Moiré Band Theory for M-Valley Twisted Transition Metal Dichalcogenides

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Chemistry

Background:

• Group IV and IVB trigonal transition metal dichalcogenides (TMDs) MX$_{2}$ (M=Zr, Hf, Sn and X=S, Se) possess unique electronic properties in monolayer form.
• These properties include conduction band minima near the Brillouin zone M points and minimal spin-orbit coupling, leading to six low-energy conduction band states.

Purpose of the Study:

• To propose twisted bilayers of specific TMDs as novel moiré materials.
• To investigate the electronic behavior and valley polarization phenomena in these engineered moiré systems.

Main Methods:

• Derivation of emergent moiré-periodic Hamiltonians from small-unit-cell density functional theory (DFT) calculations.
• Analysis of single-particle decoupling of flavor sectors in twisted bilayers.
• Theoretical modeling of valley-projected Hamiltonians with valley-dependent mass anisotropies.

Main Results:

• Accurate description of twisted bilayer TMDs using emergent moiré Hamiltonians.
• Identification of time-reversal invariant Hamiltonians with significant valley-dependent mass anisotropies.
• Prediction that spontaneous valley polarization is signaled by anisotropy in transport.

Conclusions:

• Twisted TMD bilayers offer a new platform for exploring moiré physics.
• Anisotropy in transport serves as a distinct signature for valley polarization in these systems.
• This work provides an alternative to anomalous Hall and magnetic circular dichroism signals for detecting valley polarization.