Fundamental optical properties of α-RuCl3

Electronic and phonon excitations in α−RuCl3

“…These electronic transitions between the lower and upper Hubbard band, obviously are almost invisible in IR reflectivity, but are certainly detectable in transmission experiments. It is unclear, whether in addition to this transition across the correlation gap, single-ion excitations between ruthenium d levels, as reported in literature [32,33,35], still can be identified experimentally. To complete this survey on optical experiments on α-RuCl 3 , very recently, the THz conductivity has been reported by Little et al [39] for frequencies between 1 and 7 meV.…”

“…Group factor analysis then gives 5 IR active modes, namely 2 A 2u modes, polarized parallel to the c axis, which is the stacking direction and 3 E u modes polarized perpendicular to the c axis, within the stacks. In this early work, in the farinfrared (FIR) reflectivity spectra one optical mode with a transverse eigenfrequency close 300 cm -1 (37.2 meV) was observed [33]. Utilizing transmission experiments, Refs [32] and [33] also reported IR forbidden d-d transitions at 290, 510 and 710 meV within the ruthenium 4d bands.…”

“…At room temperature infrared (IR) bands close to 188, 274, and 315 cm -1 , corresponding to 23.3, 34.0 and 39.1 meV, were identified. Later on, Binotto et al [32] and Guizzetti et al [33] reported further optical investigations on α-RuCl 3 . Because of the strongly two-dimensional character of the molecular stacks, D 3d symmetry was assumed for the isolated sandwich of the layered structure.…”

“…The main difference of these interpretation in terms of band structures [37], compared to the single ion interpretation presented in Ref. [33,35,36], concerns the excitations below 1 eV: Only the inclusion of moderate electronic correlations can establish an insulating ground state and the dominating structure in the dielectric loss function then signals the transition across the Mott-Hubbard gap. Taking into account electronic correlations with a Hubbard U ~ 1.5 eV induces a gap in the density of states at the Fermi level and splits the density of states of the t 2g level into a broad lower and a narrow upper Hubbard band, with a transition energy of about 250 meV [37], in good agreement with the conductivity results [34].…”

Magnetic order inα−RuCl3: A honeycomb-lattice quantum magnet with strong spin-orbit coupling

“…[32][33][34][35][36] While early transport studies described α-RuCl 3 as a small gap semiconductor, later spectroscopic studies seem to favor a Mott insulator description. 36 These conflicting viewpoints were resolved in a recent study, in which it was pointed out that the Mott insulating behavior of α-RuCl 3 arises from the combined effect of band narrowing due to SOC and moderate size electron correlation.…”