RIVERSIDE, California – Materials that have excess electrons are usually conductors. However, moire patterns – interference patterns that typically occur when one object with a repeating pattern is placed over another with a similar pattern – can suppress electrical conductivity, a study led by physicists at the University of California, Riverside, has found.
In the laboratory, the researchers coated a single layer of tungsten disulfide (WS2) on a single layer of tungsten diselenide (WSe2) and aligned the two layers with each other to create large-scale moiré patterns. Atoms in both ZS2 and WSe2 the layers are arranged in a two-dimensional honeycomb lattice with periodicity or recurring intervals, much less than 1 nanometer. But when the two grids are aligned at 0 or 60 degrees, the composite material creates a moiré pattern with a much higher periodicity of about 8 nanometers. The conductivity of this 2D system depends on how many electrons are placed in the moiré sample.
“We found that when the moire-pattern is partially filled with electrons, the system exhibits several isolation states as opposed to the conduction states expected from conventional understanding,” said Yongtao Cui, assistant professor of physics and astronomy at UC Riverside, who led the research team. “The charge percentages have been found to be simple fractions such as 1/2, 1/3, 1/4, 1/6, etc. The mechanism for such isolation states is the strong interaction between electrons that limits mobile electrons to local moiré cells. This understanding can help in developing new ways to control conductivity and discover new superconducting materials. “
The results of the study appear today in Physics of nature.
Moiré samples generated on WS composite material2 and WSe2 it can be imagined that there are wells and ridges similarly arranged in the shape of honeycombs.
“WS2 and WSe2 they have a slight mismatch in terms of lattice size, making them ideal for producing moiré patterns, ”Cui said.“ Furthermore, the coupling between the electrons becomes strong, meaning the electrons “talk to each other” as they move along the ridges and wells. “
Typically, when a small number of electrons are placed in a 2D layer such as WS2 or WSe2, have enough energy for free and random travel, making the system a conductor. Cui’s laboratory found that when moiré lattices are formed using both WSs2 and WSe2, resulting in a periodic pattern, the electrons begin to slow down and bounce off each other.
“Electrons don’t want to be close to each other,” said Xiong Huang, the first author and PhD student in Cui’s microwave nanoelectronics lab. “When the number of electrons is such that one electron occupies each moiré hexagon, the electrons remain locked and can no longer move freely. The system then acts like an insulator.”
Cui compared the behavior of such electrons to social distancing during a pandemic.
“If one can imagine hexagons being homes, all the electrons are indoors, one per house and not moving around the neighborhood,” he said. “If we don’t have one electron per hexagon, but we have 95% occupancy of the hexagon, which means some nearby hexagons are empty, then the electrons can still move a little through the empty cells. Then the material is not an insulator. It acts like a bad conductor.”
His lab was able to fine-tune the number of electrons in the WS2– WSe2 lattice composite to change the average occupancy of the hexagon. His team found that isolation states occurred when the average occupancy was less than one. For example, for an occupancy of one-third, electrons occupied every other hexagon.
“Using the analogy of social distancing, instead of a 6-foot separation, you now have a separation of, say, 10 feet,” Cui said. “So when one electron occupies a hexagon, it forces all adjacent hexagons to be empty to conform to a stricter rule of social distancing. When all electrons adhere to this rule, they form a new pattern and occupy a third of the total hexagons in which they again lose freedom of movement. which leads to an insulating state. “
The study shows that similar behaviors can occur for other occupancy fractions, such as 1/4, 1/2, and 1/6, each corresponding to a different pattern of occupation.
Cui explained that these isolation states are caused by a strong interaction between electrons. It is, he added, Coulomb repulsion, the repulsive force between two positive or two negative charges, as described in Coulomb’s law.
He added that it is known that strong electronic interactions in 3D materials lead to various exotic electronic phases. For example, they are likely to contribute to the creation of unconventional high-temperature superconductivity.
“The question we still don’t have an answer to is whether 2D structures, like the ones we used in the experiments, can produce high-temperature superconductivity,” Cui said.
Next, his group will work on characterizing the power of electronic interactions.
“The strength of the electron interaction largely determines the insulation state of the system,” Cui said. “We’re also interested in the possibility of manipulating the force of electron interaction.”
Cui and Huang were funded by National Science Foundation grants, Hellman Scholarships, and SHINES Seed Grants.
They were joined in the study by Tianmeng Wang, Shengnan Miao, Zhipeng Li, Zhen Lian, and Su-Fei Shi at Rensselaer Polytechnic Institute in New York City; Chong Wang and Di Xiao of Carnegie Mellon University in Pennsylvania; Takashi Taniguchi and Kenji Watanabe of the National Institute of Materials Science in Japan; and Satoshi Okamoto of Oak Ridge National Laboratory in Tennessee. Huang, T. Wang, Miao, and C. Wang equally contribute to the research.
The research paper is entitled “Correlated insulation states on fractured WS fillings2/ WSe2 Moire? Lattice. “
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