Hidden and unexpected quantum behavior uncovered in simple iron-iodide material

Neutrons piece together a 40-year puzzle behind iron-iodide's mysterious magnetism.

According to quantum research, some simpler materials might already have advanced properties that scientists couldn’t see.

Recently such advanced and unexpected quantum behavior was uncovered in simple iron-iodide material (FeI2). The study by the scientists from Georgia Tech and the University of Tennessee–Knoxville was possible using a combination of neutron scattering experiments and theoretical physics calculations at the Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL).

The material iron-iodide was discovered in 1929. It was intensively studied during the 1970s and 80s: scientists observed some peculiarity or unconventional modes of behavior. At that time, they didn’t have the resources to understand why they saw it fully.

Scientists, in this study, were aware that there was something unsolved that was strange and interesting in this material. Hence, they decided to revisit this problem and hoped to provide some new insights.

Martin Mourigal, professor of physics at Georgia Tech, said, “For a long time, our quest in quantum materials has been to find exotic phases, but the question we asked ourselves in this research is ‘Maybe the phase itself is not exotic, but what if its excitations are?’ And indeed, that’s what we found.”

The neutron has a magnetic moment that couples to spatial variations of magnetization on the atomic scale. Neutrons are, therefore, ideally suited to the study of magnetic structures and the fluctuations and excitations of spin systems.

When scientists exposed the iron-iodide material to a beam of neutrons, they saw not one but two different quantum fluctuations emanating simultaneously. At first, scientists were expecting to see single excitation or band of energy associated with a magnetic moment from a single electron. Still, they were in awe to see two different quantum fluctuations that allowed them to see hidden fluctuation very clearly.

Xiaojian Bai, the paper’s first author, said, “We could measure its entire excitation spectrum, but we still didn’t understand why we saw such abnormal behavior in a classical phase.”

To seek the answers, scientists collaborated with theoretical physicist Cristian Batista, Lincoln Chair Professor at the University of Tennessee–Knoxville and deputy director of ORNL’s Shull Wollan Center. The collaboration modeled the behavior of the mysterious quantum fluctuation. After performing additional neutron experiments using the CORELLI and SEQUOIA instruments at SNS, they were able to identify the mechanism that was causing it to appear.

Batista said, “What theory predicted and what we were able to confirm with neutrons, is that this exotic fluctuation happens when the spin direction between two electrons is flipped, and their magnetic moments tilt in opposite directions. When neutrons interact with the spins of the electrons, the spins rotate in synchronicity along a certain direction in space. This choreography triggered by neutron scattering creates a spin wave.”

“In different materials, electronic spins can take on many different orientations and spin choreographies that create different kinds of spin waves. In quantum mechanics, this concept is known as “wave-particle duality,” wherein the new waves are regarded as new particles and are typically hidden to neutron scattering under normal conditions.”

“In a sense, we’re looking for dark particles. We can’t see them, but we know they’re there because we can see their effects or the interactions they’re having with the particles that we can see.”

Bai said“In quantum mechanics, there’s no distinction between waves and particles. We understand the particle’s behavior based on the wavelength, and that’s what neutrons allow us to measure.”

“Now that we understand how this exotic behavior works in a relatively simple material, we can imagine what we could find in more complicated ones. This new understanding has motivated us, and hopefully, it will encourage the scientific community to investigate more of these kinds of materials which will undoubtedly lead to more exciting physics.”

Journal Reference:
  1. Xiaojian Bai et al., Hybridized quadrupolar excitations in the spin-anisotropic frustrated magnet FeI2, Nature Physics (2021). DOI: 10.1038/s41567-020-01110-1

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