Researchers at the Department of Energy’s Oak Ridge National Laboratory used neutron scattering to determine whether the atomic structure of a specific material could accommodate a new state of matter called spiral spin liquid. By tracking tiny magnetic moments called “spin” on the honeycomb lattice of a layered iron trichloride magnet, the team found the first 2D system to host a helical spin liquid.
This discovery provides a test bed for future studies of the physical phenomena that can drive next-generation information technologies. These include fractons, or collective quantized vibrations, that could hold promise in quantum computing, and skyrmions, or new magnetic spin textures, that could promote high-density data storage.
“Materials harboring helical spin liquids are of particular interest because of their potential for use in the generation of quantum spin liquids, spin textures, and fracton excitations,” said ORNL’s Shang Gao, who led the study published in Physical examination letters.
A long-standing theory predicts that the honeycomb lattice may host spiral spin fluid—a new phase of matter in which spins form fluctuating corkscrew-like structures.
However, until this study, experimental evidence for this phase in a 2D system was lacking. A 2D system comprises a layered crystalline material where the interactions are stronger in the plane than in the stacking direction.
Gao identified iron trichloride as a promising platform to test the theory, which was proposed over a decade ago. He and co-author Andrew Christianson of ORNL approached Michael McGuire, also of ORNL, who has done extensive work growing and studying 2D materials, and asked if he would synthesize and characterize a sample of iron trichloride for neutron diffraction measurements. As 2D graphene layers exist in bulk graphite as pure carbon honeycomb lattices, 2D iron layers exist in bulk iron trichloride as 2D honeycomb layers. “Previous reports have suggested that this interesting honeycomb material can exhibit complex magnetic behavior at low temperatures,” McGuire said.
“Each iron honeycomb layer has chlorine atoms above and below it, forming chlorine-iron-chlorine sheets,” McGuire said. “The chlorine atoms above one plate interact very weakly with the chlorine atoms at the bottom of the next plate through the van der Waals bond. This weak bond means that materials like this can be easily peeled off in very thin layers, often down to a single plate. This is useful for developing devices and understanding the evolution of quantum physics from three dimensions to two dimensions. »
In quantum materials, electron spins can behave collectively and exotically. If one rotation moves, everyone reacts—an entangled state that Einstein called “spooky action at a distance.” The system remains in a state of frustration—a fluid that maintains disorder as electron spin constantly changes direction, forcing other entangled electrons to oscillate in response.
The first neutron diffraction studies of ferric chloride crystals were performed at ORNL 60 years ago. Today, ORNL’s vast expertise in materials synthesis, imaging, neutron scattering, theory, simulation, and computation enables groundbreaking exploration of quantum magnetic materials, driving the development of a new generation for information security and storage.
Mapping spin motions in helical spin fluid was made possible by experts and tools from the Spallation Neutron Source and High Flux Isotope Reactor, DOE Office of Science user facilities at ORNL. Critical to the success of neutron scattering experiments were ORNL co-authors: Clarina dela Cruz, who led experiments using HFIR’s POWDER diffractometer; Yaohua Liu, who performed experiments using SNS’s CORELLI spectrometer; Matthias Frontzek, who performed experiments involving HFIR’s WAND2 diffractometer; Matthew Stone, who performed experiments on SNS’s SEQUOIA spectrometer; and Douglas Abernathy, who performed experiments with SNS’s ARCS spectrometer.
“The neutron scattering data from our SNS and HFIR measurements provided compelling evidence for a liquid spiral phase,” Gao said.
“Neutron scattering experiments measured how neutrons exchange energy and momentum with the sample, making it possible to infer magnetic properties,” said co-author Matthew Stone. He described the magnetic structure of a helical spin-fluid: “It looks like a topographical map of a group of mountains with a group of rings going outwards. If you were to walk along a ring, all the spins would point in the same direction. But if you going outwards and going through different rings, you will see these spins start to turn around their axes. That is the spiral.
“Our study shows that the concept of a spiral spin liquid is viable for the broad class of honeycomb lattice materials,” said co-author Andrew Christianson. “This gives the community a new opportunity to explore spin textures and new excitations, such as fractons, which can then be used in future applications, such as quantum computing. »