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Illustration of the experimental setup at the basis of a powerful instrument for the study and development of atomically thin materials. Two layers of this material were compressed between the flat tips of two diamonds, thereby regulating the moir potential, a key parameter illustrated by the hills and valleys in the top layer. Credit: Matheus JS Matos, Universidade Federal de Ouro Preto

Physicists have been fascinated by systems composed of materials only one or a few layers of atoms thick. When a few sheets of these two-dimensional materials are stacked together, a geometric pattern called a moiré pattern can form. In these so-called moiré patterns, new exotic phenomena can occur, including superconductivity and unconventional magnetism.

As a result, a better understanding of what happens at the interface between each sheet to cause these phenomena could lead to heady applications in new electronics and much more.

Now an international team of scientists led by physicists from MIT presents a powerful new tool for quantifying and controlling a key parameter in moiré patterns. It involves applying extreme pressure to a moiré pattern as light passes through it, then analyzing the effects with Raman spectroscopy, a common laboratory technique.

Equally important to the work is a theoretical model that provides a framework for understanding the experimental data.

The work is reported in Nature Nanotechnology.

“The technique we developed to probe these moiré systems is methodologically similar to X-ray crystallography methods on proteins that allow biologists to know where the atoms are in a protein and how the protein will function,” says Riccardo Comin, of the Class of 1947 Assistant Professor of Career Development of Physics at MIT.

The parameter the team can now measure, known as the moir potential, ‘will tell us what physics can be achieved in a particular stack of two-dimensional materials. It is one of the most important pieces of information we need to predict whether a given material will exhibit exotic physics. or not,” continues Comin, who is also affiliated with MIT’s Materials Research Laboratory.

Equally important, the technique also allows the team to “tune” or control potential moir to potentially achieve different exotic phenomena.

Matthew Yankowitz, an assistant professor of physics at the University of Washington who was not involved in the work, says: “Pressure has recently emerged as a promising technique for regulating the properties of these [moir] materials because it directly modifies the strength of the moiré potential. By studying the optical properties of a semiconducting moiré bilayer under pressure, the team unlocked a new means of probing and manipulating the effects of a moiré superlattice. This work sets the stage for further advances in our understanding and control of the strongly correlated states of matter that arise in semiconductor moiré systems.”

The work reported in Nature Nanotechnology is the result of a collaboration between researchers from MIT, Universidad Nacional Autnoma de Mxico (UNAM) and three Brazilian federal universities: Universidade Federal de Minas Gerais (UFMG), Universidade Federal de Ouro Preto (UFOP) and Universidade Federal Fluminense (UFF).

Extreme pressure, tiny samples

The experimental setup the team developed for applying extreme pressure to a moiré material, in this case composed of two ultra-thin sheets of a transition metal dichalcogenide, involves compressing the material between two diamond points. The setup and sample sizes are incredibly small. For example, the diameter of the chamber in which this occurs is similar to the width of a human hair. “And we need to precisely position our two-dimensional sample in it, so it’s a little tricky,” says Martins, the job lead to develop the setup.

Those dimensions are needed to create the extreme pressure exerted on the sample, which is similar to the pressure the Eiffel Tower would exert sitting on top of a one-inch square piece of paper. Another analogy: the pressure is about 50,000 times the pressure of the air around us.

Experiments and theory

The team then shone light through the sample and collected the emitted light. “Light leaves a certain energy within the material, and this energy can be associated with different things,” Martins said. In this case, the team focused on energy in the form of vibrations. “By measuring the difference between photon energies [light particles] by going in and out of the material, we can probe the energy of the vibrations created in the material,” he continues.

The intensity of the light escaping from the material associated with those vibrations, in turn, indicates how strongly the electrons in one atomically thin sheet communicate with the electrons in the other. The stronger these interactions are, the greater the chances of exotic phenomena occurring. “The moir potential is basically the strength of that coupling between the 2D layers,” says Comin.

Says Martins: “By comparing the experimental improvement in the intensity of the light output associated with these vibrations, against the calculations of our theoretical model, we were able to obtain the strength of the moiré potential and its evolution with pressure.”

The theoretical model, developed by Ruiz-Tijerina, is itself very sophisticated. Says Comin: “It’s a complex model because it involves atoms, it involves electrons, and it’s a so-called large supercell model. This means that you don’t just model a single quantity, such as a single atom with its electrons, but a large collection of them. Really see the dynamics of the atoms while they’re still interacting with the electrons around them.”

Ruiz-Tijerina concludes, “When the experiment shows what you predicted, or when your model can actually reproduce what the experiments measure, it’s a feeling like no other.”

More information:
Pimenta Martins et al. Pressure regulation of minibands in MoS2/WSe2 heterostructures detected by moir phonons, Nature Nanotechnology (2023). DOI: 10.1038/s41565-023-01413-3

About the magazine:
Nature Nanotechnology

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