting from an assumed antiferromagnetic intralayer exchange. However, density-functional theory calculations suggest a ferromagnetic intralayer exchange which would render such a frustration impossible [2]. We have performed high-resolution neutron spectroscopy experiments and calculated the excitation spectrum for various dimer models. Our results suggest that the intralayer exchange is ferromagnetic, while there exist at least three different dimer types in BaCuSi₂O₆. We conclude that the existence of different dimer types might lead to 2D behavior in close vicinity to the quantum critical point.

[1] C. E. Sebastian et al., Nature 441, 617 (2006).
[2] V. V. Mazurenko et al., PRL 112, 107202 (2014).