Sub-gap optical response in the Kitaev spin-liquid candidate α-RuCl₃

Area of Science:

• Condensed Matter Physics
• Materials Science
• Spectroscopy

Background:

• Alpha-RuCl3 is a layered honeycomb material exhibiting a structural phase transition.
• Understanding THz and spin-orbital excitations is crucial for materials with potential Kitaev physics.
• The transition involves a change in stacking sequence from monoclinic (high-T) to rhombohedral (low-T) structure.

Purpose of the Study:

• To investigate the THz and sub-gap optical response of alpha-RuCl3 across its structural phase transition.
• To understand temperature and symmetry-induced changes in THz and spin-orbital excitations.
• To explore the low-energy electronic structure of ruthenium 4d5 electrons.

Main Methods:

• Detailed optical experiments including THz, far-infrared, and mid-infrared spectroscopy.
• Complementary thermal expansion measurements.
• Analysis of temperature and symmetry-dependent changes in optical response.

Main Results:

• Observed a two-step hysteresis in the structural phase transition with significant reflectivity changes.
• Complex dielectric loss spectrum in the THz regime, potentially indicating Kitaev physics remnants.
• Identified unusual orbital and electronic excitations, with most spin-orbital excitations largely unaffected by the phase transition.

Conclusions:

• The symmetry of individual molecular layers appears to dominate the eigenfrequencies of most spin-orbital excitations.
• Specific THz and phonon modes show sensitivity to the structural phase transition and symmetry changes.
• The study provides insights into the unsolved low-energy electronic structure of alpha-RuCl3.