A second-order phase transition is associated with emergence of an "orderparameter" and a spontaneous symmetry breaking. For the heavy fermionsuperconductor URu$_2$Si$_2$, the symmetry of the order parameter associatedwith its ordered phase below 17.5 K has remained ambiguous despite 30 years ofresearch, and hence is called "hidden order" (HO). Here we use polarizationresolved Raman spectroscopy to specify the symmetry of the low energyexcitations above and below the HO transition. These excitations involvetransitions between interacting heavy uranium 5f orbitals, responsible for thebroken symmetry in the HO phase. From the symmetry analysis of the collectivemode, we determine that the HO parameter breaks local vertical and diagonalreflection symmetries at the uranium sites, resulting in crystal field stateswith distinct chiral properties, which order to a commensurate chiralitydensity wave ground state.
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机译：二阶相变与“阶数参数”的出现和自发的对称破坏有关。对于重铁氧超导体URu $ _2 $ Si $ _2 $，尽管有30年的研究，但与17.5 K以下的有序相相关的有序参数的对称性仍然不明确，因此被称为“隐藏阶”（HO）。在这里，我们使用偏振分辨拉曼光谱来指定HO跃迁之上和之下的低能激发的对称性。这些激发涉及相互作用的重铀5f轨道之间的跃迁，造成HO相中对称性的破坏。通过对集体模态的对称性分析，我们确定HO参数破坏了铀位点处的局部垂直和对角反射对称性，从而产生了具有不同手性的晶体场态，从而形成了相应的手性密度波基态。
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