Magnetic fluctuations with a small gap in a nearly perfect Kagome Heisenberg antiferromagnet revealed by thermal transport*

Two-dimensional Spin-1/2 Kagome Heisenberg antiferromagnets are arguably the most promising model to realize quantum spin liquid state with geometric frustrations. Over the past decades, enormous work had been dedicated to one such material, the Herbertsmithite ZnCu₃(OH)₆Cl₂ [1]. However, there is even no consensus on whether its spin-gap exists or not, partially due to the intrinsic disorder of Herbertsmithite crystals. Here we report on low-temperature thermal conductivity of an  alternative Kagome Heisenberg antiferromagnet YCu₃[OH(D)]_6.5Br_2.5 (YCOB), which is free from the disorder problem [2]. YCOB is dominated by the nearest neighbor spin interaction of ~ 56 K, while no magnetic order was detected down to 2 K [2]. We found that its thermal conductivity is purely phonon-like at the low temperature limit without a residual linear term, thus ruling out gapless spin excitations. However, at a temperature as low as 250 mK, the thermal conductivity deviates from the phononic power-law behavior, indicating scattering from spin excitations sets in. Furthermore, the spin excitation spectrum can be turned by magnetic fields. We propose a field-closing of the small spin gap in YCOB.

[1] M. R. Norman, Rev. Mod. Phys, 88, 041002 (2016)

[2] J. B. Liu et. al., arXiv:2107.12712 (2021)