Unconventional magnetic oscillations and thermodynamic evidence for Dirac spin liquid state in a kagome Mott insulator

Quantum spin liquid is a state where no magnetic order can form even at absolute zero temperature. Spin-1/2 Kagome lattice antiferromagnet is a natural playground for such a state, where on the shared-corner triangles, the third spin cannot minimize energy with both neighbors, giving rise to geometrical frustration. Recently, a Kagome antiferromagnet YCu₃(OH)₆Br₂[Br_1−y(OH)_y] (YCOB) appeared as a quantum spin liquid candidate, where no magnetic order was found down to 50 mK. The low-temperature specific heat shows a T² dependence at zero field, suggesting the possibility that the ground state of the material is a Dirac quantum spin liquid. In this study, we report observation of a highly non-trivial plateau state at magnetization equal to 1/9 Bohr magneton per magnetic ion [1]. We also observed exotic magnetic oscillations in torque close to the 1/9 plateau, reminiscent of quantum oscillations from Landau level quantization in metals. The temperature dependence of the oscillation amplitude follows the Fermi-Dirac distribution. In addition, we report T-linear dependence as well as double peak structures in specific heat C/T near the 1/9 plateau, indicating a finite fermionic density of state [2]. Our observations in torque and thermodynamic measurements provide evidence for a quantum spin liquid ground state in YCOB, whose excitations are fermionic spinons with a Dirac-like spectrum coupled to an emergent gauge field.