This “Quantum” Material Fooled Scientists – but It’s Actually Something Even Stranger

A material thought to be a quantum spin liquid actually exhibits a newly identified magnetic state caused by competing ferromagnetic and antiferromagnetic interactions.

Materials that enter a quantum spin liquid phase attract significant attention because of their unusual properties and potential applications in quantum computing. However, the quantum world often presents surprising results. A recent study published in Science Advances and co-led by Rice University physicist Pengcheng Dai found that cerium magnesium hexalluminate (CeMgAl₁₁O₁₉) does not actually form a quantum spin liquid, even though earlier observations seemed to point in that direction.

“The material had been classified as a quantum spin liquid due to two properties: observation of a continuum of states and lack of magnetic ordering,” said Bin Gao, co-first author and a research scientist at Rice. “But closer observation of the material showed that the underlying cause of these observations wasn’t a quantum spin liquid phase.”

In insulating materials such as CeMgAl₁₁O₁₉, magnetic ions like cerium can adopt one of two arrangements: ferromagnetic or antiferromagnetic. In a ferromagnetic state, an ion tends to influence nearby ions to align in the same direction, leading to a structure where the magnetic moments point the same way. In an antiferromagnetic state, neighboring ions orient in opposite directions, creating an alternating pattern. Researchers can observe how these magnetic arrangements form by cooling the material to temperatures near absolute zero.

Ferromagnetic vs. Antiferromagnetic States in Magnetic Materials

At extremely low temperatures, conventional nonquantum materials settle into a single low-energy configuration once their ions align in one of these magnetic states. Because all the ions adopt either the ferromagnetic or anti-ferromagnetic arrangement, researchers typically observe just one stable low-energy state.

Quantum spin liquids behave differently. Even at temperatures near absolute zero, these materials do not settle into a single configuration. Instead, quantum mechanical effects allow them to continuously shift between multiple low-energy states. This dynamic behavior produces a continuum of states rather than a single one. It also prevents the material from developing a fixed magnetic pattern, meaning both ferromagnetic and anti-ferromagnetic arrangements appear instead of a single ordered structure.

CeMgAl11O19 displayed both of these features. It showed no clear magnetic ordering and produced a continuum of states. However, detailed analysis revealed that this behavior did not originate from a true quantum spin liquid. Instead, it resulted from a degeneracy of states caused by competing ferromagnetic and antiferromagnetic interactions within the material.

Investigating CeMgAl₁₁O₁₉’s Unusual Magnetic Behavior

“We were interested in this material, which had a collection of characteristics we hadn’t seen before,” said Tong Chen, co-first author and a research scientist at Rice. “It was not a quantum spin liquid, yet we were observing what we thought were quantum spin liquid-associated behaviors.”

To understand the source of the unusual signals, the research team used neutron scattering experiments along with additional measurements. Their results strongly suggested that the boundary separating the ferromagnetic and antiferromagnetic states in CeMgAl₁₁O₁₉ is weaker than in most magnetic materials. Because of this weaker boundary, the magnetic ions can more easily switch between the two possibilities.

As a result, the ions do not lock into a single ordered configuration. Instead, some adopt ferromagnetic alignment while others remain antiferromagnetic within the same structure. This mixture prevents the formation of long-range magnetic order.

The absence of ordering also increases the number of possible low-energy configurations available to the material. When cooled to near absolute zero, CeMgAl₁₁O₁₉ can settle into any one of several low-energy states. This produces a range of observable states that resemble the continuum typically associated with quantum spin liquids. However, because the material is not truly in that phase, it cannot move between these states once it settles into one.

Discovery of a New Magnetic State of Matter

“The material’s unique ability to ‘choose’ between different low-energy states produced observational data very similar to a quantum spin liquid state,” said Dai, corresponding author on this study. “This is a new state of matter that, to our knowledge, we are the first to describe.”

Dai noted that discoveries like this highlight how much scientists still have to learn about quantum materials. “It underscores the importance of careful observation and thorough investigation of your data.”

Reference: “Spin excitation continuum from degenerate states in the mixed ferro-antiferromagnetic exchange system CeMgAl₁₁O₁₉” by Bin Gao, Tong Chen, Chunxiao Liu, Mason L. Klemm, Shu Zhang, Zhen Ma, Xianghan Xu, Choongjae Won, Gregory T. McCandless, Karthik Rao, Naoki Murai, Seiko Ohira-Kawamura, Stephen J. Moxim, Jason T. Ryan, Xiaozhou Huang, Xiaoping Wang, Manh Duc Le, Emilia Morosan, Julia Y. Chan, Sang-Wook Cheong, Oleg Tchernyshyov, Leon Balents and Pengcheng Dai, 6 March 2026, Science Advances.
DOI: 10.1126/sciadv.aed7778

This article was supported by the U.S. Department of Energy’s Basic Energy Sciences (DE-SC0012311, DE-SC0026179), the Robert A. Welch Foundation (C-1839), and the visitor program at the Center for Quantum Materials Synthesis. This article was funded by the Gordon and Betty Moore Foundation’s EPiQS initiative (GBMF6402) and by Rutgers.

Researchers received individual support from the Gordon and Betty Moore Foundation through the Emergent Phenomena in Quantum Systems program; the National Natural Science Foundation of China (12204160); the National Research Foundation of Korea, Ministry of Science and ICT (2022M3H4A1A04074153); and the Welch Foundation (AA-2056-20240404). The neutron scattering experiment at the MLF of J-PARC was performed under proposal No. 2022B0242. This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by Oak Ridge National Laboratory.

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