Exotic topological materials are surprisingly common


February 27, 2019

(Nanowerk NewsIn a major step towards a research area that won the Nobel Prize for Physics in 2016, an international team discovered that substances with exotic electronic behaviors called topological materials are indeed quite common, and include everyday elements like arsenic and gold. The team has created an online catalog (www.topologicalquantumchemistry.com) to facilitate the design of new topological materials using periodic table elements.

These materials have unexpected and strange properties that have changed scientists' understanding of how electrons behave. Researchers hope that these substances can form the basis of future technologies, such as low power devices and quantum computing.

"Once the analysis was done and all errors corrected, the result was surprising: more than a quarter of all materials exhibit some kind of topology," said B. Andrei Bernevig, a senior author of the article ("A Complete Catalog high-quality topological materials ") and professor of physics at Princeton. "Topology is ubiquitous in materials, not esoteric."

Topological materials are intriguing because their surfaces can conduct electricity without resistance, so they are potentially faster and more energy efficient than current technologies. Its name comes from an underlying theory that is based on topology, a branch of mathematics that describes objects by their ability to be stretched or bent.

The beginnings of theoretical understanding of these states of matter formed the basis of the 2016 Nobel Prize for Physics, divided between Professor F. Duncan Haldane of Princeton University, Professor of Physics Sherman Fairchild, J. Michael Kosterlitz of Brown University, and David. J. Thouless, University of Washington-Seattle.

So far, only a few hundred of the more than 200,000 known inorganic crystalline materials were characterized as topological and were believed to be anomalies.

"When it's complete, this catalog will usher in a new era of topological design," Bernevig said. "This is the beginning of a new type of periodic table where compounds and elements are indexed by their topological properties rather than by more traditional means."

The international team included Princeton researchers; the International Center of Physics Donostia, in San Sebastian, Spain; the Basque IKERBASQUE Foundation for Science; the University of the Basque Country; cole Normale Suprieure Paris and the French National Center for Scientific Research (CNRS); and the Max Planck Institute for Solid Chemical Physics.

The team investigated about 25,000 inorganic materials whose atomic structures are accurately known experimentally and classified in the database of inorganic crystal structures. The results show that instead of being rare, more than 27% of the materials in nature are topological.

The newly created database allows visitors to select elements from the periodic table to create compounds that the user can explore for their topological properties. More materials are being analyzed and placed in a database for future publication.

Two factors allowed the complex task of topologically classifying the 25,000 compounds.

First, two years ago, some of the present authors developed a theory, known as topological quantum chemistry and published in Nature in 2017, which allowed the classification of the topological properties of any material from the simple knowledge of the positions and nature of its atoms.

Second, in the current study, the team applied this theory to compounds in the database of inorganic crystal structures. In doing so, the authors needed to plan, write and modify a large number of computerized instructions to calculate the electron energies in the materials.

We had to go into these old programs and add new modules that would compute the required electronic properties, said Zhijun Wang, who was a postdoctoral researcher at Princeton and is now a professor at the National Laboratory of Condensed Matter Physics in Beijing and the Institute of Physics, Chinese Academy of Sciences.

We then needed to analyze these results and calculate their topological properties based on our newly developed topological quantum chemistry methodology, "said Luis Elcoro, a professor at the University of the Basque Country in Bilbao, Spain.

The authors have written several sets of codes that obtain and analyze the topology of electrons in real materials. The authors made these codes available to the public through the Bilbao Crystallographic Server (http://www.cryst.ehu.es/cgi-bin/cryst/programs/topological.pl). With the help of the Max Planck Supercomputer Center in Garching, Germany, the researchers then applied their codes to the 25,000 compounds.

"Computationalally, it was incredibly intense," said Nicolas Regnault, professor at the Normale Suprieure in Paris, and director of research at CNRS, the French National Center for Scientific Research. "Fortunately, the theory has shown us that we need to compute only a fraction of the data we needed previously. We need to see what the electron does only in part of the parameter space to get the system topology."

"Our understanding of materials became much richer because of this classification," said Maia Garcia Vergniory, a researcher at the Donostia International Physics Center in San Sebastian, Spain. "It's really the last line of understanding of the properties of materials."

Claudia Felser, a professor at the Max Planck Institute for Solid Chemical Physics in Dresden, Germany, previously predicted that even gold is topological. "Many of the properties of the material we know, such as the color of gold, can be understood through topological reasoning," Felser said.

The team is now working to classify the topological nature of additional compounds in the database. The next steps involve identifying the compounds with the best versatility, conductivity and other properties, and verifying experimentally their topological nature. "One can then dream of a complete topological periodic table," said Bernevig.


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