​​​​​​​Intertwined: Charge and Magnetism Intertwine in the Kagome Material | Rice News | News and Media Relations


HOUSTON – (September 14, 2022) – Physicists have discovered a material in which atoms are arranged in a way that frustrates the movement of electrons so much that they engage in a collective dance where their electronic and magnetic natures seem to both compete and cooperate in unexpected ways.

Colors are used to illustrate the charge density wave patterns that occur at extremely low temperatures in magnetic iron-germanium crystals. The material is an example of a kagome lattice metal with a crystal lattice arrangement of atoms in hexagons (colors) and triangles (black). The lattice layout impedes the movement of electrons (blue and silver spheres), giving rise to collective behavior like the charge density wave. (Illustration courtesy of Jiaxin Yin, Ming Yi and Pengcheng Dai.)

Led by physicists at Rice University, the research was published online today in Nature. In experiments at Rice, Oak Ridge National Laboratory (ORNL), SLAC National Accelerator Laboratory, Lawrence Berkeley National Laboratory (LBNL), University of Washington (UW), Princeton University, and University of California at Berkeley, The researchers studied pure-germanium iron crystals and discovered standing waves of fluid electrons appeared spontaneously in the crystals when they were cooled to an extremely low temperature. Curiously, the charge density waves appeared while the material was in a magnetic state, which it had transitioned to at a higher temperature.

“A charge density wave usually occurs in materials that have no magnetism,” said study co-corresponding author Pengcheng Dai of Rice. “Materials that have both a charge density wave and magnetism are actually rare. Even rarer are those where the charge density wave and the magnetism “talk” to each other, as they seem to do in this case.

“Usually the charge density wave occurs at the same time as the magnetism or at a higher temperature than the magnetic transition,” he said. “This particular case seems to be special, because the charge density wave actually occurs at a temperature much lower than the magnetism. We don’t know of any other example where this actually happens in a material like this that has a kagome network. This suggests that it could be related to magnetism.

The iron-germanium crystals used in the experiments were grown in Dai’s lab and exhibit a distinct arrangement of atoms in their crystal lattice reminiscent of patterns found in Japanese kagome baskets. The equilateral triangles in the lattice force the electrons to interact, and because they hate being close to each other, this forcing frustrates their movements. The forcing increases as temperatures drop, giving rise to collective behaviors like the charge density wave.

Ming Yi
Ming Yi (Photo by Jeff Fitlow/Rice University)

The study’s co-corresponding author, Ming Yi, also from Rice, said: ‘The charge density wave is like waves forming on the surface of the ocean. It only forms when the conditions are met. In this case, we observed it when a unique saddle-like feature appeared in the quantum states in which electrons are allowed to live. The connection to the magnetic order is that this charge density wave only occurs when magnetism causes the saddle to appear. . This is our assumption. »

The experiments offer tantalizing insight into the properties physicists will find in quantum materials that exhibit both topological features and those resulting from strongly correlated electronic interactions.

In topological materials, quantum entanglement patterns produce “protected” states that cannot be erased. The immutable nature of topological states is of growing interest in quantum computing and spintronics. The earliest topological materials were nonconductive insulators whose shielded states allowed them to conduct electricity in limited ways, such as on 2D exterior surfaces or along 1D edges.

Pengcheng Dai
Pengcheng Dai (Photo by Jeff Fitlow/Rice University)

“In the past, topological materials were very loosely correlated types,” said Yi, assistant professor of physics and astronomy at Rice. “People have used these materials to really understand the topology of quantum materials, but the challenge now is to find materials where we can take advantage of both topological states and strong electronic correlations.”

In strongly correlated materials, the interactions of billions and billions of electrons give rise to collective behaviors such as unconventional superconductivity or continual fluctuations between magnetic states in quantum spin liquids.

“For weakly correlated materials like original topological insulators, first principle calculations work very well,” Yi said. “Just based on how the atoms are arranged, you can calculate what kind of band structure to expect. There is a very good path from a material design point of view. You can even predict material topology.

Physics graduate student Xiaokun Teng
Xiaokun Teng (Photo by Jeff Fitlow/Rice University)

“But strongly correlated materials are more difficult,” she said. “There is a lack of connection between theory and measurement. So not only is it hard to find materials that are both strongly correlated and topological, but when you find and measure them, it’s also very hard to relate what you’re measuring to a theoretical model that explains what’s going on. pass.

Yi and Dai said kagome lattice materials could lead the way.

“At some point you want to be able to say, ‘I want to make a material with particular behaviors and properties,'” Yi said. means to make direct predictions, based on the crystal structure, what kind of band structure you will get and therefore what phenomena can occur based on that band structure.It has many of the right ingredients.

Dai is the Sam and Helen Worden Professor of Physics and Astronomy. Dai and Yi are each members of the Rice Quantum Initiative and the Rice Center for Quantum Materials (RCQM).

Study co-authors include Xiaokun Teng, Lebing Chen, Kelly Neubauer, Bin Gao and Yaofeng Xie of Rice; Feng Ye of ORNL; Elliott Rosenberg, Zhaoyu Liu and Jiun-Haw Chu from UW; Jia-Xin Yin, Yu-Xiao Jiang and Zahid Hasan of Princeton; Ji Seop Oh of Rice and UC Berkeley; Robert Birgeneau of UC Berkeley; Makoto Hashimoto and Donghui Lu of SLAC; and LBNL’s Chris Jozwiak, Aaron Bostwick and Eli Rotenberg.

Research at Rice was funded by the National Science Foundation (2100741), the Welch Foundation (C-1839, C-2024), the Department of Energy (DE-SC0021421), and the Gordon and Betty Moore Foundation (GBMF9470).

Peer-reviewed article

“Discovery of a charge density wave in a kagome lattice antiferromagnet” | Nature | DOI: 10.1038/s41586-022-05034-z

Xiaokun Teng, Lebing Chen, Feng Ye, Elliott Rosenberg, Zhaoyu Liu, Jia-Xin Yin, Yu- Xiao Jiang, Ji Seop Oh, M. Zahid Hasan, Kelly J. Neubauer, Bin Gao, Yaofeng Xie, Makoto Hashimoto, Donghui Lu , Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Robert J. Birgeneau, Jiun-Haw Chu, Ming Yi and Pengcheng Dai


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LEGEND: Colors are used to illustrate the charge density wave patterns that occur at extremely low temperatures in magnetic iron-germanium crystals. The material is an example of a kagome lattice metal with a crystal lattice arrangement of atoms in hexagons (colors) and triangles (black). The lattice layout impedes the movement of electrons (blue and silver spheres), giving rise to collective behavior like the charge density wave. (Illustration courtesy of Jiaxin Yin, Ming Yi and Pengcheng Dai.)


CAPTION: Pengcheng Dai (Photo by Jeff Fitlow/Rice University)

CAPTION: Ming Yi (Photo by Jeff Fitlow/Rice University)

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