Reschke Stephan, Mayr Franz, Widmann Sebastian, von Nidda Hans-Albrecht Krug, Tsurkan Vladimir, Eremin Mikhail V, Do Seung-Hwan, Choi Kwang-Yong, Wang Zhe, Loidl Alois
Experimental Physics V, Center for Electronic Correlations and Magnetism, University of Augsburg, 86135 Augsburg, Germany.
J Phys Condens Matter. 2018 Nov 28;30(47):475604. doi: 10.1088/1361-648X/aae805. Epub 2018 Nov 6.
We report detailed optical experiments on the layered compound α-RuCl focusing on the THz and sub-gap optical response across the structural phase transition from the monoclinic high-temperature to the rhombohedral low-temperature structure, where the stacking sequence of the molecular layers is changed. This type of phase transition is characteristic for a variety of tri-halides crystallizing in a layered honeycomb-type structure and so far is unique, as the low-temperature phase exhibits the higher symmetry. One motivation is to unravel the microscopic nature of THz and spin-orbital excitations via a study of temperature and symmetry-induced changes. The optical studies are complemented by thermal expansion experiments. We document a number of highly unusual findings: A characteristic two-step hysteresis of the structural phase transition, accompanied by a dramatic change of the reflectivity. A complex dielectric loss spectrum in the THz regime, which could indicate remnants of Kitaev physics. Orbital excitations, which cannot be explained based on recent models, and an electronic excitation, which appears in a narrow temperature range just across the structural phase transition. Despite significant symmetry changes across the monoclinic to rhombohedral phase transition and a change of the stacking sequence, phonon eigenfrequencies and the majority of spin-orbital excitations are not strongly influenced. Obviously, the symmetry of a single molecular layer determines the eigenfrequencies of most of these excitations. Only one mode at THz frequencies, which becomes suppressed in the high-temperature monoclinic phase and one phonon mode experience changes in symmetry and stacking. Finally, from this combined terahertz, far- and mid-infrared study we try to shed some light on the so far unsolved low energy (<1 eV) electronic structure of the ruthenium 4d electrons in α-RuCl.
我们报告了关于层状化合物α-RuCl的详细光学实验，重点研究了从单斜高温结构到菱面体低温结构的结构相变过程中的太赫兹和能隙以下的光学响应，在此相变过程中分子层的堆叠顺序发生了变化。这种类型的相变是各种结晶为层状蜂窝型结构的三卤化物的特征，并且到目前为止是独特的，因为低温相具有更高的对称性。一个动机是通过研究温度和对称性引起的变化来揭示太赫兹和自旋轨道激发的微观本质。光学研究由热膨胀实验补充。我们记录了许多非常不寻常的发现：结构相变的特征性两步滞后现象，伴随着反射率的急剧变化。太赫兹频段的复杂介电损耗谱，这可能表明存在基塔耶夫物理的残余。基于最近的模型无法解释的轨道激发，以及一种电子激发，它出现在刚好跨越结构相变的狭窄温度范围内。尽管从单斜相到菱面体相的相变过程中对称性发生了显著变化，并且堆叠顺序也发生了改变，但声子本征频率和大多数自旋轨道激发并没有受到强烈影响。显然，单个分子层的对称性决定了这些激发中大多数的本征频率。只有一个太赫兹频率的模式在高温单斜相中被抑制，并且一个声子模式经历了对称性和堆叠的变化。最后，通过这种结合了太赫兹、远红外和中红外的研究，我们试图对α-RuCl中钌4d电子迄今为止尚未解决的低能（<1 eV）电子结构有所了解。
