When two regular grids are overlaid imperfectly on each other, they form a pattern of bright and dark bands called a moiré pattern. Such patterns are often found in daily life, such as when looking through two sets of railings on a walkway.
Atoms in crystalline solids, which have regular spacings of around 10-10 meters, can also create moiré patterns. When 2D materials – atomically thin sheets of atoms – are stacked on top of each other, they can form a structure called a moiré superlattice, as seen in the image below. Moiré superlattices can have a much larger period than the individual atomic layers (10-8 to 10-7 meters). In recent years, they have been extensively investigated by physicists, who believe that they may exhibit interesting properties not found in typical materials.
A Moiré pattern formed by stacking two sheets with different periods. In the moiré superlattice studied by the researchers, the blue circles represent the W atoms in WSe2, and the red circles represent the Mo atoms in MoS2. Figure credit: Q. Tan.
Now, researchers have found the best evidence to date that moiré superlattices can behave as novel forms (or “phases”) of matter. Writing in Nature Materials, a team led by Associate Professor Weibo Gao, from NTU’s School of Physical and Mathematical Sciences, report that they have found several “correlated electron phases” in moiré superlattices formed by tungsten diselenide (WSe2) and molybdenum disulfide (MoS2). As the phases can be easily changed by varying the electrical voltage, these materials may be useful for creating advanced electrical devices, or as models to help scientists understand more complicated materials.
When Electrons Get Together
Correlated electron phases are exotic state of matter in which the motion of each electron is strongly influenced by (or “correlated with”) all the other electrons. In a conventional material, by contrast, electrons move independently of each other. Physicists have discovered a wide variety of correlated electron phases, including superconductors (where correlations allow electric currents to flow with zero resistance) and Mott insulators (where correlations have the opposite effect of blocking electric currents).
In recent years, researchers have been especially interested in correlated electron phases found in 2D materials called transition metal dichalcogenides (TMDs), such as WSe2 and MoS2. Because 2D materials are made out of single atomic layers, researchers can manipulate them to an extraordinary degree, such as applying strong electric fields or mechanical strains, in order to force the materials into different phases. Several correlated electron phases, such as Mott insulators, have been found in TMDs in this way.
What if two TMD layers are combined into a moiré superlattice? According to the new experiments, the TMD superlattice can exhibit new correlated electron phases not found in single-layer TMDs, marked by correlations between electrons moving in different atomic sheets.
“When it comes to TMDs, 1 + 1 is more than 2,” says Assoc. Prof. Gao, a Provost’s Chair at NTU and the project’s lead investigator. “It turns out that stacking two layers gives you a material with much richer properties.”
The NTU team in their lab. Front row: Qinghai Tan (left) and Weibo Gao (right). Back row: Xuran Dai (left) and Abdullah Rasmita (right). Photo credit: M. Y. Hor.
Shedding New Light on Moiré Superlattices
To probe the TMD superlattices, which were created at the National Institute for Materials Science in Japan, Assoc. Prof. Gao’s group used an experimental technique known as photoluminescence spectroscopy. This method relies on the detection of “interlayer excitons”: composite particles consisting of an electron in one layer bound to a hole (a missing electron) in the other layer.
Interlayer excitons are created when the superlattice is illuminated with a high-power laser. Subsequently, an interlayer exciton can decay when its electron hops to the other layer and recombines with the hole. This process is accompanied by the release of a photon (a particle of light).
An interlayer exciton consists of a hole (h), or missing electron, in one atomic layer, and an excess electron (e) in the other layer. When the electron hops into the other layer and recombines with the hole, the interlayer exciton is destroyed and a photon (wiggly purple line) is emitted. Figure credit: A. Rasmita, et al.
With the help of theoretical calculations from Prof. Allan H. MacDonald at the University of Texas at Austin, the researchers showed that when the TMD superlattice is in a correlated phase, the interlayer excitons are more likely to release a photon when they decay. Physicists refer to this phenomenon as “suppression of nonradiative decay” resulting in an increase in “quantum efficiency”. Hence, a measured enhancement in the intensity of the emitted light signals the existence of a correlated electron phase.
Based on this approach, Assoc. Prof. Gao’s team systematically studied the behavior of the TMD superlattices under different conditions. They found that when the samples are placed on an electrode, varying the voltage causes different correlated electron phases to appear. While some of these are similar to the Mott insulator phase previously observed in single-layer TMDs, others are distinct and seem to involve correlations between electrons in different layers, not just within the individual layers.
“Demonstrating electrically controlled correlated electron phases in a moiré superlattice is an important achievement in this research field,” says Dr Qinghai Tan, a research fellow at NTU and the first author of this paper. According to him, such superlattices can be used by scientists as a model system, helping them learn more about the general properties of correlated electron phases. “Normally, studying correlated electron phases requires highly specialized equipment and experimental conditions. Our setup is simpler, and we can easily study multiple phases in one sample. Also, our photoluminescence technique lets us study samples that are not uniform, like a correlated phase that forms over part of a sample.”
Looking ahead, the NTU team aims to look for other unusual phenomena that have been speculated to exist in moiré superlattices, such as exciton crystals and exciton Bose-Einstein condensates. “This is just the beginning of the moiré superlattice story, and more exciting findings are on the way,” says Assoc. Prof. Gao.
This work has been selected as the Featured research article in Nature Materials, and has also been written about in a Nature Materials News & Views article.