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Strong Electron Interactions Play a Key Role in Superconductivity

An unexpected discovery in spring 2018 relating to superconductivity in a novel material gained considerable attention from the scientific community.

A team led by Princeton physicist Ali Yazdani has shown that strong electron interactions play a key role in the superconductivity that has been discovered in graphene, a material made up of single-layer sheets of carbon atoms. Here, two graphene sheets stacked on each other with a twist make a long-wavelength moiré pattern. (Image credit: Kai Fu for Yazdani Lab, Princeton University)

Developed by layering one carbon sheet over another and winding the top sheet at a “magic” angle, the material allowed electrons to flow without any resistance—a feature that can radically increase energy-efficient power transmission and bring in a range of novel technologies.

Now, the latest experiments performed at Princeton University provide clues at how this new material—called magic-angle twisted graphene—leads to superconductivity.

In the latest issue of the journal Nature, researchers at Princeton University have provided strong evidence that the superconducting behavior is due to powerful interactions that occur between electrons. This provides a better understanding of the rules followed by electrons during the emergence of superconductivity.

This is one of the hottest topics in physics. This is a material that is incredibly simple, just two sheets of carbon that you stick one on top of the other, and it shows superconductivity.

Ali Yazdani, Study Senior Author and Class of 1909 Professor of Physics, Princeton University

It is not known how superconductivity actually emerges—a mystery that laboratories worldwide are attempting to solve. The domain even has a name called “twistronics.”

Part of the exhilaration is that, when compared to current superconductors, the material can be studied more easily because it includes only one type of atom—carbon—and just a couple of layers.

The main thing about this new material is that it is a playground for all these kinds of physics that people have been thinking about for the last 40 years.

B. Andrei Bernevig, Professor of Physics, Princeton University

Bernevig specializes in theories to describe complex materials.

Within the novel material, the superconductivity seems to work by an underlying different mechanism from conventional superconductors, which currently are employed in strong magnets and other restricted applications.

The innovative material shares similarities with high-temperature, copper-based superconductors—known as cuprates—that were identified in the 1980s. This finding of cuprates fetched the Nobel Prize in Physics in 1987.

The novel material contains a pair of atomically thin sheets of carbon called graphene. This material was also the subject of a Nobel Prize in Physics, in 2010. It features a flat honeycomb pattern, similar to a sheet of chicken wire.

Earlier in March 2018, Pablo Jarillo-Herrero and his group from the Massachusetts Institute of Technology placed another graphene layer atop the first and subsequently rotated the top sheet by the “magic” angle of around 1.1°. Earlier, physicists had predicted that this angle causes new electron interactions; however, it came as a surprise when MIT researchers showed superconductivity.

When the overlapping chicken-wire patterns are observed from above, they provide a flickering effect called “moiré,” which emerges when a pair of geometrically regular patterns overlap, and which was once famous in the fashions and fabrics of 17th- and 18th-century royals.

The moiré patterns result in profoundly novel properties that are observed in normal materials. Most of the standard materials fall into a spectrum ranging from insulating to conducting. While insulators capture electrons in energy levels or pockets that make sure they are held in place, metals contain energy states that allow electrons to move from one atom to another. In both examples, electrons take up different energy levels and do not engage or interact in collective behavior.

However, in the case of twisted graphene, energy states produced by the physical structure of the moiré lattice prevent the electrons from standing apart and force them to engage.

It is creating a condition where the electrons can’t get out of each other’s way, and instead they all have to be in similar energy levels, which is a prime condition to create highly entangled states.

Ali Yazdani, Study Senior Author and Class of 1909 Professor of Physics, Princeton University

The question addressed by the team was whether this entanglement has any kind of association with its superconductivity. In addition, most of the simple metals are superconductors, but all the high-temperature superconductors identified so far, including the cuprates, exhibit highly entangled states induced by mutual repulsion between electrons.

The powerful interaction that occurs between electrons seems to be significant for achieving higher temperature superconductivity.

In order to deal with this question, Princeton University researchers utilized a scanning tunneling microscope that is sensitive enough to image separate atoms on a surface. The researchers examined samples of magic-angle twisted graphene in which they applied a voltage to a nearby electrode to control the number of electrons.

The analysis revealed microscopic data on how electrons behave in a twisted bilayer graphene, while a majority of other studies have tracked only macroscopic electrical conduction, to date.

When the researchers dialed the number of electrons to very high or very low concentrations, they noticed that electrons behaved nearly autonomously, as they would in modest metals. Conversely, at the major concentration of electrons where superconductivity was detected in this system, the electrons abruptly showed signs of powerful interaction and entanglement.

At the concentration where superconductivity developed, the researchers discovered that the electron energy levels turned out to be surprisingly broad—signals that validate the robust interaction and entanglement. Nevertheless, Bernevig highlighted that while these experiments pave the way for more research, additional work needs to be performed to get a deeper understanding of the type of entanglement that is taking place.

There is still so much we don’t know about these systems,” he stated. “We are nowhere near even scraping the surface of what can be learned through experiments and theoretical modeling.”

Other researchers who contributed to the study are Takashi Taniguchi and Kenji Watanabe of the National Institute for Material Science in Japan; first author and graduate student Yonglong Xie, postdoctoral research associate Xiaomeng Liu, postdoctoral research fellow Berthold Jäck, and graduate student Cheng-Li Chiu in Yazdani’s research group; and Biao Lian in Bernevig’s research group.


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