The work, published in Science Advances, was co-led by Rice University physicist Pengcheng Dai. The research team set out to understand why CeMgAl11O19, an insulating magnet, appeared to show both a continuum of low energy excitations and a lack of magnetic ordering, two hallmarks that had led to its earlier classification as a quantum spin liquid. They combined neutron scattering experiments with other measurements to probe the material at temperatures near absolute zero.
In insulating magnets such as CeMgAl11O19, magnetic ions like cerium can adopt one of two basic configurations. They can align in a ferromagnetic arrangement, where neighboring spins point in the same direction, or in an antiferromagnetic arrangement, where neighboring spins point in opposite directions. Under typical conditions, interactions between ions favor one pattern or the other, and as the material is cooled toward absolute zero the system settles into a single, ordered low energy configuration.
For conventional nonquantum magnets, this ordering means that all ions share either a ferromagnetic or an antiferromagnetic pattern, producing one dominant ground state when observed at very low temperatures. In contrast, a genuine quantum spin liquid does not freeze into a single arrangement even near absolute zero. Instead, quantum fluctuations drive continuous transitions among many nearly degenerate low energy states, leading to a broad continuum of excitations and an absence of long range magnetic order.
CeMgAl11O19 initially seemed to fit the quantum spin liquid profile because experiments detected both a continuum of spin excitations and no clear magnetic ordering. However, detailed analysis of the excitation spectrum showed that the underlying mechanism was different. Rather than arising from quantum spin liquid physics, the continuum in CeMgAl11O19 stems from a dense set of nearly degenerate states created by competition between ferromagnetic and antiferromagnetic exchange interactions within the material.
According to co-first author and Rice research scientist Bin Gao, the material had been categorized as a quantum spin liquid because of the observed continuum and the absence of ordering, but closer inspection revealed that these signatures could be explained without invoking a quantum spin liquid phase. Co-first author Tong Chen, also at Rice, noted that CeMgAl11O19 displayed an unusual combination of characteristics that motivated the team to revisit its classification. Their goal was to determine how the material could show behaviors associated with quantum spin liquids while not actually being in that phase.
Neutron scattering measurements were central to resolving this puzzle. By bombarding CeMgAl11O19 with neutrons, the researchers mapped how its magnetic ions respond across a range of energies and momenta. They found that the boundary between ferromagnetic and antiferromagnetic configurations in this compound is unusually weak. This softness allows cerium ions in the same crystal to adopt both ferromagnetic and antiferromagnetic states instead of locking into a single ordered pattern, which naturally produces the observed lack of long range magnetic order.
Because the magnetic ions do not settle into a uniform arrangement, the system gains access to many low energy configurations. As the sample is cooled toward absolute zero, it can select among these different nearly degenerate states, generating a spectrum of excitations that looks like the continuum measured in quantum spin liquid candidates. The crucial distinction is that, in this nonquantum state, once the material enters a particular low energy configuration it does not exhibit quantum-driven transitions among the different states.
Dai said that the material's ability to effectively choose among multiple low energy states leads to experimental data that closely resemble quantum spin liquid behavior. He and his colleagues argue that this represents a previously unrecognized state of matter created by mixed ferro-antiferromagnetic exchange interactions. To their knowledge, CeMgAl11O19 is the first documented example of this specific nonquantum state, highlighting how classical competition between magnetic interactions can imitate quantum phenomena.
Research Report:Spin Excitation Continuum from Degenerate States in the Mixed Ferro-Antiferromagnetic Exchange System CeMgAl11O19
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