Antiferromagnetic Resonance and Terahertz Continuum in α-RuCl_{3}

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Magnetism

Background:

• $\alpha$-RuCl$_{3}$ is a zigzag antiferromagnet with potential Kitaev physics.
• Understanding its magnetic excitations is crucial for exploring exotic quantum states.

Purpose of the Study:

• To investigate the magnetic excitations in $\alpha$-RuCl$_{3}$ using terahertz spectroscopy.
• To characterize the behavior of antiferromagnetic resonance (AFMR) under varying temperature and magnetic fields.

Main Methods:

• Time-domain terahertz spectroscopy was employed to measure optical absorption.
• Measurements were conducted as a function of temperature (T), magnetic field (B), and photon energy (ℏω).
• Polarized measurements were used to probe symmetry breaking.

Main Results:

• A sharp AFMR peak at 2.56 meV was observed below the Néel temperature (7 K) at zero magnetic field.
• Polarized measurements revealed broken threefold rotational symmetry in the honeycomb plane from 2 to 300 K.
• Applied magnetic fields broadened and shifted the AFMR to lower frequencies, indicating a transition to a spin-disordered phase.

Conclusions:

• The observed peak is identified as a zero-wave-vector magnon excitation.
• The study provides insights into the magnetic phase transitions and excitation spectrum of $\alpha$-RuCl$_{3}$.
• An upper bound was placed on the contribution of magnetic excitation continua to dc susceptibility.