Heat transport of the kagome Heisenberg quantum spin liquid candidate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>YCu</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>OH</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>6.5</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>Br</mml:mi><mml:mrow><mml:mn>2.5</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>: Localized magnetic excitations and a putative spin gap
Xiaochen Hong, Mahdi Behnami, Yuan Long, Boqiang Li, Wolfram Brenig, B. Büchner, Yuesheng Li, C. Heß
Abstract
The spin-1/2 kagome 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 ${\mathrm{ZnCu}}_{3}{(\mathrm{OH})}_{6}{\mathrm{Cl}}_{2}$, which unfortunately suffers from the notorious orphan spin problem caused by magnetic disorders. Here, we turn to ${\mathrm{YCu}}_{3}{(\mathrm{OH})}_{6.5}{\mathrm{Br}}_{2.5}$, recently recognized as another host of a globally undistorted kagome ${\mathrm{Cu}}^{2+}$ lattice free from orphan spins, and thus a more feasible system for studying the intrinsic kagome quantum spin liquid physics. Our high-resolution low-temperature thermal conductivity measurements yield a vanishingly small residual linear term of $\ensuremath{\kappa}/T$ ($T\ensuremath{\rightarrow}0$), and thus clearly rule out itinerant gapless fermionic excitations. An 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, an analysis of the magnetic field impact on the thermal conductivity reveals a field closing of the spin gap, while the excitations remain localized.