Scientists Discover Exciton Behavior in Van der Waals Magnets

A research group led by scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory has discovered details about the formation and behavior of moving, microscopic, particle-like objects called “excitons” in a class of materials known as van der Waals magnets.

Their work helps to build a picture of the complex relationship between the optical and magnetic properties of these materials, which exhibit intriguing characteristics that could one day lead to entirely new technologies based on magnetism, such as information storage.

The study is described in a paper published 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-rays 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 like a positively charged particle) that are coupled together and move as a unit. Discovery of excitons in NiPS₃ has generated considerable interest in this particular Van der Waals material.

This is due to the possible strong connection between excitons and the underlying magnetic structure, suggesting a path to understanding, and perhaps even controlling, excitons via magnetism. But despite several studies, scientists have so far failed to uncover the structure and motion of excitons in NiPS.₃.

The group addressed this challenge by using an X-ray technique called resonant inelastic X-ray scattering (RIXS), available on the NSLS-II Soft Inelastic X-ray Scattering (SIX) beamline. This state-of-the-art experimental facility was designed to use NSLS-II’s ultra-bright X-ray beams to study the electronic properties of solid materials, revealing energetic 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 asked 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 in many directions. At SIX, scientists can “capture” these photons and measure their momentum and energy with extremely high resolution. With this information, they can work backward to study the properties of electrons and holes in the material using software developed at Brookhaven.

They found that the formation and propagation of excitons through the NiPS₃ The crystal is governed by a physical principle called the Hund exchange interaction. This rule dictates the energy of different electron spin configurations, the tiny “up” or “down” magnetic moment carried by each electron. In NiPS₃This Hund exchange provides the energy necessary for the formation of the exciton.

The researchers also found that the exciton scatters in the crystal in a way similar to a type of spin perturbation called a “double magnon,” another quasiparticle. Magnons, which are collective excitations of electron spins in a crystal lattice, are another facet of the intertwined electronic and magnetic behaviors in van der Waals magnets.

“In the coming years, as instrumentation and techniques such as RIXS and electron microscopy are further developed, we hope to be able to take even better measurements of NiPS.₃” said Wei He, a postdoctoral researcher and first author of the study. “We believe this material has exceptional potential to pave the way for using Hund’s magnetic excitons to realize new forms of controllable magnetic information.”