Recently, Prof. Jie Ma’s research team from School of Physics and Astronomy, SJTU published a research paper titled “Field-induced quantum spin disordered state in spin-1/2 honeycomb magnet Na₂Co₂TeO₆” in Nature Communications, a top international journal.

Magnets with significant spin-orbital couplings (SOCs) have become a new playground for quantum magnetism in recent years thanks to their potential of hosting novel quantum phases of matter. A prominent example is the Kitaev model, an exactly solvable spin model that features a topological quantum spin liquid (QSL) ground state. Microscopically, such a model could emerge from magnetic insulators with competing, spin anisotropic exchange interactions. The essential prerequisites are that (a) the magnetic ions have spin-orbital entangled, Kramers degenerate ground states and (b) they are arranged on a suitable lattice, with the two-dimensional (2D) honeycomb lattice being the simplest example. So far, the search for the material incarnations of the Kitaev model has been focused on 4d/5d transition mental based systems due to their relatively strong SOCs. The examples include H₃LiIr₂O₆, α-Li₂IrO₃, α-Na₂IrO_3, and α-RuCl₃. To describe these materials, the Kitaev model has been extended to a generalized Heisenberg-Kitaev (H-K) model with five symmetry-allowed terms, Kitaev term K, off-diagonal symmetric exchange term G and Γ′, nearest-neighbor (NN) Heisenberg coupling J, and the third NN Heisenberg coupling J₃.

More recently, the theoretical studies suggest that Kitaev physics could also be found in honeycomb magnets made of cobalt, a 3d transition metal. A pertinent material is the honeycomb magnet Na₂Co₂TeO₆ (NCTO). We have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO’s quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (^ b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets. Its origin, in addition to the related spin structure and spin dynamics, calls for future experimental work on single crystals and theoretical studies.

Dr. Gaoting Lin is the first author of the paper. Prof. Jie Ma from SJTU and Prof. Yuan Wan from Institute of Physics, CAS are the co-corresponding authors. This work is supported by the financial support from the National Key Research Development Program of China, the National Science Foundation of China, and the project funded by China Postdoctoral Science Foundation. A portion of this work was supported by the High Magnetic Field Laboratory of Anhui Province. The INS experiment was performed at the MLF of J-PARC, Japan and NEAT II, HZB, Deutschland.

Paper link: https://www.nature.com/articles/s41467-021-25567-7