Understanding Multiferroic Hexagonal Manganites by Static and Ultrafast Optical Spectroscopy

Multiferroic hexagonal manganites ReMnO3studied by optics are reviewed. Their electronic structures were revealed by static linear and nonlinear spectra. Two transitions located at~1.7 eV and~2.3 eV have been observed and attributed to the interband transitions from the lower-lying Mn3+dxy/dx2-y2anddxz/dyzstates to the Mn3+d3z2-r2state, respectively. These so-called d-d transitions exhibit a blueshift as decreasing temperatures and an extra blueshift nearTN. This dramatic change indicates that the magnetic ordering seriously influences the electronic structure. On the other hand, the ultrafast optical pump-probe spectroscopy has provided the important information on spin-charge coupling and spin-lattice coupling. Because of the strongly correlation between electronic structure and magnetic ordering, the amplitude of the initial rising component inΔR/Rshows striking changes at the vicinity ofTN. Moreover, the coherent optical and acoustic phonons were observed on optical pump-probe spectroscopy. Both the amplitude and dephasing time of coherent phonons also exhibit significant changes atTN, which provide the evidence for spin-lattice interaction in these intriguing materials.