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.

Abstract-Electronic and phonon excitations in α − RuC l 3

S. Reschke, F. Mayr, Zhe Wang, Seung-Hwan Do, K.-Y. Choi, and A. Loi

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.165120

We report on terahertz (THz), infrared reflectivity, and transmission experiments for wavenumbers from 10 to $8000c{m}^{-1}$ ($\sim 1\mathrm{meV}-1\mathrm{eV}$) and for temperatures from 5 to 295 K on the Kitaev candidate material $\alpha -\mathrm{RuC}{l}_3$. As reported earlier, the compound under investigation passes through a first-order structural phase transition, from a monoclinic high-temperature to a rhombohedral low-temperature phase. The phase transition shows an extreme and unusual hysteretic behavior, which extends from 60 to 166 K. In passing this phase transition, in the complete frequency range investigated, we found a significant reflectance change, which amounts to almost a factor of two. We provide a broadband spectrum of dielectric constant, dielectric loss, and optical conductivity from the THz to the mid-infrared regime and study in detail the phonon response and the low-lying electronic density of states. We provide evidence for the onset of an optical energy gap, which is on the order of 200 meV, in good agreement with the gap derived from measurements of the dc electrical resistivity. Remarkably, the onset of the gap exhibits a strong blue shift on increasing temperatures.

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