A new class of moiré materials based on layered semiconductors of transition metal dichalcogenides (TMDs) is capturing the spotlight in the search for exotic quantum phases. A review article published in National Science Review by a joint team led by Prof. Fengcheng Wu from Wuhan University and Prof. Allan H. MacDonald from the University of Texas at Austin highlights how these materials are emerging as a rich platform for exploring a diverse range of strongly correlated and topological phases of matter.
When two layers of the same TMD semiconductor, such as MoTe₂ or WSe₂, are stacked and twisted at small angles, they form a moiré superlattice that leads to flat electronic bands near the valence band edge. These bands are characterized by a nontrivial topological index, the Chern number. The key feature of these "Chern flat bands" is that their electron's kinetic energy is dramatically reduced, making Coulomb interactions the dominant energy scale. This interplay between strong electron correlations and topology transforms these systems into quantum simulators, capable of realizing many-body phases once thought to be purely theoretical.
The experimental breakthroughs in this field have been truly exceptional. Researchers have observed not only integer but also fractional quantum anomalous Hall effects at zero external magnetic field, where the latter effect has not been observed previously. Quantum spin Hall insulators with helical edge states have also been identified, alongside exotic metallic phases such as the anomalous Hall metal and zero-field composite Fermi liquids. Intriguingly, unconventional superconductivity has been discovered near the fractional quantum anomalous Hall states. The ability to tune these phases in situ using gate voltage and displacement fields allows for precise control over quantum phase transitions in a single device. The review article offers a comprehensive overview of the experimental observations on the diverse quantum phases.
The authors also provide an in-depth analysis of the theoretical mechanism that underlies these phenomena. They examine key concepts such as band topology, electron interactions, symmetry breaking, and charge fractionalization. Looking ahead, the authors suggest that with advancements in material quality, these systems could give rise to even more exotic phenomena, such as non-Abelian quasiparticles and topological superconductivity, unlocking new possibilities for quantum computing applications.
