Optical Spectroscopy Investigations of Double-Perovskite (Y, Nd)BaMn₂O₆ Single Crystals
Rea Divina Mero1*, Hsiang-Lin Liu1, Shigeki Yamada2
1Department of Physics, National Taiwan Normal University, Taipei, 11677, Taiwan
2Department of Material System Science, Yokohama City University, Yokohama, 236-0027, Japan
* Presenter:Rea Divina Mero, email:readivina@gmail.com

We investigate the electronic structure and lattice dynamics of double-perovskite YBaMn₂O₆ and NdBaMn₂O₆ single crystals using spectroscopic ellipsometry and Raman scattering spectroscopy. These materials are interesting because of their complex magnetic and charge/orbital ordered phase transitions. Room temperature optical absorption spectrum of YBaMn₂O₆ shows three bands around 1.50, 4.05, and 5.49 eV. NdBaMn₂O₆ also exhibits three bands, which are slightly shifted to lower energies at 0.87, 3.87, and 5.03 eV. The lower energy band is assigned to the on-site d-d transition of Mn3+. The other two bands are associated with the charge-transfer transition between O 2p to Mn 3d states. Room temperature Raman scattering spectrum of YBaMn₂O₆ shows nine phonon modes at approximately 162, 211, 250, 265, 304, 335, 470, 496, and 610 cm-¹. Phonon modes below 300 cm-¹ involve the motion of Y and Ba atoms. Phonon modes between 300 and 400 cm-1 are due to mix vibrations of stretching, bending, and tilting, while the 496 cm-¹ mode is mainly due to Jahn-Teller distortion. The peak at 610 cm-1 is assigned to the breathing modes of the oxygen atoms in the MnO₆ octahedra. NdBaMn₂O₆ exhibits two strong phonon modes near 320 and 500 cm-1, which are assigned to the bending of the MnO₆ octahedra tilt and Jahn-Teller distortion, respectively. With decreasing temperature, these phonon modes show a shift of the peak position to higher frequencies and a narrowing the resonance linewidth. Notably, new phonon peaks appear in the spectra range between 200 and 400 cm-¹ for YBaMn₂O₆ below 180 K. Furthermore, the magnetic-order-induced splitting of the 610 cm-¹ breathing mode is observed. NdBaMn₂O₆ reveals the appearance of breathing mode at low temperature. Anomalies in the phonon frequency and linewidth are observed near the structural and magnetic phase transition temperatures. Our results highlight a strong spin-phonon coupling in these materials.

Keywords: strongly correlated systems, spin-phonon coupling, optical properties, Raman scattering, double perovskite