标题： 范德华材料中的量子涨落 $\rm NiPS_3$ 显示英文标题

标题： Quantum Fluctuations in the van der Walls material $\rm NiPS_3$

摘要： 我们展示了量子磁体$\rm NiPS_3$在包含双线性和双二次自旋相互作用的最小蜂窝自旋哈密顿量的锯齿状基态附近的磁激发谱。 我们的分析使用了适用于自旋 S=1 系统的多玻色子广义自旋波理论，揭示了线性水平下的两种正常模式。 较低能量的模式对应于单个磁振子模式，与光谱学实验结果一致。 在没有单离子各向异性的情况下，该模式在布里渊区角落具有一个戈德斯通模。 当引入单离子各向异性时，锯齿相的全局 U(1) 对称性被破坏，导致出现能隙。 较高能量的模式对应于两磁振子涨落，在广义自旋波理论的谐波水平上出现。 由于双线性自旋相互作用，该模式形成一个有能隙的平带，而当考虑双二次相互作用时，它变得色散。 较高能量的色散与四极涨落有关，这在序参量在 SU(3) 空间中涨落的磁体中是可行的。 谱分析得到了序参量的量子修正，并检测到了$\rm NiPS_3$二极相中的不稳定性。 在实验中识别最高能量分支可能有助于了解$\rm NiPS_3$及其他范德华磁体中的隐藏向列序。 显示英文摘要

摘要： We present the magnetic excitation spectrum of the quantum magnet $\rm NiPS_3$ near the zig-zag ground state of a minimal honeycomb spin Hamiltonian that includes bilinear and biquadratic spin interactions. Our analysis, using a multi-boson generalized spin wave theory suited for spin S=1 systems, revealed two normal modes at the linear level. The one at lower energy corresponds to a single magnon mode, consistent with results from spectroscopy experiments. Without single-ion anisotropy, this mode features a Goldstone mode at the corner of the Brillouin zone. When single ion anisotropy is introduced, the zig-zag phase's global U(1) invariance is broken, resulting in a gap. The higher energy mode corresponds to two-magnon fluctuations, which appear at the harmonic level in the generalized spin wave theory. This mode forms a gapped flat band due to bilinear spin interactions and becomes dispersive when biquadratic interactions are considered. The higher energy dispersion is related to quadrupolar fluctuations, which are feasible in magnets where the order parameter fluctuates in the SU(3) space. The spectrum analysis yielded quantum corrections to the order parameter and detected instabilities in the $\rm NiPS_3$ dipolar phases. Identifying the highest energy branch in experiments could provide insight into hidden nematic orders in $\rm NiPS_3$ and other van der Waals magnets.