Long-Wavelength Modulation in Two-Dimensional Mott Insulator 1T-NbSe₂

Monolayer 1T-phase NbSe₂, as a two-dimensional triangular-lattice Mott insulator, has drawn enormous attention since it was predicted to be a candidate for quantum spin liquids (QSLs). The Mott insulating nature arises from the charge density wave with Star-of-David motifs, and each cluster of Star-of-David hosts one localized magnetic moment to form a triangular spin lattice. Despite theoretical prediction and experimental demonstration of a Mott gap and Kondo resonance, direct evidence of QSLs is still absent. In this work, monolayer 1T-NbSe₂ was grown with molecular beam epitaxy method and investigated with scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS). Surprisingly, enhanced density of states at the edge of the Mott gap was shown by STS measurement, and long-wavelength modulation patterns were exhibited by dI/dV maps around lower/upper Hubbard band energies. Such unusual phenomena are attributed to spinon Fermi surface and support the existence of QSLs.