Physicists have discovered a material in which atoms are arranged in a way that impedes 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.
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 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, which 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.
The study’s co-corresponding author, Ming Yi, also of Rice, says that “the charge density wave is like waves forming on the surface of the ocean. It only forms when the conditions are united. In this case, we observed it when a unique feature in the form of a saddle appeared in the quantum states that electrons are allowed to live in. The connection to the magnetic order is that this wave of charge density only occurs when magnetism causes the saddle to appear.This is our hypothesis.
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.
“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 the original topological insulators, first-principle calculations work very well,” Yi said. “Just based on how the atoms are arranged, you can calculate what type of band structure to expect. There is a very good path from a materials design perspective. You can even predict the topology of materials .”
“But strongly correlated materials are more difficult,” she said. “There is a disconnect between theory and measurement. So not only is it hard to find materials that are both strongly correlated and topological, but when you find them and measure them, it’s also very hard. to connect what you are measuring with a theoretical model that explains what is happening.”
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. “I think kagome is a good platform in this direction, because there are ways to make direct predictions, based on the crystal structure, what type of band structure you will get and therefore what phenomena may occur depending on of that band structure. It has a lot of the right ingredients.”
Newly discovered magnetic interactions could lead to new ways to manipulate electron flow
Pengcheng Dai, Discovery of the charge density wave in a kagome lattice antiferromagnet, Nature (2022). DOI: 10.1038/s41586-022-05034-z. www.nature.com/articles/s41586-022-05034-z
Provided by Rice University
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