Magnetic excitations and optical transitions in the multiferroic spin-1/2 system LiCu$_2$O$_2$

TL;DR

This study investigates magnetic excitations in the multiferroic LiCu$_2$O$_2$ using THz spectroscopy, revealing multiple optically active modes, their magnetic field dependence, and polarization characteristics, with no electromagnon detected in the studied range.

Contribution

First detailed THz spectroscopic analysis of magnetic excitations in LiCu$_2$O$_2$, identifying mode behavior and polarization dependence in a multiferroic spin-1/2 system.

Findings

Eight optically active modes observed below 30 cm$^{-1}$

Mode number increases with magnetic field

No electromagnon detected above 4 cm$^{-1}$

Abstract

Magnetic excitations in cycloidal magnet LiCu$_2$O$_2$ are explored using THz absorption spectroscopy in magnetic fields up to 12 T. Below the spin ordering temperature we observe eight optically active transitions in the spin system of LiCu$_2$O$_2$ in the energy range from 4 to 30 cm$^{-1}$. In magnetic field the number of modes increases and the electric polarization flop is seen as a change in magnetic field dependence of mode energies. The polarization dependence of two of the modes in zero magnetic field fits the selection rules for the cycloid tilted by $\theta=41\pm1^{\circ}$ from the bc plane. For the remaining six modes electric and magnetic dipole approximations cannot explain the observed polarization dependence. We do not see the electromagnon in the explored energy range although there is evidence that it could exist below 4 cm$^{-1}$.