Coexisting Charge Density Waves in Twisted Bilayer NbSe₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid State Physics

Background:

• Twisted bilayers of 2D materials offer a tunable platform for exploring broken symmetry phases.
• Investigating twisted bilayers composed of monolayers with inherent ordered states can lead to novel phenomena.
• Niobium diselenide (NbSe2) monolayers exhibit charge density wave (CDW) order.

Purpose of the Study:

• To investigate the atomic structure and emergent phenomena in twisted bilayer NbSe2.
• To understand the interplay between constituent monolayer CDW states and bilayer stacking.
• To predict the ground state CDW configurations in twisted bilayer NbSe2.

Main Methods:

• Utilized first-principles density-functional theory (DFT) calculations.
• Analyzed the atomic structure of twisted bilayer NbSe2 at various stacking configurations.
• Simulated the charge density wave states within the twisted bilayer system.

Main Results:

• Identified the coexistence of distinct CDW states in the twisted bilayer NbSe2 ground state.
• Observed monolayer-like 3x3 triangular and hexagonal CDW states in low-energy stacking regions.
• Found stripe CDW states localized in the domain walls surrounding these low-energy regions.

Conclusions:

• The ground state of twisted bilayer NbSe2 hosts a complex interplay of different CDW phases.
• Stacking configurations significantly influence the resulting CDW order in the bilayer.
• These findings provide testable predictions for scanning tunneling microscopy (STM) experiments and highlight potential for complex CDW engineering.