Electromagnon by chiral spin dynamics in the triangular lattice antiferromagnet

From high field electron spin resonance measurements in illuminating polarized light, we have revealed the existence of electromagnon, i.e., magnon excitation by oscillatory electric fields of light, in the field-induced 1/5-plateau phase of the triangular lattice antiferromagnet ${\mathrm{CuFeO}}_2$. We indicate that peculiar magnon modes, which generate uniform fluctuation of the vector spin chirality at wave vector $k=0$, appear in the magnetic ordered phase with a collinear spin structure on triangular lattice. Our experimental results demonstrate that such magnon modes couple with an electric component of light, leading to the emergence of the electromagnon. Moreover, the measurements in circularly polarized light exhibit an anomalous behavior that circular dichroism, which is usually found in magnetic resonance, is absent in the resonance signal of the electromagnon. The microscopic mechanism of the electromagnon in ${\mathrm{CuFeO}}_2$ is also discussed.

Figure 1

(a) Frequency-field diagram of the ESR resonance points observed in ${\mathrm{CuFeO}}_2$ for $H\parallel \left[001\right]$. Solid curves show theoretical ESR modes. Dashed lines are the transition field. The inset shows a triangular layer, built by edge sharing of upward (u) and downward (d) elementary triangles. Precession motion of the sublattice moments for (b) ${\omega }_{\mathrm{b}+}$, (c) ${\omega }_{5\mathrm{u}+}$, and (d) ${\omega }_{3\mathrm{u}}$ modes.

Figure 2

Polarization dependence of the ESR spectra at 162 GHz and 1.5 K, for which oscillatory fields are applied along (a) the $\left[110\right]$ and $\left[001\right]$ axes, and (b) the $\left[1\overline{1}0\right]$ and $\left[001\right]$ axes. Upper curves are the spectra observed in an experimental setup for the oscillatory electric field perpendicular to the $\left[001\right]$ axis, whereas lower ones are those for the oscillatory magnetic field perpendicular to the $\left[001\right]$ axis. The ESR absorption areas are highlighted. Dashed lines are transition fields.

Figure 3

ESR spectra observed in illuminating circularly polarized light at 162 GHz and 1.5 K. Upper and lower curves are the spectrum, observed in right- and left-handed circularly polarized light, respectively. The ESR absorption areas are highlighted.

Figure 4

Left panel: Snapshots of the precession motion of the sublattice moments for the ${\omega }_{5\mathrm{u}+}$ mode. The right upper and lower panels depict the snapshot at instants, when the sublattice moments are laying in a plane perpendicular to the $\left[110\right]$ axis. Ellipses are the plane, in which the sublattice moments are laying. Cones, drawn by dashed curves, are loci of the precession motion. The right-handed proper screw texture with the screw axis along the $\left[110\right]$ direction, depicted in the upper panel, is converted to the left-handed texture by changing phases of the precession motion by ${180}^{\circ }$, as shown in the lower panel.