Magnetic materials believed to host a quantum spin liquid have drawn strong interest because of their potential to reveal exotic states of matter and advance quantum computing. However, appearances in the quantum world can be misleading. A new study published in Science Advances and co-led by Rice University’s Pengcheng Dai shows that cerium magnesium hexalluminate (CeMgAl11O19), once thought to belong to this rare category, is not actually a quantum spin liquid.
“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.”
How Magnetic States Normally Behave
In insulating materials such as CeMgAl11O19, magnetic ions like cerium can adopt one of two arrangements: ferromagnetic or anti-ferromagnetic. In a ferromagnetic state, ions align in the same direction, with each one encouraging its neighbors to do the same. In an anti-ferromagnetic state, neighboring ions point in opposite directions, creating a different kind of ordered pattern.
Scientists can observe these arrangements by cooling materials to temperatures close to absolute zero. Under these conditions, conventional materials settle into a single, stable low energy state. Because all ions align in the same type of arrangement, researchers typically see just one configuration.
What Makes Quantum Spin Liquids Different
Quantum spin liquids behave in a very different way. Instead of settling into one fixed state, they continuously shift between multiple low energy states through quantum effects. This leads to a spread, or continuum, of observable states rather than a single one. It also results in a lack of magnetic ordering, since both ferromagnetic and anti-ferromagnetic tendencies can appear at the same time.
CeMgAl11O19 showed both of these key features. It lacked clear magnetic order and displayed a continuum of states, which initially pointed to a quantum spin liquid. However, a closer look revealed a different explanation. The observed continuum came from a degeneration of states caused by competing ferromagnetic and antiferromagnetic interactions, not from quantum 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.”
A Subtle Magnetic Competition
To uncover what was really happening, the team used neutron scattering along with other precise measurements. They found that the boundary between ferromagnetic and anti-ferromagnetic behavior in this material is unusually weak. This allows the magnetic ions to move more freely between the two states instead of locking into a single pattern.
As a result, some ions behave ferromagnetically while others behave anti-ferromagnetically within the same structure. This mixed arrangement prevents the system from forming a single ordered state and instead creates many possible low energy configurations. When cooled to near absolute zero, the material can settle into any one of these configurations, producing a range of observed states that resemble the continuum seen in quantum spin liquids. However, unlike a true quantum spin liquid, once the material settles into one state, it remains there and does not transition between states.
“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.”
A Reminder of Quantum Complexity
The discovery highlights how complex and surprising magnetic systems can be. Even when a material appears to match the expected signatures of a quantum state, the underlying physics may tell a different story.
This unique material, Dai added, is a good reminder of how much we don’t know about the quantum realm. “It underscores the importance of careful observation and thorough investigation of your data.”
Funding and Research Support
The neutron scattering and AC magnetic susceptibility work at Rice was supported by the U.S. Department of Energy’s Basic Energy Sciences (DE-SC0012311, DE-SC0026179). The single crystal growth work was supported by the Robert A. Welch Foundation (C-1839). Crystal growth by BG, XX, and SWC at Rutgers University was supported by the visitor program at the Center for Quantum Materials Synthesis, funded by the Gordon and Betty Moore Foundation’s EPiQS initiative (GBMF6402) and by Rutgers. The theoretical work done by CL and LB was supported by the DOE, Office of Science, BES (DE-FG02-08ER46524) and the Simons Collaboration on Ultra-Quantum Matter. 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.
