Abstract-Field evolution of magnons in α − RuCl 3 by high-resolution polarized terahertz spectroscopy

Liang Wu, A. Little, E. E. Aldape, D. Rees, E. Thewalt, P. Lampen-Kelley, A. Banerjee, C. A. Bridges, J.-Q. Yan, D. Boone, S. Patankar, D. Goldhaber-Gordon, D. Mandrus, S. E. Nagler, E. Altman, and J. Orenstein

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.094425

The Kitaev quantum spin liquid (KSL) is a theoretically predicted state of matter whose fractionalized quasiparticles are distinct from bosonic magnons, the fundamental excitation in ordered magnets. The layered honeycomb antiferromagnet $\alpha \text{−}{\mathrm{RuCl}}_3$ is a KSL candidate material, as it can be driven to a magnetically disordered phase by application of an in-plane magnetic field, with $H_c\sim 7T$. Here, we report a detailed characterization of the magnetic excitation spectrum of this material by high-resolution time-domain terahertz spectroscopy. We observe two sharp magnon resonances whose frequencies and amplitudes exhibit a discontinuity as a function of applied magnetic field, as well as two broader peaks at higher energy. Below the Néel temperature, we find that linear spin wave theory can account for all of these essential features of the spectra when a $C_3$-breaking distortion of the honeycomb lattice and the presence of structural domains are taken into account.

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Abstract-Antiferromagnetic Resonance and Terahertz Continuum in α − RuCl 3

A. Little, Liang Wu, P. Lampen-Kelley, A. Banerjee, S. Patankar, D. Rees, C. A. Bridges, J.-Q. Yan, D. Mandrus, S. E. Nagler, and J. Orenstein

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.227201

We report measurements of optical absorption in the zigzag antiferromagnet $\alpha \text{−}{\mathrm{RuCl}}_3$ as a function of temperature $T$, magnetic field $B$, and photon energy $\hslash \omega$ in the range $\sim 0.3$–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.

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Abstract-Antiferromagnetic resonance and terahertz conductivity in \alpha-RuCl3

A. Little, Liang Wu, P. Lampen-Kelley, A. Banerjee, S. Pantankar, D. Rees, C. A. Bridges, J. -Q. Yan, D. Mandrus, S. E. Nagler, and J. Orenstein

https://journals.aps.org/prl/accepted/78076Y70Qb613566849547557f2aaf480ab45f021

We report measurements of optical absorption in the zig-zag antiferromagnet \alpha-RuCl3 as a function of temperature, T, magnetic field, B, and photon energy, \hbar\omega in the range \sim 0.3 to 8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that 3-fold rotational symmetry is broken in the honeycomb plane from 2 K to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the N\'{e}el temperature of 7 K at B=0 that we identify as magnetic-dipole excitation of a zero-wavevector magnon, or antiferromagnetic resonance (AFMR). With application of B, the AFMR broadens and shifts to lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From 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.

Abstract-Antiferromagnetic resonance and terahertz conductivity in α−RuCl3

A. Little, Liang Wu, P. Lampen-Kelley, A. Banerjee, S. Pantankar, D. Rees, C. A. Bridges, J.-Q. Yan, D. Mandrus, S. E. Nagler, J. Orenstein

(Submitted on 24 Apr 2017)

https://arxiv.org/abs/1704.07357

Spin-1/2 moments in the antiferromagnetic Mott insulator α-RuCl3 are coupled by strongly anisotropic bond-dependent exchange interactions on a honeycomb lattice. Intense study of α-RuCl3 by inelastic scattering has been driven by the proposal that its low energy excitations may be adiabatically connected to the Majorana quasiparticles that emerge in the exact solution of the Kitaev spin liquid model. In this work, we report optical absorption measurements using time-domain terahertz spectroscopy in the range 0.8 to 7.5 meV that reveal new features of the low-energy spectrum of α-RuCl3. Above a sharp threshold photon energy of ∼1 meV, we observe a continuum of absorption that we assign to spin-driven electric dipole fluctuations. Polarized measurements demonstrate optical anisotropy, showing that 3-fold rotational symmetry is broken in α-RuCl3 at 300 K and down to low temperature. Below 7 K we observe the onset of a single, narrow antiferromagnetic resonance mode above the continuum, enabling the first measurements of the absolute spectral weight and lifetime of the q=0 magnon in α-RuCl3.