Crystal field excitations of new low energy spin-1/2 triangular antiferromagnets

Despite being frustrated, the triangular lattice antiferromagnet is expected to develop long-range magnetic order and spin-wave excitations at low temperatures. Theoretical studies suggest that the magnetic spectrum shows small deviations with respect to linear spin-wave theory with the band of excitations expected to be renormalized downward and broadened for the case of quantum spin-1/2. In contrast recent experiments on the spin-1/2 triangular antiferromagnet Ba3CoSb2O9 reveal major differences between the measured excitations and spin-wave theory, including 3 bands of excitations and a continuum that extends to much higher energies (6 times the exchange energy). Because of the unexpected nature of these results it is necessary to verify them in other materials, thus we plan to investigate the spectrum of the new effective spin-1/2 triangular lattice antiferromagnets Ba3CoNb2O9 and Ba3CoTa2O9. We also plan to investigate the crystal field levels of their magnetic Co2+ ions to determine their ground state wavefunction and identify the presence of any anisotropies.尽管存在磁阻挫，三角晶格反铁磁体（triangular lattice antiferromagnet）在低温环境下有望形成长程磁有序（long-range magnetic order）与自旋波激发（spin-wave excitations）。理论研究表明，针对量子自旋1/2（quantum spin-1/2）体系，其磁谱相对于线性自旋波理论（linear spin-wave theory）存在小幅偏移，激发带预计会向下重整化且发生展宽。与之形成鲜明对比的是，近期针对自旋1/2三角反铁磁体Ba3CoSb2O9开展的实验显示，实测激发信号与线性自旋波理论存在显著差异：包括3条激发带，以及延伸至远高于该能量尺度（为交换能（exchange energy）的6倍）的连续谱。鉴于上述结果具有出乎意料的特性，有必要在其他同类材料中对其进行验证，因此本研究计划探究新型有效自旋1/2三角晶格反铁磁体Ba3CoNb2O9与Ba3CoTa2O9的谱学特性。此外，我们还将对其磁性Co²+离子的晶体场能级（crystal field levels）展开研究，以确定其基态波函数（ground state wavefunction）并识别是否存在各向异性（anisotropies）。