Advanced materials with more novel characteristics are almost always developed by adding more elements to the list of raw materials. But quantum research shows that some simpler materials may have advanced characteristics that scientists have not discovered until now.
Researchers from Georgia Institute of Technology and the University of Tennessee at Knoxville discovered hidden and unexpected quantum behaviors in a fairly simple iron iodide material (FeI2) discovered nearly a century ago. The behavior of this material was studied by neutron scattering experiment and theoretical physical calculation in Oak Ridge National Laboratory of the Ministry of Energy.
The team's discovery-published in the journal Nature Physics-solved a 40-year puzzle about the mysterious behavior of this material and can be used as a map to open the treasure house of quantum phenomena in other materials.
"Our findings are largely out of curiosity," said Bai, the first author of the paper. Dr. Bai received his Ph.D. from Georgia Institute of Technology and is currently a postdoctoral researcher, where he studied magnetic materials with neutrons. "I was part of the 20 19 doctoral thesis project and stumbled upon this iron iodide material. I am trying to find a compound with a magnetic triangular lattice arrangement, which shows the so-called' frustration magnetism'. "
In ordinary magnets, such as magnets on refrigerators, the electrons of this material are arranged like arrows, either pointing in the same direction-up or down-or alternately up and down. The direction in which electrons point is called "spin". In more complex materials, such as iron iodide, electrons are arranged in a triangular grid, and the magnetic forces between the three magnetic points collide with each other, and it is uncertain which direction to point to-therefore, "magnetic frustration".
"When I read all the literature, I noticed that this compound iron iodide was discovered in 1929 and was deeply studied in the 1970s and 1980s," Bai said. "At that time, they saw some strange or unconventional behavior patterns, but they didn't have real resources to fully understand why they saw them." Therefore, we know that some strange and interesting problems have not been solved. Compared with 40 years ago, we have more powerful experimental tools, so we decided to re-examine this problem and hope to provide some new insights. "
Quantum materials are usually described as systems that behave strangely and violate the laws of classical physics—for example, solid materials behave like liquids and particles move like water, and they will not freeze or stop moving even at freezing temperature. Understanding how these strange phenomena work or their underlying mechanisms is the key to promoting electronics and developing other next-generation technologies.
"In quantum materials, two things are very interesting: the phase states of matter, such as liquids, solids and gases, and the excitation of these phase states, such as sound waves. Similarly, spin waves are excited by magnetic solid materials, "said Martin Morigal, a physics professor at Georgia Institute of Technology. "For a long time, our exploration of quantum materials has been looking for strange phases, but the question we ask ourselves in this study is:' Maybe the phase itself is not obviously strange, but what if its excitation is strange?' "This is indeed our discovery."
Neutrons are ideal detectors for studying magnetism, because they are like miniature magnets, which can be used to interact with other magnetic particles and excite them without affecting the atomic structure of materials.
Bai was exposed to neutrons when he was a graduate student at Georgia Institute of Technology. Mourigal has been a neutron scattering user of ORNL Qualcomm Isotope Reactor (HFIR) and Spallation Neutron Source (SNS) for several years. Use the user facilities of the Science Office of the U.S. Department of Energy to study various quantum materials and their strange behaviors.
When Bai and Morigar exposed iron iodide materials to neutron beams, they expected to see a specific excitation or energy band related to the magnetic moment of a single electron. But what they see is not one, but two different quanta fluctuating at the same time.
"Neutrons let us see this hidden fluctuation very clearly, and we can measure its entire excitation spectrum, but we still don't understand why we see this abnormal behavior at an obvious classical stage," Bai said.
To find the answer, they turned to theoretical physicist Christian Batista, a professor of Lincoln at the University of Tennessee, Knoxville, and the deputy director of the ORNL Shuwolan Center. Shuerworland Center is a joint research institute of neutron science, providing additional neutron scattering resources and expertise for visiting researchers.
A small piece of iron iodide sample taken from Bai (above) is installed and prepared for neutron scattering experiment to measure the basic magnetic excitation of materials. Source: ORNL
With the help of Batista and his team, the team was able to model the mysterious quantum wave mathematically. After CORELLI and SEQUOIA instruments of SNS conducted additional neutron experiments, they were able to determine the mechanism that caused it.
Batista said: "The theoretical prediction and what we can confirm with neutrons is that this strange fluctuation will occur when the spin direction between two electrons is reversed and their magnetic moments are tilted in the opposite direction." "When neutrons interact with the spins of electrons, the spins rotate synchronously in a certain direction in space. This dance caused by neutron scattering produces spin waves. "
He explained that in different materials, the spin of electrons can show different directions and spin actions, thus producing different kinds of spin waves. In quantum mechanics, this concept is called "wave-particle duality", and new waves are regarded as new particles, which are usually hidden in neutron scattering under normal conditions.
"In a sense, we are looking for dark particles," Batista added. "We can't see them, but we know they are there because we can see their influence or their interaction with the particles we can see."
"In quantum mechanics, there is no difference between waves and particles. We understand the behavior of particles based on wavelength, which neutrons allow us to measure, "Bai said." "
Mourigal compares the way neutrons detect particles to waves around rocks on the ocean surface.
Morigel said: "In still water, we can't see the rocks on the seabed until the waves pass it." Only by using neutrons to generate as many waves as possible can Xiao Jian identify rocks, or in this case, make hidden waves visible through Christian's theory.
The application of quantum magnetic behavior has promoted the progress of technology, such as nuclear magnetic resonance imaging machine and magnetic hard disk storage, which has promoted personal computing. More exotic quantum materials may accelerate the next wave of technology.
In addition to Bai, Morigal and Batista, the authors of this paper include Zhang Shangshun,,,, Zhou Haidong, Matthew Si Tong, Alexander Kolesnikov and.
Since their discovery, the team has used these insights to develop and test a wider range of materials, which they expect will produce more promising results.
"When we introduce more ingredients into a material, we will also increase potential problems, such as disorder and heterogeneity. If we really want to understand and create clean quantum mechanical systems based on materials, it may be more important to return to these simple systems than we think. "
Bai said: "This has solved the mystery of the mysterious excitation in iron iodide for 40 years." "Our advantage today is that the progress of large neutron facilities (such as SNS) allows us to basically detect the entire energy and momentum space of materials and see what happens to these strange excitations.
"Now that we know how this strange behavior works in relatively simple materials, we can imagine what we will find in more complex materials." This new understanding inspires us, and we hope it will inspire the scientific community to study more such materials, which will certainly lead to more interesting physics. "