Heat transport of the kagom\'{e} Heisenberg quantum spin liquid candidate YCu$_3$(OH)$_{6.5}$Br$_{2.5}$: localized magnetic excitations and spin gap

TL;DR

This study investigates the heat transport in the kagomé Heisenberg quantum spin liquid candidate YCu₃(OH)₆.₅Br₂.₅, revealing a gapped magnetic excitation and localized spin excitations, supporting a $ ext{Z}_2$ quantum spin liquid ground state.

Contribution

It provides the first thermal conductivity measurements on YCu₃(OH)₆.₅Br₂.₅, demonstrating a gapped spin excitation and localized magnetic excitations, advancing understanding of kagomé quantum spin liquids.

Findings

Residual thermal conductivity is nearly zero, indicating absence of itinerant gapless fermionic excitations.

Phonon scattering grows exponentially with temperature, consistent with a gapped magnetic excitation.

Magnetic field reduces the spin gap but excitations remain localized.

Abstract

The spin-1/2 kagom\'{e} Heisenberg antiferromagnet is generally accepted as one of the most promising two-dimensional models to realize a quantum spin liquid state. Previous experimental efforts were almost exclusively on only one archetypal material, the herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$, which unfortunately suffers from the notorious orphan spins problem caused by magnetic disorders. Here we turn to YCu$_3$(OH)$_{6.5}$Br$_{2.5}$, recently recognized as another host of a globally undistorted kagom\'{e} Cu$^{2+}$ lattice free from the orphan spins, thus a more feasible system for studying the intrinsic kagom\'{e} quantum spin liquid physics. Our high-resolution low-temperature thermal conductivity measurements yield a vanishing small residual linear term of $\kappa/T$ ($T\rightarrow 0$), and thus clearly rule out itinerant gapless fermionic excitations. Unusual scattering of phonons grows exponentially with temperature, suggesting thermally activated phonon-spin scattering and hence a gapped magnetic excitation, consistent with a $\mathbb{Z}_2$ quantum spin liquid ground state. Additionally, the analysis of magnetic field impact on the thermal conductivity reveals a field closing of the spin gap, while the excitations remain localized.