Exploring new physics from electronic interactions in semiconductor moiré superlattices

Semiconductor moiré superlattices are fascinating material structures that have shown promise for the study of correlated electronic states and quantum physics phenomena. These structures, made up of networks of artificial atoms arranged in a so-called moiré configuration, are highly tunable and characterized by strong electronic interactions.

Researchers at the Massachusetts Institute of Technology (MIT) recently conducted a study further exploring these materials and their underlying physics. Their article, published in Physical Examination Lettersintroduces a new theoretical framework that could inform the study of long-period moiré superlattices, characterized by weakly interacting electrons residing in different potential wells.

“Our group has been working on two-dimensional moiré semiconductor materials for five years,” Liang Fu, co-author of the paper, told Phys.org. “In these systems, electrons move in a periodic potential landscape (the moiré superlattice) and interact with each other through Coulomb repulsion.”

The main advantage of moiré semiconductor superlattices is that they can easily be manipulated in experimental settings. Specifically, physicists can control the density of the electrons they contain to change the property of their many-electron ground state.

“Most previous studies have focused on the case of the presence of one or less than one electron per moiré unit cell,” Fu said. “We decided to explore the multielectronics regime and see if there was anything new.”

Predicting the behavior of multielectronic materials can be very difficult. The main reason for this is that these systems often contain different energy scales that compete with each other.

“Kinetic energy favors a liquid electron, while interaction and potential energy favor a solid electron,” explained Aidan Reddy, first author of the paper. “The nice thing about moiré materials is that the relative strength of different energy scales can be tuned by varying the period of the moiré. Taking advantage of this tunability, we developed a theoretical framework for studying moiré systems at long period, in which the electrons residing on different potential wells are weakly coupled.

The theoretical framework introduced by this team of researchers focuses on the behavior of individual atoms in the moiré superlattice. Reddy, Fu and their colleague Trithep Devakul found that this relatively simple approach could still help shed light on various interesting phenomena in quantum physics.

Using their framework, the researchers unveiled new physics that could be observed in semiconductor-based multielectronic moiré superlattices. For example, at a filling factor n = 3 (that is, when each moiré atom in a superlattice contains three electrons), they discovered that Coulomb interactions led to the formation of what we called the “Wigner molecule”. Furthermore, under specific circumstances (that is, if their size is comparable to the moiré period), they showed that these Wigner molecules could form a unique structure known as an emergent Kagome lattice.

The interesting self-organized electronic configurations described in this research team’s paper may soon be explored in more detail in follow-up studies. Furthermore, these newly discovered configurations could serve as inspiration for other physicists, allowing them to study charge ordering and quantum magnetism in a regime quite unfamiliar to conventional materials.

“The most remarkable finding of our work is that, with particular filling factors, electrons self-organize into striking configurations (Wigner molecules) due to a balance between the energy scales involved. Our Wigner’s solid prediction was confirmed experimentally”, Trithep added.

In the short term, the researchers plan to study the quantum phase transition between electronic Wigner solids and electronic liquids.

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