Maharaj D D, Sala G, Stone M B, Kermarrec E, Ritter C, Fauth F, Marjerrison C A, Greedan J E, Paramekanti A, Gaulin B D
Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
Phys Rev Lett. 2020 Feb 28;124(8):087206. doi: 10.1103/PhysRevLett.124.087206.
We report time-of-flight neutron spectroscopy and neutron and x-ray diffraction studies of the 5d^{2} double perovskite magnets, Ba_{2}MOsO_{6} (M=Zn,Mg,Ca). These materials host antiferromagnetically coupled 5d^{2} Os^{6+} ions decorating a face-centered cubic (fcc) lattice and are found to remain cubic down to the lowest temperatures. They all exhibit thermodynamic anomalies consistent with a single phase transition at a temperature T^{}, and a gapped magnetic excitation spectrum with spectral weight concentrated at wave vectors typical of type-I antiferromagnetic orders. However, while muon spin resonance experiments show clear evidence for time-reversal symmetry breaking below T^{}, we observe no corresponding magnetic Bragg scattering signal. These results are shown to be consistent with ferro-octupolar symmetry breaking below T^{*}, and are discussed in the context of other 5d double perovskite magnets and theories of exotic orders driven by multipolar interactions.
我们报告了对5d²双钙钛矿磁体Ba₂MOsO₆（M = Zn、Mg、Ca）的飞行时间中子光谱以及中子和X射线衍射研究。这些材料包含反铁磁耦合的5d²Os⁶⁺离子，它们排列在面心立方（fcc）晶格上，并且在最低温度下仍保持立方结构。它们都表现出与在温度T处的单一相变相一致的热力学异常，以及具有集中在I型反铁磁序典型波矢处的光谱权重的带隙磁激发光谱。然而，虽然μ子自旋共振实验显示出在T以下时间反演对称性破缺的明确证据，但我们未观察到相应的磁布拉格散射信号。结果表明这些结果与T*以下的铁八极对称性破缺相一致，并在其他5d双钙钛矿磁体以及由多极相互作用驱动的奇异序理论的背景下进行了讨论。
