Role of magnon-magnon interaction in optical excitation of coherent two-magnon modes

Abstract

Two-magnon modes are terahertz-frequency magnetic excitations in antiferromagnets, governed by exchange interactions and involving magnons from the entire Brillouin zone. The ability to couple to light promotes two-magnon modes as contenders for ultrafast optical manipulation of the magnetic state, beyond conventional zone-center magnonics. While magnon-magnon interactions are known to critically shape the two-magnon line in spontaneous Raman scattering spectra, their role in coherent time-domain excitations remains unexplored. We report a detailed experimental and theoretical study of the influence of magnon-magnon interactions on coherent two-magnon modes in a cubic antiferromagnet excited via Impulsive Stimulated Raman scattering. We reveal the nontrivial evolution of coherent magnetic dynamics in the time domain and the corresponding spectrum and compare it with the spontaneous Raman scattering spectrum. By extending the spin-correlations based theory for two-magnon modes, we derive a unified description of their spectra in Raman Scattering and Impulsive Stimulated Raman Scattering and highlight the role of magnon-magnon interactions.

Figures (4)

• Figure 1: (a) Crystal and magnetic structure of RbMnF3. Blue and red arrows indicate spins in two sublattices aligned along the $\langle111\rangle$ direction. (b) Schematic representation of magnon dispersion and light interaction with a two-magnon mode. The figure depicts inelastic light scattering via a two-magnon excitation at the X point of the Brillouin zone. The color gradient schematically represents an effect of magnon-magnon interaction. (c) The time-resolved pump-probe experimental scheme: linearly polarized at an angle $\beta$ optical pump pulse excites spin dynamics via ISRS, which is detected via transient ellipticity of the optical probe pulse. $\Delta t$ is time delay between pump and probe. (d) RS experimental scheme in back-scattering geometry with $z \parallel [001]$. Incident continuous-wave (CW) beam is polarized along the $x \parallel [100]$ axis. In the parallel ($xx$) and cross-polarized ($xy$) configurations of the scattered beam is polarized along the $x$ and $y \parallel [010]$ axes, respectively.
• Figure 2: (a) Laser-induced probe ellipticity $\Delta\phi$ as a function of the delay time $\Delta t$ (open blue symbols) measured in RbMnF3 at $T = 5$ K. Lines -- The inverse Fourier transform of the calculated ISRS spectra of 2M mode [Eq. \ref{['eq:ellipticity']}] obtained taking into account the magnon-magnon interaction (blue line), and without magnon-magnon interaction (red line). (b) Transient probe ellipticity $\Delta\phi(\Delta t)$ measured at different combinations of the pump $\beta$ and probe $\alpha$ polarization angles. (c) Polarization dependences of the intensity of the 2M mode line in measured RS spectra in parallel [$z(xx)\overline{z}$] (solid orange symbols) and crossed [$z(xy)\overline{z}$] (solid green symbols) configurations. The open symbols are the experimental data reflected relative to the horizontal axis, assuming the angular symmetry.
• Figure 3: (a) 2M mode spectra at 5 K. Open blue symbols are the Fourier transform of experimental transient probe ellipticity [Fig. \ref{['fig:Time_trace']}(a)], solid blue line is the calculated ISRS 2M spectrum taking into account the magnon-magnon interaction [Eq. \ref{['eq:ellipticity']}]. Open orange symbols are the experimental RS 2M spectrum, solid orange line is the calculated RS spectrum taking into account the magnon-magnon interaction [Eq. \ref{['eq:scattering']}]. Dashed lines are a calculated spectrum of 2M mode in RS (orange) and ISRS (blue) without magnon-magnon interaction. All spectra were normalized by the maximum of magnitude. (b) Pump fluence $F_{p}$ dependence of the FFT ISRS amplitude. The line is a linear fit. (c) Integral amplitude (blue symbols) of the calculated ISRS 2M spectrum [Eq. \ref{['eq:ellipticity']}] and ratio of the amplitudes of the features $P_{2}/P_{1}$ (green symbols) vs pump duration $\tau_\mathrm{p}$. The lines are guides to the eye.
• Figure 4: Experimemntal 2M mode spectra as obtained at different temperatures from (a) the Fourier transform of experimental transient probe ellipticity, and (b) RS measurements in $z(xx)\overline{z}$ configuration.