Resonant inelastic x-ray scattering study of α -RuCl₃: a progress report

Abstract

Ru M 3 -edge resonant inelastic x-ray scattering (RIXS) measurements of α-RuCl 3 with 27 meV resolution reveals a spin-orbit exciton without noticeable splitting. We extract values for the spin-orbit coupling constant (λ = 154 ± 2 meV) and trigonal distortion field energy (|∆| < 65 meV) which support the j eff = 1/2 nature of α-RuCl 3 . We demonstrate the feasibility of Medge RIXS for 4d systems, which allows ultra high-resolution RIXS of 4d systems until instrumentation for L-edge RIXS improves.

Multiple spin-orbit excitons and the electronic structure of α−RuCl3

“…Note that our experimental splitting = 42(4) meV is still smaller than the vibronic peak width, and one may speculate whether the considerable overlap of the two excitations explains the reduced dichroism. With the currently available resolution, RIXS data measured at the Ru M edge at 300 K support our assignment of peak A but cannot resolve the splitting [29].…”

“…Considering the full 4d shell including the e g orbitals with 10 Dq = 2.2 eV [30,39], we find λ = 0.16 eV. In the literature on α-RuCl 3 , typical values for λ fall in the range 0.13-0.15 eV [29,37,42,44,58], but a precise experimental value was missing thus far.…”

“…In the light of these far-reaching results, it is surprising that the low-energy electronic structure of α-RuCl 3 remains controversial. In the related iridates, the approximate validity of the j = 1/2 scenario is well established by resonant inelastic x-ray scattering (RIXS) data of the spin-orbit exciton [23][24][25][26][27], i.e., the excitation to j = 3/2, but high-resolution RIXS at the Ru L or M edges is challenging [28,29]. In α-RuCl 3 , conflicting energies of 145, 195, and 231 meV were reported for the spin-orbit exciton by Raman, inelastic neutron scattering, and Ru M edge RIXS, respectively [14,29,30].…”

“…In the related iridates, the approximate validity of the j = 1/2 scenario is well established by resonant inelastic x-ray scattering (RIXS) data of the spin-orbit exciton [23][24][25][26][27], i.e., the excitation to j = 3/2, but high-resolution RIXS at the Ru L or M edges is challenging [28,29]. In α-RuCl 3 , conflicting energies of 145, 195, and 231 meV were reported for the spin-orbit exciton by Raman, inelastic neutron scattering, and Ru M edge RIXS, respectively [14,29,30]. Furthermore, several groups reported on infrared absorption bands at 0.30, 0.53, and 0.75 eV [4,[30][31][32][33][34][35].…”

“…The sketch neglects the noncubic crystal field. = 39 meV [43], density functional theory, = 37 meV [45,46], and M edge RIXS, < 40 meV [29]. Note that ≈ 180 meV would strongly mix j = 1/2 and 3/2 states, again with dramatic consequences for the exchange interactions [7,47].…”