Study predicts new anomalous quantum crystal in fractionally packed moiré superlattices

Moiré superlattices, structures that arise when two layers of two-dimensional (2D) materials are superimposed with a small twist angle, have been the subject of much physics study. Indeed, they have recently been found to host fascinating new and previously unobserved physical phenomena and exotic phases of matter.

Researchers from California State University, Northridge, Stockholm University, and the Massachusetts Institute of Technology (MIT) have recently predicted the emergence of a new anomalous quantum state of matter in fractionally packed moiré superlattice stripes. Their paper, published in Physical Exam Letterspredicts the existence of this state of matter in the twisted semiconductor bilayer 𝑡MoTe₂.

“Moire materials harbor a variety of electronic phases, including topological quantum liquids and electronic crystals,” Liang Fu, a co-author of the paper, told Phys.org. “Generally speaking, crystallization and topology are rooted in the particle and wave aspect of an electron, respectively.”

Inspired by recent studies on moiré superlattices, Fu and his colleagues set out to explore the dual nature of electrons in these materials. After calculations and brainstorming, they predicted the emergence of a topological electronic crystal in these materials that had never been observed before.

“Our main goals were to understand what new quantum phases can be realized, given the distinct features of moiré superlattice systems with richer interplay between kinetic energy and interaction, and how to characterize them,” said Donna Sheng, co-author of the paper.

The new state of matter discovered by the research team exhibits an intriguing combination of ferromagnetism, charge ordering, and topology. This combination of properties is highly unusual, as typically topology and local charge ordering compete and are not observed together.

“This class of states may be quite common in moiré superlattices, with telltale experimental signatures including a surprisingly large quantized zero-field Hall conductance,” said Emil J. Bergholtz, co-author of the paper.

“What makes this even more remarkable is that strong Coulomb interactions are at the origin of this state. Without these interactions, the system would behave like a simple metal. However, the topology of the strongly interacting system nevertheless manifests itself in terms of effectively non-interacting fermions in the form of a Chern insulating state.”

The team’s prediction of this new state of matter is based on extensive numerical calculations, using data from studies examining twisted bilayer semiconductors. The researchers also created a simple phenomenological model that captures the key qualitative features of the new state, providing a better understanding of its underlying physics.

“Our study has identified a new and unexpected phase of matter that combines different aspects of quantum phenomena that occur in strongly interacting materials such as crystallization and topology,” said Ahmed Abouelkomsan, co-author of the paper.

“This phase competes with neighboring phases such as the composite Fermi liquid phase which does not exhibit crystallization. Our results therefore serve as a guide for current experiments on moiré materials which attempt to identify possible underlying phases.”

This recent study opens new possibilities for the study of exotic phases of matter in moiré superlattices. Recent studies have brought together the experimental observation of a quantum anomalous Hall crystal in twisted bilayer-trilayer graphene, which closely resembles the state predicted by the team from California State University, Northridge, Stockholm University, and MIT.

In their next studies, Fu, Sheng, and their colleagues plan to continue investigating the state of matter they predicted and hopefully unveil other exotic states in moiré superlattices. Based on their results, they predict that whole Chern insulator crystals with fractional filling of the moiré band play an important role in the phenomenology of moiré superlattices.

“Such states had been observed under a finite magnetic field before our work and have since been observed at zero field in several graphene-based moiré systems,” said Aidan Reddy, co-author of the paper.

“This phenomenology raises many theoretical questions. For example, how should we think about the energy competition between these states and the fractional Chern insulators? How should we understand the relationship between the crystal filling factor and the Chern number and the Chern number of the underlying moiré bands?

“We are excited to continue to consider these and other questions.”

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