Unusual magnetic oscillations in a quantum spin liquid candidate

Quantum spin liquid (QSL) is a state where magnetic order can not exist due to strong frustration. The two-dimensional spin-1/2 Heisenberg Kagome lattice antiferromagnet (AFM) is an ideal playground. Recently, a promising Dirac spin liquid candidateYCu₃(OH)₆Br₂[Br_1-x(OH)_x] has been discovered without magnetic order down to 50 mK. Interestingly the heat capacity is found to be proportional to temperature T² and is enhanced by magnetic field, leading to the proposal that these compounds are spin liquids with Dirac spinons. This motivates us to study the behavior of this compound under intense magnetic fields B. Here we report we observed an unconventional plateau at magnetization equal to 1/9 Bohr magneton per magnetic ion in magnetization measurements [1]. More surprisingly, magnetic oscillations were observed in this robust insulator which are roughly periodic in B instead of 1/B, while their temperature dependence follows the Lifshitz-Kosevich formula, a defining characteristic of quantum oscillations due to fermions. These phenomena are consistent with a QSL state whose excitations are fermionic spinons with a Dirac-like spectrum. Our results provide strong evidence that fractionalized particles in QSL have been observed.

[1] G. Zheng et al., Unconventional Magnetic Oscillations in Kagome Mott Insulators. arXiv preprint arXiv:2310.07989, (2023).