Optical Studies on the Phase Transitions in YBaMn2O6 Single Crystals.

The double perovskite YBaMn2O6 exhibited complex structural, magnetic, and charge/orbital ordering phase transitions. In this paper, we investigated the correlation between the temperature-dependent optical response and complex phase transitions of YBaMn2O6 single crystals through spectroscopic ellipsometry and Raman scattering spectroscopy. The room temperature optical absorption spectrum of YBaMn2O6 revealed three bands of approximately 1.50, 4.05, and 5.49 eV. The lowest optical absorption band was assigned to on-site d-d transitions in Mn ions, whereas the other two optical features were assigned to charge-transfer transitions between the 2p states of O and 3d states of Mn. The room temperature Raman scattering spectrum revealed 25 phonon modes. Notably, the MnO6 octahedral tilting and bending modes between 360 and 440 cm-1 increased in intensity at temperatures <200 K. Furthermore, several new phonon peaks appeared at temperatures <200 K, which were associated with charge ordering. Additionally, the magnetic order-induced changes were observed in the breathing modes, with reduced intensity of the 620 cm-1 and a substantial enhancement of the 644 cm-1 phonon peaks. The Jahn-Teller mode at approximately 496 cm-1 exhibited strong hardening at temperatures <200 K, which indicated a linear correlation with the square of the magnetic susceptibility and thus revealed the occurrence of spin-phonon coupling. Anomalies in the phonon frequency, line width, and intensity observed near the phase transition temperatures highlighted the importance of lattice-charge-spin interactions in YBaMn2O6.

Anomalous boron isotope effects on electronic structure and lattice dynamics of CuB2O4.

Copper metaborate had a unique crystal structure and exhibited noteworthy magnetic phase transitions at 21 and 10 K. The electronic structure and lattice dynamics of copper metaborate Cu11B2O4 single crystals were investigated and compared with the optical properties of CuB2O4, to assess the boron isotope effect. The optical absorption spectrum at room temperature revealed two charge-transfer bands at approximately 4.30 and 5.21 eV with an extrapolated direct optical band gap of 3.16 ± 0.07 eV. Compared with the data on CuB2O4, the electronic transitions were shifted to lower energies upon the replacement of a heavier boron isotope. The band gap was also determined to be lower in Cu11B2O4. Anomalies in the temperature dependence of the optical band gap were observed below 21 K. Furthermore, 38 Raman-active phonon modes were identified in the room-temperature Raman scattering spectrum of Cu11B2O4, which were also observed in CuB2O4 with a shift to lower frequencies. No broadening caused by isotopic changes was observed. As the temperature decreased, phonon frequencies shifted to higher wavenumbers and the linewidth decreased. Anomalous softening in the Raman peaks below 21 K was also revealed.

Optical Study of the Electronic Structure and Lattice Dynamics of NdBaMn2O6 Single Crystals.

We investigated the electronic structure and lattice dynamics of double perovskite NdBaMn2O6 single crystals through spectroscopic ellipsometry and Raman scattering spectroscopy. The optical absorption band centered at approximately 0.88 eV was assigned to on-site d-d transitions in Mn, whereas the optical feature at approximately 4.10 eV was assigned to charge-transfer transitions between the 2p state of O and 3d state of Mn. Analysis of the temperature dependence of the d-d transition indicated anomalies at 290 and 235 K. The activated phonon mode, which appeared at approximately 440 cm-1 alongside with the enhancement of the 270 cm-1 phonon mode, coupled strongly to the metal-insulator transition at 290 K, which was associated with a charge/orbital ordering. Moreover, the MnO6 octahedral breathing mode at 610 cm-1 exhibited softening at a temperature lower than 235 K (temperature of the antiferromagnetic phase transition), which revealed the strong coupling between the lattice and magnetic degrees of freedom. The spin-phonon coupling constant obtained was λ = 2.5 cm-1. These findings highlight the importance of charge-orbital-spin interactions in establishing NdBaMn2O6 phases with novel properties.