Florida State University scientists study quantum properties of cesium-vanadium-antimonide.

In quantum physics, the name Kagome, an ancient design seen in traditional Japanese basket-weaving, has been borrowed by scientists to describe a class of ferromagnetic quantum materials with an atomic structure closely resembling this distinctive lattice pattern.

A new Florida State University (FSU) study published in Nature Communications focuses on how a particular Kagome metal interacts with light to generate what are known as plasmon polaritons - nanoscale-level linked waves of electrons and electromagnetic fields in a material, typically caused by light or other electromagnetic waves.

Kagome is characterized by a symmetrical pattern of interlaced triangles.

Since 2019, physicists have been striving to better understand Kagome metal properties and its potential applications. The atomic lattice in a Kagome material has layered overlapping triangles and large hexagonal voids where magnetic effects can cause electrons to flow in more organized patterns.

The Kagome material explored in the study is made of three elements – cesium, vanadium, and antimony.

Kagome metals have mystified scientists for their ability to exhibit collective behavior when cooled below room temperature, including superconductivity, which allows a material to conduct electricity without the loss of energy.

In natural metals, electrons have no organization. In a Kagome superconductor, when the material is cooled to 3 Kelvin (approximately minus 454 degrees Fahrenheit), the electrons begin to move in pairs, like couples at a dance. At 100 Kelvin, the electrons arrange themselves in the shape of ripples.

During this study, the researchers, led by FSU Assistant Professor of Physics Guangxin Ni, examined the metal cesium-vanadium-antimonide (CsV3Sb5) to better understand the properties that make it a promising contender for more precise and efficient photonic technologies.

The researchers identified for the first time the existence of plasmons in CsV3Sb5 and found that the wavelength of those plasmons depends upon the metal's thickness. They also found that changing the frequency of a laser on the metal caused the plasmons to behave differently, allowing them to move from surface confinement to spread throughout the material. The waves also traveled more effectively, with less energy loss (a pattern called hyperbolic bulk).

Courtesy of Guangxin Ni

A diagram showing plasmon waves moving through the Kagome metal cesium vanadium antimonide.

"Hyperbolic plasmon polaritons can offer a range of amazing nano-optical features and abilities," Ni said. "They have the potential to boost optical communication systems, allow for super-clear imaging beyond current limits and make photonic devices work better. They could also be useful for sensing things like environmental changes and medical diagnostics because they react strongly to their surroundings. These qualities make them key for advancing future optical and photonic technologies."

To explore how plasmons interacted with the metal, the researchers grew cesium-vanadium-antimonide crystals, placing thin flakes of the material onto specially prepared gold surfaces.

By using lasers to perform scanning infrared nano-imaging, they observed how the waves of electrons interacting with electromagnetic fields changed. Recent advances in imaging technology at the nano-scale level helped the researchers complete and record their work.

"What makes CsV3Sb5 interesting is how it interacts with light on a very small scale, what's known as nano-optics," said lead author Hossein Shiravi, a graduate research assistant at the FSU-headquartered National High Magnetic Field Laboratory. "We found that over a wide range of infrared light frequency, the correlated electrical properties within the metal triggered the formation of hyperbolic bulk plasmons."

The hyperbolic pattern translates to less energy loss.

"Electronic losses typically encountered in conventional metals have previously complicated efforts to observe exotic light-matter coupling effects, including hyperbolic polaritons," Ni said. "This is part of what makes this an exciting breakthrough."

The study was conducted in collaboration with researchers from the University of California Santa Barbara, Oak Ridge National Laboratory in Tennessee, Tsinghua University in China, and Germany's University of Stuttgart, Leipzig University, and Institute of Ion Beam Physics and Materials Research. At FSU, this research is supported by funding from the U.S. Department of Energy and National Science Foundation.