A research group led by scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory has uncovered details about the formation and behavior of mobile, microscopic, particle-like objects called “excitons” in a class of materials known as van der Waals magnets. Their work helps establish a picture of the complex relationship between the optical and magnetic properties of these materials, which display intriguing characteristics that could one day lead to brand-new technologies based on magnetism, such as information storage.
The study is described in a paper in the April 25, 2024, online edition of the journal Nature Communications.
The researchers studied the crystalline material nickel phosphorus trisulfide (NiPS3) using the National Synchrotron Light Source II (NSLS-II), a DOE Office of Science user facility located at Brookhaven. NSLS-II produces intense beams of X-ray light that are used to study a wide range of materials and biological samples, from battery compounds to proteins.
An exciton consists of an electron and a “hole” — a space in a crystal that lacks an electron and behaves as a positively charged particle — that are coupled together and move as a unit. Discovering excitons in NiPS3 has sparked significant interest in this particular van der Waals material. This is due to the possible strong link between the excitons and the underlying magnetic structure, suggesting a route to understanding, and perhaps even controlling, excitons via magnetism. But despite several studies, scientists have been so far unable to uncover the exciton structure and motion in NiPS3.
The group tackled this challenge using an X-ray technique known as resonant inelastic X-ray scattering (RIXS), available at NSLS-II's Soft Inelastic X-ray Scattering (SIX) beamline. This experimental station was designed to use NSLS-II's ultrabright X-ray beams to study the electronic properties of solid materials, revealing energy behaviors at very high resolution.
“What is the fundamental nature of an exciton? How does it interact with magnetism? These are two of the questions we looked to RIXS to help us answer,” said Brookhaven physicist Mark Dean, one of the paper’s authors.
In RIXS, X-ray photons hit electrons in the material and scatter off in many directions. At SIX, scientists can “catch” these photons and measure their momenta and energies with extremely high resolution. With this information, using software developed at Brookhaven, they can work backward to study the properties of electrons and holes in the material.
They found that exciton formation and propagation through the NiPS3 crystal is governed by a physics principle called Hund’s exchange interaction. This rule dictates the energy of different configurations of electron spin, which is carried by every electron as either an "up" or "down" magnetic moment. In NiPS3, this Hund’s exchange provides the energy necessary for exciton formation.
The researchers also found that exciton dispersal through the crystal resembles a type of spin disturbance called a “double-magnon,” another quasiparticle. Magnons are collective excitations of electron spins in a crystal lattice and represent another facet of intertwined electronic and magnetic behaviors in van der Waals magnets.
“In the coming years, as instrumentation and techniques like RIXS and electron microscopy are further developed, we expect to be able to take even better measurements of NiPS3,” said postdoctoral researcher Wei He. “We believe that this material has outstanding potential for opening a pathway for using magnetic Hund’s excitons to realize new forms of controllable magnetic information.”
The research team also included Johnny Pelliciari and Valentina Bisogni from Brookhaven; Krzysztof Wohlfeld from the University of Warsaw; Steve Johnston from University Tennessee Knoxville; Edoardo Baldini from University Texas Austin; Matteo Mitrano from Harvard University.
This study was supported by DOE Office Science University Warsaw United States Army Research Office National Science Foundation Brookhaven Laboratory Directed Research Development funding.
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