Magnetic materials thought to host quantum spin liquids are of intense interest because of their potential to reveal exotic states of matter and advance quantum computing. However, appearances in the quantum world can be misleading. New research published in scientific progress Rice University’s Pengcheng Dai and colleagues have shown that cerium magnesium hexaaluminate (CeMgAl11O19), once thought to belong to this rare category, is not actually a quantum spin liquid.
“Two properties classified this material as a quantum spin liquid: the observation of a continuum of states and the lack of magnetic order,” said co-first author Bing Gao, a researcher at the Rice Institute. “However, when we looked closely at the material, we found that the quantum spin liquid phase was not the root cause of these observations.”
How do magnetic states usually behave?
In insulating materials such as CeMgAl11O19, magnetic ions such as cerium can adopt one of two configurations: ferromagnetic or antiferromagnetic. In the ferromagnetic state, the ions align in the same direction, and each ion encourages neighboring ions to align in the same direction. In the antiferromagnetic state, adjacent ions point in opposite directions, forming different kinds of regular patterns.
Scientists can observe these arrangements by cooling matter to temperatures close to absolute zero. Under such conditions, conventional materials settle into a single, stable, low-energy state. All ions align in the same type of configuration, so researchers typically only see one configuration.
Differences between quantum spin liquids
Quantum spin liquids behave very differently. Rather than settling into one fixed state, they continuously transition between multiple low-energy states due to quantum effects. This results in a spread, or continuum, of observable states rather than a single state. It also results in a lack of magnetic order, as both ferromagnetic and antiferromagnetic tendencies can appear simultaneously.
CeMgAl11O19 exhibited both of these important features. It lacks a clear magnetic order and exhibits a continuum of states, initially indicating a quantum spin liquid. But a closer look reveals a different explanation. The observed continuum does not result from quantum behavior, but from a degeneracy of states caused by competing ferromagnetic and antiferromagnetic interactions.
“We were interested in this material, which had a set of characteristics that had never been seen before,” said co-lead author Tong Chen, a researcher at Rice. “It wasn’t a quantum spin liquid, but we were observing behavior that we think is associated with quantum spin liquids.”
subtle magnetic competition
To find out what was really going on, the team used neutron scattering and other precise measurements. They discovered that the boundary between ferromagnetism and antiferromagnetism in this material is unusually weak. This allows the magnetic ions to move more freely between the two states, rather than being locked into a single pattern.
As a result, some ions behave ferromagnetically and others antiferromagnetically within the same structure. This mixed configuration prevents the system from forming a single ordered state and instead creates many possible low-energy configurations. Upon cooling to near absolute zero, the material settles into one of these configurations, producing a series of observed states similar to the continuum found in quantum spin liquids. However, unlike true quantum spin liquids, 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 has led to observational data that closely resemble the quantum spin liquid state,” said Dai, the study’s corresponding author. “This is a new state of matter and, to our knowledge, the first we have described.”
reminds us of quantum complexity
This discovery highlights how complex and surprising magnetic systems are. Even if matter appears to match the expected characteristics of a quantum state, the underlying physics may tell a different story.
This unique material is a great reminder of how little we know about the quantum realm, Dai added. “It highlights the importance of looking at the data carefully and investigating it thoroughly.”
Funding and research support
Research on neutron scattering and alternating current magnetic susceptibility at Rice was supported by the U.S. Department of Energy Fundamental Energy Sciences (DE-SC0012311, DE-SC0026179). Single crystal growth research was supported by the Robert A. Welch Foundation (C-1839). Crystal growth by BG, XX, and SWC at Rutgers University was supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative (GBMF6402) and the Rutgers University-funded Center for Quantum Materials Synthesis Visitor Program. The theoretical work performed by CL and LB was supported by the DOE, Office of Science, BES (DE-FG02-08ER46524) and the Simons Collaboration on Superquantum Materials. The researchers received individual support from the Gordon and Betty Moore Foundation through the Emergent Phenomena in Quantum Systems Program. National Natural Science Foundation of China (12204160); National Research Foundation of Korea, Ministry of Science Information and Communication (2022M3H4A1A04074153); and Welch Foundation (AA-2056-20240404). Neutron scattering experiments at J-PARC MLF were conducted based on proposal number 2022B0242. This study used the resources of the Spallation Neutron Source, a DOE Office of Science User Facility operated by Oak Ridge National Laboratory.
