Moiré quantum chemistry: charge transfer in transition metal dichalcogenide superlattices

Abstract

Transition metal dichalcogenide (TMD) bilayers have recently emerged as a robust and tunable moiré system for studying and designing correlated electron physics. In this work, we provide an electronic structure theory that maps long-period heterobilayer TMD superlattices to diatomic crystals with cations and anions. We find the interplay between moiré potential and Coulomb interaction leads to filling-dependent charge transfer between AA and AB stacking regions several nanometers apart. We show the insulating state at half filling found in recent experiments on WSe2/WS2 is a charge-transfer insulator rather than a Mott-Hubbard insulator. Our work reveals the richness of simplicity in moiré quantum chemistry.

The Orbitally Selective Mott Phase in Electron Doped Twisted TMDs: A Possible Realization of the Kondo Lattice Model

“…At small twist angles θ M π the superlattice constant is given by a M ≈ a/θ M , where θ M ≡ √ δ 2 + θ 2 , δ is the miss-match between the layers taken from Ref. [21] and θ accounts for any additional twist. In this limit we have a M a, which justifies the use of a simple triangular periodic potential constructed out of the six smallest harmonics…”

“…This is because of the relatively small spin-oribt splitting of the bare conduction bands around the K and K points [46], which leads to a total of four electron pockets, at close proximity, including spin and valley [see different effective masses. 1 Crucially, this splitting is of the order of tens of meV's [46] and is thus, in the same energy scale as the expected moiré potential depth [20,21]. The two parabolic band minima form two pairs of timereversal partners, which we denote by a specie index τ = a, b (colored "blue" and "red" in the figure, respectively).…”

“…In contrast, semiconducting transition metal dichalcogenides (TMDs) subject to moiré potentials are naively expected to have a simpler microscopic picture. The electronic properties of such devices are captured by the Hubbard model on the triangular lattice [20,21]. Mott insulators and generalized Wigner crystals have been experimentally observed [22][23][24][25], as well as possible indications of superconductivity [22].…”

“…The two parabolic band minima form two pairs of timereversal partners, which we denote by a specie index τ = a, b (colored "blue" and "red" in the figure, respectively). We then subject these states to a moiré potential, assumed to be induced by a second layer [20,21]…”

“…, where δ is the lattice mismatch and θ accounts for any additional twist. We also assume for simplicity that the potential is uniform across platforms and given by V 0 = 15 meV [20,21].…”