Scientists Discover a New Magnetic Quasiparticle

Neutron scattering reveals a new way for magnetic oscillations to stick together.

Image courtesy of Stephen Wilson and Rebecca Dally
Top: oscillating moments in a spin chain, forming a magnon. Bottom: neutron scattering data (left) and corresponding theoretical models (right) in sodium manganese oxide corresponding to one-, two-, and three-magnon bound states.

The Science

Scientists often come to understand how materials behave by disturbing them and watching them respond. Just as the tension of a taut string can be understood by plucking it, magnetism in materials can be explored by studying the oscillations of the magnetic effects. Studying these magnetic oscillations, or “magnons,” helps to explain why magnetic materials behave as they do. Magnons can also be treated as interesting new particles (i.e. quasiparticles, disturbances that behave like particles). Scientists have long predicted that magnons can interact and combine to form new quasiparticles. Now, scientists have used neutron scattering to find these multiple-magnon “bound states” in real materials.

The Impact

Scientists predict that interactions between magnons create new three-magnon bound states. However, until now, scientists have not observed more than two magnons bound together in a real material. This lack of observations has called into question the existence of three-magnon bound states. In this work, a material called sodium manganese oxide represents the first antiferromagnetic (moments aligned antiparallel) material that hosts a three-magnon bound state. This demonstrates that such quasiparticles are stable enough to form. Using multiple-magnon bound states as carriers of quantum information is important for future quantum technologies, and this work provides a model material for studying the formation and properties of these new quasiparticles.

Summary

The bedrock of our understanding of quantum magnetism is built on models of one-dimensional magnetic chains. Exact analytical solutions can be used to predict the properties in such chains, making them ideal testbeds for fundamental models of how magnets work. One prediction from these models is that, in special settings, oscillations of magnetic moments, or magnons, in chains can be thought of as quasiparticles that are attracted to one another. Scientists predict that these magnons will bind together in ever greater numbers, making new quasiparticles, and two-magnon bound states have been previously observed in materials containing one-dimensional chains of spins. Within the layered antiferromagnetic material sodium manganese oxide, magnetic frustration causes the magnetism to be predominantly one-dimensional, making it an excellent material for searching for new forms of magnon bound states. In this new work, neutron scattering measurements revealed that a native three-magnon quasiparticle exists in sodium manganese oxide. This opens a new window for testing fundamental theories of interactions in one-dimensional antiferromagnets and furthering our understanding of quantum magnetism. These findings are relevant for informing experiments searching for similar quasiparticles in cold-atomic gases and for paving the way for their use as carriers of quantum information in future quantum technologies.

Contact

Stephen D. Wilson
Professor, Materials Department, University of California, Santa Barbara
email:  stephendwilson@ucsb.edu phone:  805-893-2008

Leon Balents
Professor, Kavli Institute for Theoretical Physics, University of California, Santa Barbara
email:  balents@physics.ucsb.edu phone:  805-893-6381

Funding

Experimental and theoretical work was supported by the Department of Energy Office of Science, Basic Energy Sciences. The research benefited from the use of the Spallation Neutron Source, a BES user facility operated by Oak Ridge National Laboratory.

Publications

Dally, R.L., et al., “Three-Magnon Bound State in the Quasi-One-Dimensional Antiferromagnet α-NaMnO2” Physical Review Letters 124, 197203 (2020). [DOI:10.1103/PhysRevLett.124.197203]

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