Investigating field-induced magnetic order in Han Purple by neutron scattering up to 25.9 T

TL;DR

This study used high-field neutron scattering to explore the magnetic order in BaCuSi$_2$O$_6$ under magnetic fields up to 25.9 T, revealing insights into its quantum critical behavior and layered structure effects.

Contribution

First neutron scattering investigation of BaCuSi$_2$O$_6$ in fields above 23 T, demonstrating the technique's capability to study field-induced quantum phase transitions.

Findings

Magnetic order parameter follows 3D quantum critical scaling.

Quasi-2D interactions are not dominant enough to produce 2D physics.

Magnetic excitation dispersion matches zero-field interaction parameters.

Abstract

BaCuSi$_2$O$_6$ is a quasi-two-dimensional (2D) quantum antiferromagnet containing three different types of stacked, square-lattice bilayer hosting spin-1/2 dimers. Although this compound has been studied extensively over the last two decades, the critical applied magnetic field required to close the dimer spin gap and induce magnetic order, which exceeds 23 T, has to date precluded any kind of neutron scattering investigation. However, the HFM/EXED instrument at the Helmholtz-Zentrum Berlin made this possible at magnetic fields up to 25.9 T. Thus we have used HFM/EXED to investigate the field-induced ordered phase, in particular to look for quasi-2D physics arising from the layered structure and from the different bilayer types. From neutron diffraction data, we determined the global dependence of the magnetic order parameter on both magnetic field and temperature, finding a form consistent with 3D quantum critical scaling; from this we deduce that the quasi-2D interactions and nonuniform layering of BaCuSi$_2$O$_6$ are not anisotropic enough to induce hallmarks of 2D physics. From neutron spectroscopy data, we measured the dispersion of the strongly Zeeman-split magnetic excitations, finding good agreement with the zero-field interaction parameters of BaCuSi$_2$O$_6$. We conclude that HFM/EXED allowed a significant extension in the application of neutron scattering techniques to the field range above 20 T and in particular opened new horizons in the study of field-induced magnetic quantum phase transitions.