Observation of unconventional charge order during the transition from Mott-insulator to charge-transfer-insulator in α-RuCl_3

The magnetic nature of the Kitaev candidate material α-RuCl_3 has been widely studied but its charge properties are less known. In the present work, we systematically investigate the scanning tunneling microscopy/spectroscopy (STM/STS) on few-layer α-RuCl_3 in proximity to graphite. While a single-layer α-RuCl_3 has a large Mott gap of order 2 eV in spite of charge transferring to the heterointerface from graphite, surface of 2-monolayers (MLs) and 3-MLs samples have a reduced charge gap of order of 200 meV. Furthermore, clear signals of lattice and charge incommensurate super-modulations are observed in 2-MLs α-RuCl_3 with negative sample biases outside the charge gap. The dI/dV spectra indicates that 2-MLs and 3-MLs α-RuCl_3 are turned from Mott insulator into charge transfer insulator, while the charge super-modulation suggests the existence of charge density wave induced by incommensurate dipole order.