Li Yuan's group and collaborators in the School of Physics studied neutron scattering of cobalt-based hexagonal honeycomb lattice magnetic materials

Recently, Li Yuan, a long-term associate professor at the Center for Quantum Materials Science of the School of Physics of Peking University, and his collaborators used inelastic neutron scattering experiments and high-quality single crystal samples to completely determine the spin excitation spectrum
of a quantum spin liquid candidate material for the first time in the world.
The relevant research results are based on "Excitations in the ordered and paramagnetic states of" (Excitations in the ordered and paramagnetic states of honeycomb magnet Na 2 Co₂TeO₆), published online in Physical Review Letters
.

"Magnetoresistive frustration" means that the magnetic interactions in crystalline materials are contradictory, which is not conducive to the formation
of classical magnetic order.
The resulting enhanced quantum effects make magnetoresistive systems considered promising to achieve novel states
of matter such as "quantum spin liquids".
In 2006, Alexei Kitaev proposed a theoretically rigorously solvable quantum spin liquid model, later known as the Kitaev honeycomb model, which achieves magnetoresistance by introducing three Ising interactions orthogonal to each other on the sides of three different orientations of the two-dimensional honeycomb lattice
.
Over the past decade, the search for Kitaev quantum spin liquids has led to a wave
of research in compounds of elements such as iridium (Ir), ruthenium (Ru) and cobalt (Co).
Magnetic order tends to remain in real-world materials at low temperatures, suggesting that magnetic interactions in the materials deviate from the ideal scenario
in the Kitaev honeycomb model.
If these deviations can be accurately identified, it will help to implement targeted regulation of materials, so as to achieve the ultimate goal
of suppressing magnetic order and inducing the liquid state of quantum spin.

In recent years, Li Yuan's group and collaboratorshave carried out a series of studies
on Na2Co2_TeO6, a honeycomb lattice material.
In previous studies, they carefully characterized the abundant phase transition phenomenon of materials at low temperatures [PRB101, 085120 (2020); PRB103, L180404（2021）】
。 Recently, using inelastic neutron scattering experiments and high-quality single crystal samples, the research team has completely determined the spin excitation spectrum
of the material for the first time in the world.
The results of the low-energy range show that there is a magnetic interaction at the distance of the "third neighbor" in the system, which is an important supplement
to the Kitaev cellular model, which originally only involved the nearest neighbor interaction.
At the same time, at least six dispersive overlapping spin waves were observed experimentally, which also brought clear limitations
to the magnetically ordered ground state and interaction model of the system.

(a—b) showed the fitting results of a spin wave with the lowest energy and an effective model of its third neighbor interaction (J₃); (c) The full spectrum of experimental measurements on the same momentum path is shown, from which at least six non-overlapping spin wave dispersions can be seen

Yao Weiliang, a graduated doctoral student from the Center for Quantum Materials Science of the School of Physics of Peking University, is the first author of the paper, and Li Yuan and Yao Weiliang are the co-corresponding authors
.
The key neutron scattering experiment was completed
with the cooperation of two spectrometer scientists from J-PARC Neutron Source in Japan.

The above research work is supported
by the National Key Research and Development Program of China, the National Natural Science Foundation of China and the Collaborative Innovation Center for Quantum Matter.