Relation between Kitaev magnetism and structure in α−RuCl3

Abstract

Raman scattering has been employed to investigate lattice and magnetic excitations of the honeycomb Kitaev material α-RuCl3 and its Heisenberg counterpart CrCl3. Our phonon Raman spectra give evidence for a first-order structural transition from a monoclinic to a rhombohedral structure for both compounds. Significantly, only α-RuCl3 features a large thermal hysteresis, consistent with the formation of a wide phase of coexistence. In the related temperature interval of 70 − 170 K, we observe a hysteretic behavior of magnetic excitations as well. The stronger magnetic response in the rhombohedral compared to the monoclinic phase evidences a coupling between the crystallographic structure and low-energy magnetic response. Our results demonstrate that the Kitaev magnetism concomitant with fractionalized excitations is susceptible to small variations of bonding geometry.

The range of non-Kitaev terms and fractional particles in α-RuCl3

“…1a, we collected both the Stokes and anti-Stokes spectra of bulk α-RuCl 3 from 10 K to 300 K. Our Rayleigh scattering half width is 2.3 meV, enabling measurement down into the low energy regime. The temperature dependent spectra show a clear magnetic excitation continuum (2.3meV ∼ 10 meV) below the first phonon, which mostly results from the Kitaev interaction and is consistent with previous predictions and measurements 11,26,34,39 Fig. 1b shows the comparison of the QMC results for the pure Kitaev limit and the Raman susceptibility at 10 and 40 K. While excellent agreement is seen at 40 K, the data at 10 K only matches the model between 6 to 10 meV.…”

“…Specifically, there is a large χ δ in the quarter circle area below 6 meV and 40 K, which can be explained as the region where non-Kitaev interactions become dominant in the response. The deviation above 150 K and below 8 meV results from the QES induced by thermal fluctuations in the system, which is well known in frustrated magnets 7,11,26,48,59 . The high energy deviation (>12meV) is from the low energy tail of the phonon.…”

“…ously, the discrepancy between the prediction of the Bose factor and the measured intensity at low temperatures was attributed to fractional statistics 26,34 . However, these works did not exclude the possibility that laser induced heating kept the measured area at a fixed temperature, while the bulk was cooled.…”

“…Here, we overcome all these previous limitations with new Raman spectra with dramatically improved signal levels, high temperature and energy resolution. Firstly, having improved the optics, our Raman measurements now obtain a signal level 18 times larger than before 11 7,11,26 . With our enhanced signal the anti-Stokes spectra for all temperatures can be compared with the Stokes response to prove the sample is in detailed balance without laser heating (see Fig.…”

“…Since Raman scattering involves a photon in and photon out, it allows one to measure both the symmetry and energy change of an excitation. Furthermore, one can choose an energy and/or symmetry channel to separate the magnetic, electronic and lattice responses 11,13,26,34,52,56,59 . The majority of the Raman experiments were performed with a custom built, low temperature microscopy setup 72 .…”