Moiré Superconductivity Emerges at Band Filling with Intervalley Coherence and Chern Numbers

The peculiar behaviour of electrons in layered materials creates opportunities for novel electronic states, and recent research focuses on understanding these phenomena in moiré topological insulators. Bo Zou, Anzhuoer Li, and Allan H. MacDonald investigate the interplay between electron interactions and the unique ‘valley’ properties within these materials, revealing how these interactions can lead to surprising states of matter. Their work explores the possibility of creating states where electrons exhibit coherence between different valleys, potentially giving rise to unusual behaviours like a fractional spin Hall effect recently observed in similar materials. This research significantly advances our understanding of correlated electron systems and could pave the way for designing new electronic devices based on these exotic quantum states.

Moiré Materials Reveal Exotic Quantum States

The study of moiré materials, created by twisting or stacking layers of two-dimensional materials, is revealing a wealth of exotic quantum phenomena. These materials exhibit flat electronic bands, enhancing interactions between electrons and giving rise to novel states of matter. Researchers are actively investigating topological phases, seeking to understand and control these unique properties. A central goal is to observe fractionalization, where electrons break down into quasiparticles with fractional charge or spin, closely linked to the search for non-Abelian anyons, particles with unusual exchange statistics that could be used in future quantum computers.

The flat bands in moiré materials amplify electron interactions, leading to a rich variety of correlated electron phenomena, including superconductivity, magnetism, and charge density waves. Current research focuses on the nature of non-Abelian states and the mechanisms driving fractionalization, exploring the role of Landau levels, quantized energy levels in a magnetic field, to create and manipulate topological phases. Understanding the interplay between competing orders, such as superconductivity and magnetism, presents a significant challenge, and identifying experimental signatures is crucial for confirming the existence of these exotic states. Recent developments suggest that the band topology in twisted materials can be controlled by polarization, and evidence supports the existence of a double quantum spin Hall phase. Researchers are also investigating the emergence of vortex phases and the possibility of non-Abelian states in the second moiré band. The intense level of activity indicates the complexity of the challenges and the potential for groundbreaking discoveries in this field.

Parity Breaking and Quantum Hall States

This research investigates the behaviour of electrons in moiré topological insulators, focusing on systems where different valleys exhibit opposite Chern numbers. The team demonstrates that these systems can host intervalley coherent states, which, under certain conditions, exhibit properties reminiscent of superconductivity and strong magnetic behaviour. Importantly, the calculations reveal the emergence of gapped states, characterised by either broken time-reversal symmetry with a quantized Hall effect, or broken parity symmetry with zero Hall conductivity. The findings suggest a potential link between these parity-broken states and the recently observed fractional quantum spin Hall effect in similar materials.

By employing a Landau level model, the researchers explored how electron interactions influence the system’s electronic properties. While the current calculations do not fully explain the observed fractional quantum spin Hall effect, they identify necessary conditions and provide a framework for understanding the underlying physics. Further research could focus on exploring the precise conditions required to observe the predicted states and their potential applications in novel electronic devices.