In Quantum Materials, Magnetism Promotes unusual Electronic Order

Magnetism can indeed play a significant role in fostering unusual electronic order in certain quantum materials. When a material is subjected to a magnetic field, it can cause the material’s electrons to align themselves in a particular way, resulting in a magnetic ordering. This magnetic ordering can then interact with the electronic properties of the material in various ways, leading to a range of interesting phenomena.

Physicists have published a slew of experimental evidence demonstrating that the ordered magnetic arrangement of electrons in iron-germanium crystals plays a critical role in the formation of an ordered electronic arrangement known as a charge density wave, which the team discovered in the material last year.

The discovery in 2022 that electrons in magnetic iron-germanium crystals could spontaneously and collectively organize their charges into a pattern with a standing wave surprised physicists. Magnetism is also caused by the collective self-organization of electron spins into ordered patterns, which rarely coexist with the patterns that produce the standing wave of electrons known as a charge density wave by physicists.

In a study published this week in Nature Physics, Rice University physicists Ming Yi and Pengcheng Dai, and many of their collaborators from the 2022 study, present an array of experimental evidence that shows their charge density wave discovery was rarer still, a case where the magnetic and electronic orders don’t simply coexist but are directly linked.

We found magnetism subtly modifies the landscape of electron energy states in the material in a way that both promotes and prepares for the formation of the charge density wave.

Ming Yi

“We found magnetism subtly modifies the landscape of electron energy states in the material in a way that both promotes and prepares for the formation of the charge density wave,” said Yi, a co-corresponding author of the study.

The study was co-authored by more than a dozen researchers from Rice; Oak Ridge National Laboratory (ORNL); SLAC National Accelerator Laboratory; Lawrence Berkeley National Laboratory (LBNL); the University of Washington; the University of California, Berkeley; Israel’s Weizmann Institute of Science; and China’s Southern University of Science and Technology.

The iron-germanium materials are kagome lattice crystals, a well-studied family of materials with 2D atom arrangements reminiscent of the weave pattern in traditional Japanese kagome baskets, which features equilateral triangles that touch at the corners.

“Kagome materials have recently taken the quantum materials world by storm,” Yi explained. “The cool thing about this structure is that the geometry imposes interesting quantum constraints on the way electrons are allowed to zoom around, somewhat analogous to how traffic roundabouts affect traffic flow and sometimes bring it to a halt.”

Electrons, by definition, avoid one another. One method is to order their magnetic states (spins that point up or down) in the opposite direction of their neighbors’ spins.

Magnetism fosters unusual electronic order in quantum material

Dai, a co-corresponding study author, said, “When put onto kagome lattices, electrons can also appear in a state where they are stuck and cannot go anywhere due to quantum interference effects.”

When electrons are unable to move, the triangular arrangement causes each to have three neighbors, and there is no way for electrons to collectively order all neighboring spins in opposite directions. Electron frustration in Kagome lattice materials has long been recognized.

The lattice restricts electrons in ways that “have a direct impact on the observable properties of the material,” according to Yi, and the team was able to use this to “probe deeper into the origins of the intertwinement of the magnetism and charge density wave” in iron-germanium.

They did so by combining inelastic neutron scattering experiments at ORNL with angle-resolved photoemission spectroscopy experiments at LBNL’s Advanced Light Source and SLAC’s Stanford Synchrotron Radiation Lightsource, as well as Yi’s lab at Rice.

“These probes allowed us to see what the electrons and the lattice were doing as the charge density wave formed,” she explained.

The findings, according to Dai, confirm the team’s hypothesis that charge order and magnetic order are linked in iron-germanium. “This is one of the few, if not the only, known examples of a kagome material in which magnetism forms first, paving the way for charges to line up,” he explained.

According to Yi, the work demonstrates how curiosity and basic research into natural phenomena can lead to applied science. “As physicists, we are always excited when we discover materials that spontaneously form some sort of order,” she explained. “This means there is a chance for us to learn about the self-organizational abilities of quantum material’s fundamental particles. Only with such knowledge can we hope to engineer materials with novel or exotic properties that we can control at will.”

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