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Physicists discover “secret sauce” behind unusual exotic properties of new quantum material

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Zero-energy electronic mode

A visualization of the electronic zero-energy states - also known as a 'Fermi surface' - from the kagome material studied by MIT's Riccardo Comin and colleagues. Credit: Comin Laboratory, MIT

The work will help with the design of other unusual quantum materials with many potential uses.
WITH Physicists and colleagues have discovered the "secret sauce" behind some of the exotic properties of a new quantum material that has transfected physicists because of these properties, which include superconductivity. Although theorists had predicted the cause of the unusual properties of the material, known as a kagome metal, it is the first time that the phenomenon behind these properties has been observed in the laboratory.

"The hope is that our new understanding of the electronic structure of a kagome metal will help us build a rich platform for discovering other quantum materials," said Riccardo Comin, Class of 1947 Career Development Assistant Professor of Physics at MIT, if group led the study. This in turn can lead to a new class of superconductors, new approaches to quantum calculation, and other quantum technologies.

The work is reported in the online edition of the journal on January 13, 2022 Natural physics.

Classical physics can be used to explain any number of phenomena that underlie our world - until things get exquisitely small. Subatomic particles such as electrons and quarks behave differently in ways that are still not fully understood. Go into quantum mechanics, the field that tries to explain their behavior and resulting effects.

The Kagome metal at the heart of the present work is a new quantum material, or one that manifests the exotic properties of quantum mechanics on a macroscopic scale. In 2018, Comin and Joseph Checkelsky, MIT's Mitsui Career Development Associate Professor of Physics, led the first study of the electronic structure of kagome metals, spurring interest in this family of materials. Members of the kagome metal family are composed of layers of atoms arranged in repeating units resembling a star of David or sheriff's mark. The pattern is also common in Japanese culture, especially as a wicker motif.

"This new family of materials has attracted a lot of attention as a rich new playground for quantum material that can exhibit exotic properties such as unconventional superconductivity, nematicity and charge density waves," says Comin.

Unusual features

Superconductivity and hints of charge density wave order in the new family of kagome metals studied by Comin and colleagues were discovered in the laboratory of Professor Stephen Wilson at the University of California, Santa Barbara, where single crystals were also synthesized (Wilson is co-author of it Natural physics paper). The specific kagome material explored in the current work is made of only three elements (cesium, vanadium and antimony) and has the chemical formula CsV3Sb5.

The researchers focused on two of the exotic properties that a kagome metal exhibits when cooled to below room temperature. At these temperatures, electrons in the material begin to exhibit collective behavior. "They talk to each other, as opposed to moving independently," Comin says.

Seong-yong Lee

MIT graduate student Seongyong Lee? loads a sample at the ARPES beamline of the Pohang light source in Korea, where a key set of measurements was taken for a study of a kagome metal. Credit: Seongyong Lee

One of the resulting properties is superconductivity, which enables a material to conduct electricity extremely efficiently. In ordinary metal, electrons behave much like people dancing individually in a room. In a kagome superconductor when the material is cooled to 3 Kelvin (~ -454 Fahrenheit) the electrons begin to move in pairs, like pairs at a dance. "And all of these pairs move in unison, as if they were part of a quantum choreography," Comin says.

At 100 Kelvin, the kagome material studied by Comin and collaborators exhibits another strange form of behavior known as charge density waves. In this case, the electrons arrange themselves in the form of ripples, much like those in a dune. "They are not going anywhere; they are stuck, ”says Comin. A peak in the ripple represents a region rich in electrons. A valley is electron-poor. "Waves with charge density are very different from a superconductor, but they are still a state where the electrons have to be arranged in a collective, highly organized way. They again form a choreography, but they no longer dance. Now they form a static pattern."

Comin notes that kagome metals are of great interest to physicists, in part because they can exhibit both superconductivity and charge density waves. "These two exotic phenomena are often in competition with each other, so it is unusual for a material to host them both."

The secret sauce?

But what is behind the emergence of these two properties? What makes the electrons start talking to each other, starting to influence each other? That's the key issue, ”said lead author Mingu Kang, a graduate student at the MIT Department of Physics, also affiliated with the Max Planck POSTECH Korea Research Initiative. It reports the physicists in Natural physics. "By exploring the electronic structure of this new material, we discovered that electrons exhibit an exciting behavior known as an electronic singularity," says Kang. This particular singularity is named after Léon van Hove, the Belgian physicist who first discovered it.

The Van Hove singularity involves the relationship between the energy and speed of the electrons. Usually, the energy of a moving particle is proportional to its velocity in another. "It's a fundamental pillar of classical physics [essentially] means the greater the speed, the greater the energy, ”says Comin. Imagine a Red Sox pitcher hitting you with a quick ball. Then imagine a child trying to do the same. The pitcher's ball would hurt much more than the child's, which has less energy.

What the Comin team found is that in a kagome metal, this rule no longer applies. Instead, electrons traveling at different speeds all have the same energy. The result is that the pitcher's fast ball would have the same physical effect as the kid's. "It's very counterintuitive," Comin says. He noted that it is challenging to relate energy to the speed of electrons in a solid and requires special instruments at two international synchrotron research facilities: Beamline 4A1 from Pohang Light Source and Beamline 7.0.2 (MAESTRO) from Advanced Light Source at Lawrence Berkeley National Lab.

Comments Professor Ronny Thomale from the University of Würzburg (Germany): "Theoretical physicists (including my group) have predicted the peculiar nature of van Hove's singularities on the kagome lattice, a crystal structure made of corner triangles. Riccardo Comin has now provided the first experimental verification of these theoretical proposals. " Thomale was not involved in the work.

When many electrons exist at once with the same energy in a material, they are known to interact much more strongly. As a result of these interactions, the electrons can mate and become superconducting or otherwise form charge density waves. "The presence of a van Hove singularity in a material that has both makes perfect sense as the common source of these exotic phenomena," Kang adds. "Therefore, the presence of this singularity is the 'secret sauce' that enables the quantum behavior of kagome metals."

The team's new understanding of the relationship between energy and velocities in the kagome material "is also important because it will enable us to establish new design principles for the development of new quantum materials," says Comin. Further, "we now know how to find this singularity in other systems."

Direct feedback

When physicists analyze data, this data must mostly be processed before a clear trend is seen. The Kagome system "however, gave us direct feedback on what was happening," Comin says. "The best part of this study was being able to see the singularity right there in the raw data."

Reference: "Twofold van Hove singularity and origin of charge order in topological kagome superconductor CsV3Sb5”By Mingu Kang, Shiang Fang, Jeong-Kyu Kim, Brenden R. Ortiz, Sae Hee Ryu, Jimin Kim, Jonggyu Yoo, Giorgio Sangiovanni, Domenico Di Sante, Byeong-Gyu Park, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Efthimios Kaxiras, Stephen D. Wilson, Jae-Hoon Park and Riccardo Comin, January 13, 2022, Natural physics.
DOI: 10.1038 / s41567-021-01451-5
Additional authors of Natural physics paper is Shiang Fang from Rutgers University; Jeung-Kyu Kim, Jonggyu Yoo and Jae-Hoon Park from Max Planck POSTECH / Korea Research Initiative and Pohang University of Science and Technology (Korea); Brenden Ortiz of the University of California, Santa Barbara; Jimin Kim from the Department of Basic Sciences (Korea); Giorgio Sangiovanni from the University of Würzburg (Germany); Domenico Di Sante from the University of Bologna (Italy) and the Flatiron Institute; Byeong-Gyu Park in Pohang Light Source (Korea); Sae Hee Ryu, Chris Jozwiak, Aaron Bostwick, and Eli Rotenberg of Lawrence Berkeley National Laboratory; and Efthimios Kaxiras of Harvard University.

This work was funded by the Air Force Office of Scientific Research, National Science Foundation, National Research Foundation of Korea, a Samsung scholarship, a Rutgers Center for Material Theory Distinguished Postdoctoral Fellowship, California NanoSystems Institute, European Union Horizon 2020 program, the German research fund, and it used the resources of Advanced Light Source, a user facility of the Department of Energy Office of Science.

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