Incommensurate phases in an S = 1/2 Ising-XXZ triangular lattice magnet

Na₂BaCo(PO₄)₂ has been celebrated as one of the best experimental realisations of the quantum spin-1/2 easy-axis XXZ model on the triangular lattice [1,2]. Highly nontrivial phases such as the long-sought spin supersolid state have been studied in this system [3]. A quantum treatment of the S=1/2 easy-axis XXZ model on the triangular lattice ground state predicts three ordered phases in a magnetic field before attaining full spin saturation [4]. These are the so-called Y, up-up-down and V phases; names reminiscent of the spin sublattice arrangements. However, our recent neutron diffraction experiments on Na₂BaCo(PO₄)₂ reveal an intriguing additional phase between V and saturation for a field applied perpendicular to the triangular planes. Surprisingly, this phase features an incommensurate propagation vector. We propose an extension of the XXZ model that captures the features of this field-induced incommensurate phase.



References: [1] R. Zhong et al., Proc. Natl Acad. Sci. USA 116, 14505 (2019). [2] N. Li et al., Nat. Commun. 11, 4216 (2020). [3] Y. Gao et al., Quantum Materials 7, 8 (2022). [4] A. V. Chubukov and D. I. Golosov, J. Phys. Condens. Matter 3, 69 (1991).