Valley charge-transfer insulator in twisted double bilayer WSe₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• Emergent physics in flat-band systems is sensitive to nearby electronic bands.
• Mott-Hubbard insulators can transform into charge-transfer insulators with additional electronic states.

Purpose of the Study:

• To introduce twisted double bilayer (TDB) WSe2 as a controllable platform for exploring correlated electronic phases.
• To investigate the interplay between K-valley and Γ-valley bands in TDB WSe2.
• To demonstrate the tunability of correlated insulating phases via gate control.

Main Methods:

• Fabrication of twisted double bilayer (TDB) WSe2 heterostructures.
• Utilizing twist angles near 60° to bring K-valley and Γ-valley bands into proximity.
• Employing gate voltage to tune the relative energy positions of the K-valley and Γ-valley bands.
• Investigating electronic properties at half-filling to observe Mott-Hubbard and charge-transfer insulating behaviors.

Main Results:

• Correlations split the Γ-valley flat band into upper and lower Hubbard bands.
• A charge-transfer insulator phase was observed between the Γ-valley upper Hubbard band and the K-valley band.
• Gate control allowed continuous tuning of the charge-transfer insulator gap.
• A continuous phase transition from a charge-transfer insulator to a metallic state was achieved.

Conclusions:

• The valley degree of freedom serves as an effective control knob for transitions between exotic correlated phases.
• TDB WSe2 provides a versatile platform for studying and manipulating correlated electron phenomena.
• This work offers new pathways for designing and realizing novel quantum states of matter.