Abstract-Sub-gap optical response across the structural phase transition in van der Waals layered α-RuCl3

S. Reschke, F. Mayr, S. Widmann, H.-A. Krug von Nidda, V. Tsurkan, M. V. Eremin, Seung-Hwan Do, K.-Y. Choi, Zhe Wang, A. Loidl

https://arxiv.org/abs/1803.04887

We report on magnetic, thermodynamic, thermal expansion, and optical experiments on the layered compound \alpha-RuCl3 focusing on the sub-gap optical response across the structural phase transition from the monoclinic high-temperature to the rhombohedral low-temperature structure, where mainly the stacking sequence of the molecular layers is changed. The temperature dependence of the complex dielectric response, including symmetry changes via the phase transition should allow the identification of the microscopic origin of the recorded phonon and spin-orbital excitations. In addition, this type of phase transition seems to be characteristic for a variety of tri-halides crystallizing in a layered honeycomb-type structure and so far is unique, as the low-temperature phase has the higher symmetry. 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. An electronic excitation, which only appears in a narrow temperature range just across the structural phase transition, and a complex dielectric loss spectrum in the THz regime, which could indicate some remnants of Kitaev physics. Despite the significant structural effects across the monoclinic to rhombohedral phase transition, phonon eigenfrequencies and the majority of spin-orbital excitations are not strongly influenced. Obviously, symmetry and binding forces of the single molecular layers only determine the eigenfrequencies of most of these excitations. Finally, from a detailed and combined terahertz, far- and mid-infrared study we try to shed some light on the so far unsolved low energy (< 1eV) electronic structure of the ruthenium 4d5 electrons in \alpha-RuCl3.