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Experiments explore the secrets of "magic" angle superconductors



  Experiments Investigate the Secrets of
A team led by Princeton physicist Ali Yazdani has shown that strong electron interactions play a key role in superconductivity, which was discovered in graphene, a material consisting of a single element Layers of carbon atoms. Here, two graphene layers stacked on one another with a rotation result in a long-wave moiré pattern. Credit: Designed by Kai Fu for Yazdani Lab, Princeton University

In the spring of 2018, the surprising discovery of superconductivity in a new material astonished the scientific community. The material was made by stacking one carbon plate on top of the other and rotating the top at a "magic" angle. This allowed electrons to flow without resistance. This feature has dramatically increased energy efficient energy transfer and introduced a variety of new technologies.

New experiments conducted at Princeton suggest how this material, known as magic-twisted graphene, leads to superconductivity. In this week's issue of the journal Nature Princeton researchers provide clear evidence that superconducting behavior is based on strong interactions between electrons and provide insights into the rules that electrons follow when superconductivity arises.

"This is one of the hottest topics in physics," said Ali Yazdani, professor of physics class from 1909 and senior author of the study. "This is an incredibly simple material, just two layers of carbon that stick together, and it shows the superconductivity."

How exactly superconductivity is created is a puzzle that is at work in laboratories around the world. The field even has the name "Twistronics".

Part of the excitement is that the material is relatively easy to study compared to existing superconductors because it has only two layers and only one type of atom – carbon.

] "The most important thing about this new material is that it's a playground for all these types of physics that people have been thinking about for the past 40 years," said B. Andrei Bernevig, a professor of physics specializing in theories to explain complex interrelations materials.

The superconductivity in the new material seems to work on a fundamentally different mechanism than conventional superconductors used today in powerful magnets and other limited applications. This new material resembles copper-based high-temperature superconductors, referred to as the cuprate in the 1980s. The discovery of cuprates led in 1987 to the Nobel Prize in Physics.

The new material consists of two atomically thin carbon layers known as graphene. Graphene, which was the subject of a Nobel Prize in physics in 2010, also has a flat honeycomb pattern, similar to a piece of chicken wire. In March 2018, Pablo Jarillo-Herrero and his team at the Massachusetts Institute of Technology put a second layer of graphene on the first and then rotated the cover sheet around the "magic" angle of about 1.1 degrees. This angle had already been predicted by physicists to produce new electron interactions. However, when MIT scientists showed superconductivity, it was a shock.

Seen from above, the overlapping chicken wire patterns produce a flickering effect known as "moiré" when two geometrically regular patterns overlap, which was once popular in the fabrics and fashions of the 17th and 18th century royals.

These moiré patterns lead to profoundly new features that are not found in ordinary materials. Most ordinary materials fall into a spectrum from insulating to conductive. Isolators trap electrons in energy pockets or levels that hold them in place, while metals contain energy states that allow electrons to whiz from atom to atom. In both cases, electrons occupy different energy levels and do not interact or participate in collective behavior.

However, in twisted graphene, the physical structure of the moiré lattice creates energy states that prevent electrons from separating and forcing them to interact. "It creates a state where the electrons can not get each other out of the way, and instead they all have to be at a similar energy level, which is a prerequisite for creating highly complex states," Yazdani said.

The Question The researchers addressed the question of whether this entanglement has any connection with their superconductivity. Many simple metals are also superconducting, but all the high-temperature superconductors discovered so far, including the cuprate, have strongly entangled states caused by mutual repulsion between electrons. The strong interaction between electrons seems to be a key to achieving higher superconductivity.

To answer this question, Princeton researchers used a scanning tunneling microscope that is so sensitive that it can image single atoms on a surface. The team scanned samples of magically-angled twisted graphene, controlling the number of electrons by applying a voltage to a nearby electrode. The study provided microscopic information on electron behavior in twisted bilayer graphene, whereas most other studies have so far only observed macroscopic electrical conductivity.

By adjusting the number of electrons to very low or very high concentrations, researchers observed electrons that behave almost independently of each other, as in simple metals. At the critical electron concentration, where superconductivity was discovered in this system, the electrons suddenly showed signs of strong interaction and interlacing.

Concentrating on superconductivity, the team found that the electron energy levels were unexpectedly broad signals that confirmed a strong interaction and entanglement. Nevertheless, Bernevig emphasized that while these experiments open the door for further investigation, further work is needed to understand the nature of the entanglement in detail.

"There is still so much we do not know about these systems," he said. "We do not even scratch the surface of what can be learned through experimentation and theoretical modeling."

The authors of the study included Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan; Ph.D. candidate and first author Yonglong Xie, postdoctoral fellow Berthold Jäck, postdoctoral researcher Xiaomeng Liu and PhD student Cheng-Li Chiu in Yazdani's research group; and Biao Lian in Bernevig's research group.


Physicists show the novel Mott state in twisted graphene bilayers at magical angles


Further information:
Spectroscopic Signatures of Multibody Correlations in Twisted Double-Layer Graphs with Magic Angle, Nature (2019). DOI: 10.1038 / s41586-019-1422-x, https://nature.com/articles/s41586-019-1422-x

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Princeton University




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Experiments explore the secrets of "magic" angle superconductors (2019, July 31)
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