Physicists find direct evidence of strong electron correlation in a 2D material for the first time

In recent years physicists have discovered materials that are able to switch their electrical character from a metal to an insulator, and even to a superconductor, which is a material in a friction-free state that allows electrons to flow with zero resistance. The electrical states of these materials can be changed by the current of electrons applied to them.

Physicists suspect that the underlying physics driving these switchable materials has something to do with the interaction felt between two negatively charged electrons. Particle repulsions have no effect on the properties of most materials. In two-dimensional materials, these quantum interactions can be a dominating influence. Understanding how electron correlations drive electrical states can help scientists engineer exotic functional materials.

Physicists at MIT and elsewhere have taken a significant step towards understanding electron correlations. In a paper that will be published today in Science, the researchers show direct evidence of electron correlations in a two-dimensional material called ABC trilayer graphene. This material has been shown to be a superconductor.

The researchers were the first to directly detect electron correlations in a special state of the material. The strength of the interactions between electrons was quantified. The results show that ABC trilayer graphene is an ideal platform to explore and possibly engineer other electron correlations, such as those that drive superconductivity.

Long Ju, assistant professor of physics at MIT, says that better understanding of the underlying physics of superconductivity will allow us to engineer devices that could change our world.

Superlattice.

An ABC trilayer graphene, stacked atop a layer of hexagonal boron nitride, is similar to the more well-studied magic-angle bilayer graphene, in that both materials involve layers of graphene and can exhibit exceptional properties when isolated. Graphene is made from a lattice of carbon atoms arranged in a hexagonal pattern. There is a slightly larger hexagonal pattern in hBN.

Three sheets of ABC trilayer graphene are stacked at the same angle and slightly offset from each other, like slices of cheese. When ABC trilayer graphene sits on hBN at a zero-degree twist angle, the resulting structure is a pattern made of periodic energy wells.

Ju says that the lattice structure sets the stage for electron correlations to have a huge impact on the material.

He and his colleagues wanted to measure the strength of ABC trilayer graphene. They created a superlattice with energy wells, each of which can hold two electrons, from a sample of the material. They applied enough power to fill each well.

The electron boost.

They looked for signs that the material was in an ideal state for electron correlations to dominate. They looked for signs of a flat band structure, where all electrons have the same energy. The noise of an environment with a wide range of energies would be too loud for the tiny energy of electron correlations to have an effect. These effects would be able to come through in a flatter environment.

The team used a unique technique to confirm that the material has a flat band. They adjusted the voltage so that only one electron was in the lattice. In this half-filled state, the material behaves like an electrical state that should be able to conduct electricity like metal, but instead, due to electron correlations, it behaves as an insulator.

Ju and his colleagues wanted to see if they could detect the effect of the correlations in a half-filled, Mott state. If they disturbed the state by moving electrons, they would see what would happen. If the electron correlations were to have any effect, they would meet resistance since electrons naturally repel each other. An electron trying to move to a neighboring well would be pushed back by the electron already in that well.

It would take an extra boost of energy to overcome the resistance. The magnitude of this boost would be a direct measure of the correlation.

The researchers used light to give the boost. They shone light on the material and looked for a peak, or a single wavelength that the material absorbed. A photon with just enough energy to kick an electron into a neighboring half-filled well corresponds to this wavelength.

The first direct detection of electron correlations in this specific material was observed by the team in their experiment. The correlation energy is the strength of the repulsive force. This was determined to be about 20 million electronvolts, or 50 million electronvolts.

The results show that the physics of this material is related to electron correlations. The parent state of unconventional superconductivity, the physics of which remains illusive, is important to Ju. The ABC trilayer graphene/hBN moiré superlattice is an ideal platform to explore and engineer the more exotic electrical states, including the unconventional superconductivity.

Ju says the team can apply their optical technique to other 2D materials to reveal similar exotic states. The material forms a foundation to understand and engineer even more robust electrical states and devices.

More information: Jixiang Yang et al, Spectroscopy signatures of electron correlations in a trilayer graphene/hBN moiré superlattice, Science (2022). DOI: 10.1126/science.abg3036 Journal information: Science

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