Two-dimensional quantum universality in the spin-1/2 triangular-lattice quantum antiferromagnet Na <sub>2</sub> BaCo(PO <sub>4</sub> ) <sub>2</sub>
Jieming Sheng, Le Wang, Andrea Candini, Wenrui Jiang, Lianglong Huang, Bin Xi, Jize Zhao, Han Ge, Nan Zhao, Ying Fu, Junfeng Ren, Jiong Yang, Ping Miao, Xin Tong, Dapeng Yu, Shanmin Wang, Qihang Liu, Maiko Kofu, Richard A. Mole, G. Biasiol, Dehong Yu, Igor Zaliznyak, Jia‐Wei Mei, Liusuo Wu
Abstract
An interplay of geometrical frustration and strong quantum fluctuations in a spin-1/2 triangular-lattice antiferromagnet (TAF) can lead to exotic quantum states. Here, we report the neutron-scattering, magnetization, specific heat, and magnetocaloric studies of the recently discovered spin-1/2 TAF Na 2 BaCo(PO 4 ) 2 , which can be described by a spin-1/2 easy axis XXZ model. The zero-field neutron diffraction experiment reveals an incommensurate antiferromagnetic ground state with a significantly reduced ordered moment of about 0.54(2) μ B /Co. Different magnetic phase diagrams with magnetic fields in the a b plane and along the easy c -axis were extracted based on the magnetic susceptibility, specific heat, and elastic neutron-scattering results. In addition, two-dimensional (2D) spin dispersion in the triangular plane was observed in the high-field polarized state, and microscopic exchange parameters of the spin Hamiltonian have been determined through the linear spin wave theory. Consistently, quantum critical behaviors with the universality class of d = 2 and ν z = 1 were established in the vicinity of the saturation field, where a Bose–Einstein condensation (BEC) of diluted magnons occurs. The newly discovered quantum criticality and fractional magnetization phase in this ideal spin-1/2 TAF present exciting opportunities for exploring exotic quantum phenomena.