Valley charge-transfer insulator in twisted double bilayer WSe2

In flat-band systems, emergent physics can be substantially modified by the presence of another nearby electronic band. For example, a Mott Hubbard insulator can turn into a charge transfer insulator if other electronic states enter between the upper and lower Hubbard bands. Here, we introduce twisted double bilayer (TDB) WSe2, with twist angles near 60 degrees, as a controllable platform in which the K-valley band can be tuned to close vicinity of the Gamma-valley moir & eacute; flat band. At half-filling, correlations split the Gamma-valley flat band into upper and lower Hubbard bands and a charge-transfer insulator forms between the Gamma-valley upper Hubbard band and K-valley band. Using gate control, we continuously move the K-valley band across the Gamma-valley Hubbard bands, and observe a tunable charge-transfer insulator gap and subsequently a continuous phase transition to a metal. The tuning of Mott Hubbard to charge-transfer insulator establishes valley degree of freedom as a suitable knob for transitions between exotic correlated phases.