Optical signature of spin-charge-lattice coupling in (Nd,Y)BaMn2O6, and CuB2O4

Abstract

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(Nd,Y)BaMn2O6, and CuB2O4 display interesting magnetic, charge/orbital, and structural phase transitions. These complex phases open up new and exciting functionalities (e.g. colossal magnetoresistance, high-temperature superconductivity, magnetocaloric effect, metal-insulator transition, etc.) that are not fully understood. Recognizing and determining the strengths of these correlations is fundamental and important for practical technological applications. Optical spectroscopy is a sensitive tool that can probe the intricate interactions between the spin, charge, and lattice of these materials. In this study, we employed spectroscopic ellipsometry and Raman scattering spectroscopy to investigate the electronic structure and lattice dynamics of (Nd,Y)BaMn2O6, and CuB2O4 single crystals.The room-temperature optical absorption spectra of NdBaMn2O6 and YBaMn2O6 revealed d–d transitions in Mn ions positioned at 0.88 and 1.50 eV, respectively. The d–d transition peak for RBaMn2O6 compounds was observed to increase as R radii were decreased. Temperature-dependent d–d transition energy of NdBaMn2O6 exhibited anomalies at the charge/orbital ordering phase transition (290 K) and the magnetic phase transition (235 K). Furthermore, the temperature-dependent Raman scattering spectra revealed anomalies at the phase transition temperatures. In NdBaMn2O6, phonon modes appeared at approximately 440 cm−1. This correlated with the spike of the 270 cm-1 phonon mode at 290 K, which is also the onset of the charge/orbital ordering. Additionally, new phonon peaks appeared at temperatures< 200 K, indicating charge and orbital ordering in YBaMn2O6. Magnetic order-induced changes were observed in the breathing and Jahn-Teller modes. The octahedral breathing mode of NdBaMn2O6 at 610 cm−1 shifted to lower frequencies below 235 K along with a reduced intensity of the 620 cm-1 and an increase of the 644 cm-1 phonon peaks. Hardening of the Jahn-Teller mode of YBaMn2O6 at approximately 496 cm-1 was observed at temperature < 200 K. These phenomena revealed the occurrence of spin-phonon coupling in NdBaMn2O6 and YBaMn2O6.The room-temperature optical absorption spectrum of CuB2O4 displayed charge-transfer transitions from the 2p states of O to the 3d states of Cu at approximately 4.49 and 5.90 eV. A direct band gap for CuB2O4 was extrapolated at 3.88 ± 0.01 eV. We noted isotopic-induced changes upon the replacement of heavier boron. Optical absorption data of Cu11B2O4 revealed a shift to lower energies in the charge-transfer bands. The band gap of Cu11B2O4 was also slightly lower than that of CuB2O4. The temperature-dependent band gap and the peak energy of charge-transfer bands exhibited anomalies through the canted antiferromagnetic ordering temperature at 21 K. The Raman phonon frequencies of Cu11B2O4 shifted lower, whereas the linewidth displayed minor discrepancies. The isotopic shift in frequencies conferred with the inverse square root dependence of the mean atomic mass. The Cu stretching vibrations of CuB2O4 and Cu11B2O4 exhibited a softening below 21 K, indicating a spin-phonon interaction.We have demonstrated the correlations between the temperature-dependent optical response and the complex phase transitions in (Nd,Y)BaMn2O6, and CuB2O4. These results highlight the intricate spin, charge, and lattice interactions in these materials.