Physicists find for the first time direct evidence of a strong electronic correlation in a 2D material. This finding could help researchers engineer exotic electrical states such as unconventional superconductivity.
In recent years, physicists have discovered materials capable of changing their electrical character from a metal to an insulator, and even a superconductor, which is a material in a frictionless state that allows electrons to flow without resistance. These materials, which include “magic angle” graphene and other synthesized two-dimensional materials, can shift electrical states based on applied voltage or electron current.
The physics behind these switchable materials is a mystery, although physicists suspect it has something to do with “electron correlations” or the effects of the felt interaction between two negatively charged electrons. These particle repulsions have little or no effect on the forming properties of most materials. But in two-dimensional materials, these quantum interactions can have a dominant influence. Understanding how electronic correlations drive electrical states can help scientists design exotic functional materials, such as unconventional superconductors.
Now the physicists of MIT and elsewhere have taken a significant step towards understanding electronic correlations. In an article published on March 17, 2022, in Scienceresearchers reveal direct evidence of electronic correlations in a two-dimensional material called the ABC trilayer graphene. This material has already been shown to change from a metal to an insulator to a superconductor.
For the first time, researchers have directly detected electronic correlations in a special insulating state of the material. They also quantified the energy scales of these correlations, or the strength of interactions between electrons. The results demonstrate that ABC trilayer graphene may be an ideal platform for exploring and possibly engineering other electron correlations, such as those that drive superconductivity.
“A better understanding of the underlying physics of superconductivity will allow us to design devices that could change our world, from lossless power transmission to magnetically levitated trains,” says lead author Long Ju, assistant professor of physics at MIT. “This material is now a very rich playground for exploring electronic correlations and building even more robust phenomena and devices.”
An ABC trilayer graphene, stacked on top of a hexagonal boron nitride layer, is similar to the more well-studied magic-angle bilayer graphene, in that both materials involve layers of graphene – a material found naturally in graphite. and can exhibit exceptional properties when isolated in its pure form. Graphene is made from a network of carbon atoms arranged in a hexagonal pattern, similar to chicken wire. Hexagonal boron nitride, or hBN, has a similar slightly larger hexagonal pattern.
In ABC trilayer graphene, three sheets of graphene are stacked at the same angle and slightly offset from each other, like layered slices of cheese. When ABC trilayer graphene rests on hBN at a zero degree twist angle, the resulting structure is a moiré pattern, or “superlattice”, composed of periodic energy wells, the configuration of which determines how electrons pass through the material.
“This lattice structure forces electrons to localize and sets the stage for electron correlations to have a huge impact on the macroscopic property of the material,” Ju said.
He and his colleagues sought to probe the ABC trilayer graphene for direct evidence of electronic correlations and to measure their strength. They first synthesized a sample of the material, creating a superlattice with energy sinks, each of which can normally hold two electrons. They applied just enough voltage to fill each well in the array.
They then looked for signs that the material was in an ideal state for electronic correlations to dominate and affect the properties of the material. They specifically looked for signs of a “flat band” structure, where all the electrons have nearly the same energy. The team reasoned that an environment hosting electrons with a wide range of energies would be too noisy for the small energy of electron correlations to have any effect. A flatter, quieter environment would allow these effects to manifest.
The team used a unique optical technique they developed to confirm that the material does indeed have a flat band. They then lowered the voltage slightly, so that only one electron occupied each well of the lattice. In this “half-filled” state, the material is considered a Mott insulator – a curious electrical state that should be able to conduct electricity like metal, but instead, due to electronic correlations, the material behaves like an insulator.
Ju and his colleagues wanted to see if they could detect the effect of these electron correlations in a half-filled Mott insulator state. They set out to see what would happen if they disrupted the state by moving electrons around. If electronic correlations have an effect, such perturbations of electronic configurations would meet resistance, since electrons naturally repel each other. For example, an electron attempting to move to a nearby well would be repelled by the electron already occupying that well, even though that well could technically accommodate an additional electron.
In order to overcome this resistance, a further increase in energy would be needed – just enough to overcome the natural repulsion of the electron. The team reasoned that the magnitude of this increase would be a direct measure of the strength of the electronic correlation.
The researchers provided the extra boost by using light. They shone light of different colors, or wavelengths, onto the material and looked for a specific peak or single wavelength absorbed by the material. This wavelength corresponded to a photon with just enough energy to send an electron into a nearby half-filled well.
In their experiment, the team indeed observed a spike – the first direct detection of electronic correlations in this specific moiré superlattice material. They then measured this peak to quantify the correlation energy, or strength of the electron’s repulsive force. They determined it to be about 20 millielectronvolts, or 1/50 of an electronvolt.
The results show that strong electronic correlations underlie the physics of this particular 2D material. Ju says the insulating Mott state is particularly important because it is the parent state of unconventional superconductivity, the physics of which remain illusory. With this new study, the team has demonstrated that the three-layer graphen/hBN moiré ABC superlattice is an ideal platform for exploring and engineering the most exotic electrical states, including unconventional superconductivity.
“Today, superconductivity only occurs at very low temperatures in a realistic setting,” notes Ju, who says the team’s optical technique can be applied to other 2D materials to reveal exotic states. similar. “If we can understand the mechanism of unconventional superconductivity, we may be able to enhance this effect at higher temperatures. This material provides a basis for understanding and designing even more robust electrical states and devices. »
Reference: “Spectroscopy signatures of electron correlations in a trilayer graphene/hBN moiré superlattice” by Jixiang Yang, Guorui Chen, Tianyi Han, Qihang Zhang, Ya-Hui Zhang, Lili Jiang, Bosai Lyu, Hongyuan Li, Kenji Watanabe, Takashi Taniguchi, Zhiwen Shi, Todadri Senthil, Yuanbo Zhang, Feng Wang and Long Ju, March 17, 2022, Science.
This research was supported, in part, by the National Science Foundation, the Simons Foundation, and the MIT Skoltech program.