Quantum anomalous Hall effect from intertwined moiré bands (2107.01796v1)

Abstract: Electron correlation and topology are two central threads of modern condensed matter physics. Semiconductor moir\'e materials provide a highly tunable platform for studies of electron correlation. Correlation-driven phenomena, including the Mott insulator, generalized Wigner crystals, stripe phases and continuous Mott transition, have been demonstrated. However, nontrivial band topology has remained elusive. Here we report the observation of a quantum anomalous Hall (QAH) effect in AB-stacked MoTe2/WSe2 moir\'e heterobilayers. Unlike in the AA-stacked structures, an out-of-plane electric field controls not only the bandwidth but also the band topology by intertwining moir\'e bands centered at different high-symmetry stacking sites. At half band filling, corresponding to one particle per moir\'e unit cell, we observe quantized Hall resistance, h/e2 (with h and e denoting the Planck's constant and electron charge, respectively), and vanishing longitudinal resistance at zero magnetic field. The electric-field-induced topological phase transition from a Mott insulator to a QAH insulator precedes an insulator-to-metal transition; contrary to most known topological phase transitions, it is not accompanied by a bulk charge gap closure. Our study paves the path for discovery of a wealth of emergent phenomena arising from the combined influence of strong correlation and topology in semiconductor moir\'e materials.

• The paper demonstrates an electric field–induced QAH phase transition in AB-stacked MoTe2/WSe2 moiré heterobilayers, marked by quantized Hall resistance.
• Magneto-transport measurements reveal chiral edge states and near-zero longitudinal resistance, evidencing nontrivial band topology.
• The findings open paths for quantum computing and low-power electronics by harnessing tunable moiré superlattices.

Quantum Anomalous Hall Effect from Intertwined Moiré Bands

The paper of electron correlation and topology remains at the forefront of condensed matter physics, with semiconductor moiré materials emerging as an efficacious platform to explore these phenomena. Central to this paper is the observation of the quantum anomalous Hall (QAH) effect within AB-stacked MoTe2/WSe2 moiré heterobilayers—a configuration that highlights the manipulation of band topology via external electric fields.

Overview of Electron Correlation and Topological Phenomena

Moiré materials have demonstrated varied electron correlation-driven states; however, achieving nontrivial band topology in TMD (transition metal dichalcogenide) heterobilayers has been a significant challenge. These moiré superlattices, specifically engineered by stacking MoTe2 and WSe2 with a near-60-degree twist, present unique opportunities for band manipulation due to tunable interlayer interactions and van der Waals forces. The paper explores how an out-of-plane electric field not only influences bandwidth but also drives the system through a topological phase transition.

Key Experimental Findings

The research reports pivotal findings through magneto-transport measurements at low temperatures, indicating the emergence of a QAH state characterized by:

• Quantized Hall resistance at $h/e^2$ and minimal longitudinal resistance at zero magnetic field, signaling the presence of chiral edge states.
• An electric-field-induced transition from a Mott insulator to a QAH insulator, notably not accompanied by the closure of a bulk charge gap.

These findings contrast the typical paradigm where charge gap closure is a haLLMark of topological transitions. The AB-stacked heterobilayer's response to the electric field is underexploited in semiconductor moiré materials, potentially paving the way for discovering further exotic states.

Implications and Future Directions in AI

This paper presents one of the earliest realizations of nontrivial band topology via external electric fields in moiré materials, with implications extending to the development of quantum computing and novel electronic devices. The demonstration of Chern insulators without external magnetic fields underscores the potential for low-power electronics utilizing topological properties.

Additionally, the results may encourage theoretical advancements in the understanding of Mott insulators transitioning directly to topological states. Theoretically, these phenomena align with adaptations of the Kane-Mele model to encompass interactions within moiré setups.

Future studies should focus on precisely characterizing the valley-spin-polarized nature of the QAH state and further explore the fascinating interplay between the electric field, topology, and electron correlation in these unique two-dimensional systems. Unraveling the robustness of the QAH effect, particularly under varying disorder levels and gate-tuning conditions, remains an essential aspect for future technological applications. As moiré superlattices continue to reveal their multifaceted physics, they hold promise for substantial breakthroughs in the broader context of quantum materials research.