Unconventional Magnetic Oscillations in a Kagome Mott Insulator

Recent observations on the Landau Level oscillations in narrow-gapped correlated insulator raises intriguing questions on the origins. However, quantum oscillations have never been observed in wide-gap insulators. In this study, we apply a strong magnetic field to a wide band gap Mott insulator in a kagome lattice, which does not show magnetic ordering down to milliKelvin temperatures despite an antiferromagnetic interaction. We observe a plateau at magnetization 1/9 Bohr magneton per magnetic ion. Furthermore, near this plateau, we observe strong oscillations in the magnetic torque, reminiscent of quantum oscillations in metals. They are approximately periodic in the magnetic field B, as opposed to 1/B in conventional metals. Furthermore, a strong angular dependence is observed for the period, which indicates an orbital origin for this effect. The temperature dependence obeys the Fermi distribution. We show that the 1/9 plateau and the associated oscillations are consistent with the appearance of a quantum-spin-liquid state whose excitations are fermionic spinons that obey a Dirac-like spectrum. The oscillations are in response to an emergent gauge field. Our results provide strong evidence that fractionalized particles coupled to the elusive emergent gauge field have been observed.