[Paper Review] Observation of integer and fractional quantum anomalous Hall effects in twisted bilayer MoTe2

Fan Xu, Zheng Sun+14 more|arXiv (Cornell University)|Aug 11, 2023

Topological Materials and Phenomena32 references64 citations

TL;DR

The paper reports transport evidence of both integer and fractional quantum anomalous Hall effects at zero magnetic field in AA-stacked twisted bilayer MoTe2, tunable by a vertical displacement field, with D-field driven transitions to topologically trivial states.

ABSTRACT

The interplay between strong correlations and topology can lead to the emergence of intriguing quantum states of matter. One well-known example is the fractional quantum Hall effect, where exotic electron fluids with fractionally charged excitations form in partially filled Landau levels. The emergence of topological moiré flat bands provides exciting opportunities to realize the lattice analogs of both the integer and fractional quantum Hall states without the need for an external magnetic field. These states are known as the integer and fractional quantum anomalous Hall (IQAH and FQAH) states. Here, we present direct transport evidence of the existence of both IQAH and FQAH states in twisted bilayer MoTe2 (AA stacked). At zero magnetic field, we observe well-quantized Hall resistance of h/e2 around moiré filling factor ν = -1 (corresponding to one hole per moiré unit cell), and nearly-quantized Hall resistance of 3h/2e2 around ν = -2/3, respectively. Concomitantly, the longitudinal resistance exhibits distinct minima around ν = -1 and -2/3. The application of an electric field induces topological quantum phase transition from the IQAH state to a charge transfer insulator at ν = -1, and from the FQAH state to a generalized Wigner crystal state, further transitioning to a metallic state at ν = -2/3. Our study paves the way for the investigation of fractionally charged excitations and anyonic statistics at zero magnetic field based on semiconductor moiré materials.

Motivation & Objective

• Motivate the realization of IQAH and FQAH states in semiconductor moiré materials without external magnetic fields.
• Demonstrate zero-field quantized Hall conductance at specific moiré fillings (ν = -1 and ν = -2/3).
• Show electric-displacement-field–driven topological phase transitions between nontrivial and trivial insulating states and metals.

Proposed method

• Fabricate AA-stacked twisted bilayer MoTe2 devices with dual gates to control moiré filling ν and displacement field D.
• Measure transport (ρxx, ρxy) at low temperatures and zero magnetic field, with field antisymmetrization/symmetrization to extract σxy.
• Extract Chern numbers from Hall conductance and Streda analysis; identify gaps via thermal activation fits.
• Use a tight-binding Wannier projection and Hartree-Fock simulations to interpret field- and filling-induced transitions.

Experimental results

Research questions

• RQ1 Can IQAH and FQAH states exist at zero magnetic field in twisted bilayer MoTe2?
• RQ2 How does vertical displacement field D influence the stability and nature of IQAH and FQAH states?
• RQ3 What are the energy gaps and transition mechanisms between nontrivial (IQAH/FQAH) and trivial insulating or metallic phases?
• RQ4 Do the observed conductance plateaus align with expected Chern numbers and Streda-derived values?
• RQ5 What is the role of disorder and sample inhomogeneity in quantization quality at ν = -1 and ν = -2/3?

Key findings

• Quantized Hall conductance of e2/h at zero field is observed around ν = -1 (IQAH) and near 3h/2e2 around ν = -2/3 (FQAH).
• Longitudinal resistance shows distinct minima at ν = -1 and ν = -2/3, indicating chiral edge transport.
• Hall conductance plateaus persist within a range of D and vanish beyond a critical Dc (≈120 mV/nm for ν = -1 and ≈15 mV/nm for ν = -2/3).
• Streda analysis yields Chern numbers |C| ≈ 1 for ν = -1 and |C| ≈ 2/3 for ν = -2/3, with some deviations due to disorder.
• D-field induces topological phase transitions from IQAH/FQAH to charge-transfer insulators, and further to metallic states at larger D.
• Energy gaps at ν = -1 and ν = -2/3 decrease near the phase boundary and then evolve with D in a manner consistent with charge-transfer and correlation-driven insulators.