Charge Transfer Gap Tuning via Structural Distortion in Monolayer 1T-NbSe2

The Mott state in 1T-TaS2 is predicted to host quantum spin liquids (QSLs). However, its insulating mechanism is controversial due to complications from interlayer coupling. Here, we study the charge transfer state in monolayer 1T-NbSe2, an electronic analogue to TaS2 exempt from interlayer coupling, using spectro-scopic imaging scanning tunneling microscopy and first-principles calculations. Monolayer NbSe2 surprisingly displays two types of star of David (SD) motifs with different charge transfer gap sizes, which are interconvertible via temperature variation. In addition, bilayer 1T-NbSe2 shows a Mott collapse by interlayer coupling. Our calculation unveils that the two types of SDs possess distinct structural distortions, altering the effective Coulomb energies of the central Nb orbital. Our calculation suggests that the charge transfer gap, the same parameter for determining the QSL regime, is tunable with strain. This finding offers a general strategy for manipulating the charge transfer state in related systems, which may be tuned into the potential QSL regime.

Automated summary

The charge transfer state in monolayer 1T-NbSe2 displays two different sizes of star of David motifs that can be interconverted through temperature variation, while bilayer 1T-NbSe2 shows a Mott collapse by interlayer coupling. Computational analysis reveals that the structural distortions of the motifs in the charge transfer state alter the effective Coulomb energies of the central Nb orbital, allowing the charge transfer gap to be tunable with strain. This finding provides a general strategy for manipulating the charge transfer state in related systems toward the potential QSL regime.