Electronic and phonon excitations in α−RuCl3

Abstract

We report on THz, infrared reflectivity and transmission experiments for wave numbers from 10 to 8000 cm -1 (~ 1 meV -1 eV) and for temperatures from 5 to 295 K on the Kitaev candidate material α-RuCl 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 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 of order 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. * alois.

Magnetic field-dependent low-energy magnon dynamics in α–RuCl3

“…2. Although further theoretical studies elucidating the nature of mode I are needed, such changes may emerge from anharmonic effects linked to the symmetry breaking in this material and an associated magnetoelastic coupling below ∼150 K [50,53,54]. Regardless of their nature, it follows from the markedly different magnetic field dependences of both branches that their assignment as a single mode cannot explain our data.…”

“…First, we consider phonons. The first-order transition from a monoclinic to a rhombohedral structure that takes place in the temperature range from 60 to 150 K in α-RuCl 3 [44,50,53] has been interpreted as evidence of a magnetoelastic coupling scheme and a natural explanation for the observed phonon anomalies in this material [53,54]. This raises the question of whether a similar mechanism could explain the unconventional temperature response of mode I, invoking a phonon picture for the ob- served resonances.…”

“…With such anharmonic effects observed in α-RuCl 3 , a natural question that arises is the relevance of magnetoelastic interactions that have been reported to prevail in this system in the temperature range of ∼60−150 K [50,53,54]. Although there is no direct evidence of a change in the crystal structure in the low-temperature regime near 7 K where the zigzag order is stabilized, it remains to be explored whether and to what extent the strong spin-lattice interactions as revealed by Raman studies and the magnon mixing behavior reported in our current work are related to one another.…”

Thermodynamic evidence of fractionalized excitations in α−RuCl3

“…Details of sample growth and characterization of α-RuCl3 are described in detail in Ref. [35,37]. The difference of the present sample batches as compared to previously investigated samples is the low magnetic ordering temperature of 6.4 K and the relatively narrow hysteresis of the structural phase transition, which extends from 131 to 163 K (see later).…”

Terahertz excitations in α−RuCl3 : Majorana fermions and rigid-plane shear and compression modes

“…1(b)] are dominated by a narrow excitation close to 2.5 meV, a broad continuum around 8.5 meV, and a strong increase beyond 12 meV. The latter results from a weak phonon mode centered close to 15 meV [27,38 ]. An additional weak excitation close to 6 meV strongly overlaps with the continuum.…”