Antiferromagnetic Resonance and Terahertz Continuum in α-RuCl3

• Published in: Physical review letters 2017-11, Vol.119 (22)
• Main Authors: Little, A., Wu, Liang, Lampen-Kelley, P., Banerjee, A., Patankar, S., Rees, D., Bridges, C. A., Yan, J. -Q., Mandrus, D., Nagler, S. E., Orenstein, J.
• Format: Article
• Language: English
• Subjects: CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY
• ISSN: 0031-9007
• DOI: 10.1103/PhysRevLett.119.227201

We report measurements of optical absorption in the zigzag antiferromagnet α-RuCl3 as a function of temperature T , magnetic field B , and photon energy ℏ ω in the range ~ 0.3 –8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that threefold rotational symmetry is broken in the honeycomb plane from 2 to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Néel temperature of 7 K at B = 0 that we identify as the magnetic-dipole excitation of a zero-wave-vector magnon, or antiferromagnetic resonance (AFMR). With the application of B , the AFMR broadens and shifts to a lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From a direct, internally calibrated measurement of the AFMR spectral weight, we place an upper bound on the contribution to the dc susceptibility from a magnetic excitation continuum.