In search of flatness in materials

Finding the right ingredients to create materials with exotic quantum properties has been a dream come true for experimental scientists, due to the endless possible combinations of different elements to synthesize.

Now, the creation of these materials could be less blind thanks to an international collaboration led by Andrei Bernevig, visiting Ikerbasque professor at the Donostia International Center for Physics (DIPC) and professor at Princeton University, and Nicolas Regnault. , from Princeton University and the Ecole Normale Supérieure Paris, CNRS, with the participation of Luis Elcoro from the University of the Basque Country (UPV / EHU).

The team conducted a systematic search for potential candidates in a huge barn of 55,000 materials. The removal process began with the identification of so-called flat-band materials, that is, electronic states with constant kinetic energy. Therefore, on a flat band, the behavior of electrons is mainly governed by interactions with other electrons. However, researchers realized that flatness is not the only requirement, because when electrons are too tightly bound to atoms, even in a flat band, they are unable to move and create interesting latitude states. “You want the electrons to look at each other, which you can do by making sure they’re lying in space. That’s exactly what topological strips put on the table,” says Nicolas Regnault.

Topology plays a crucial role in modern condensed matter physics, as suggested by the three Nobel Prizes of 1985, 1997, and 2016. It imposes the extension of certain quantum wave functions, making them insensitive to local perturbations such as the impurities. It can impose certain physical properties, such as resistance, to be quantified or lead to perfectly conductive surface states.

Fortunately, the team has been at the forefront of characterizing the topological properties of bands through their approach known as “topological quantum chemistry,” giving them an extensive database of materials as well as theoretical tools for finding topological flat bands.

Using tools ranging from analytical methods to brute force searches, the team found all the flat-band materials that were currently known in nature. This catalog of flat band materials is available online at https://www.topologicalquantumchemistry.fr/flatbands with its own search engine. “The community can now look for flat topological bands in materials. We found, out of 55,000 materials, about 700 that showed what could be potentially interesting flat bands,” says Yuanfeng Xu of Princeton University and Max Planck Institute. of Microstructural Physics, one of the two main authors of the study. “We’ve made sure that the materials we promote are promising candidates for chemical synthesis,” said Leslie Schoop of the Princeton Department of Chemistry. The team then classified the topological properties of these bands, finding out what type of delocalized electrons they contain.

Now that this extensive catalog has been completed, the team will begin cultivating the predicted materials to experimentally discover the myriad of potential new states of interaction. “Now that we know where to look, we need to grow these materials,” says Claudia Felser of the Max Planck Institute for Solid State Chemical Physics. “We have a dream team of experimenters working with us. They are eager to measure the physical properties of these candidates and see what exciting quantum phenomena will arise.”

The catalog of flat belts, published in Nature March 30, 2022 marks the end of years of research by the team. “Many people and many donor institutions and universities to whom we presented the project said it was too difficult and could never be done. It took us a few years, but we did it, “said Andrei Bernevig.

The publication of this catalog will not only reduce the serendipity in the search for new materials, but will also allow extensive searches for compounds with exotic properties, such as magnetism and superconductivity, with applications in memory devices or in transport without long power. distance. dissipation.

Financing

Funding for the project was mainly provided by an advanced grant from the European Research Council (ERC) to the DIPC (SUPERFLAT, ERC-2020-ADG).

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