| Summary: | 碩士 === 國立臺灣師範大學 === 物理學系 === 105 === We studied the optical properties of perovskite Nd0.35Sr0.65MnO3 (30 nm) (NSMO)、YBa2Cu3O7 (40 nm) (YBCO)、NSMO (30 nm)/YBCO (30 nm)heterostructure thin films and double-perovskite SmBaMn2O6 single crystal.We employed the spectroscopic ellipsometry to investigate the optical constants and electronic structures of these materials. Furthermore, we performed Raman scattering study of SmBaMn2O6. The analysis of the temperature evolution of lattice vibrational modes would give important information about lattice-charge-spin interactions.
Room temperature optical conductivity spectrum of NSMO determined from spectroscopic ellipsometry analysis shows two main bands at about 1.1 eV and 3.7 eV. The absorption peaks of 1.1 eV and 3.7 eV are due to Mn3+ d → its neighbor Mn4+ d and O 2p → Mn 3d charge-transfer transitions, respectively. The optical conductivity spectrum of YBCO determined from spectroscopic ellipsometry analysis shows three main bands at about 3.1 eV, 3.8 eV and 4.6 eV. The 3.1 eV absorption peak is due to charge-transfer transitions between O 2p and Cu 3d states. The 3.8 and 4.6 eV absorption peaks are due to charge-transfer transitions between Cu(1)3d_(3z^2-r^2 )and4p_x. The optical conductivity spectra of NSMO/YBCO heterostructure show changes of the peak position and intensity in comparison with those of the single layers, indicating that strain effect of heterostructure modifies the electronic structures of individual NSMO and YBCO layers. The observed new absorption peaks are possibly due to interface-induced effect.
Room temperature absorption spectrum of SmBaMn2O6 determined from spectroscopic ellipsometry analysis shows three main bands at about 1.3, 3.4 and 4.2 eV. The first absorption peak is due to Mn3+ d_(3x^2-r^2 ) (or d_(3y^2-r^2 ))→ its neighbor Mn4+ d. The second and third peaks are due to O 2p →Mn 3d charge-transfer transitions. Their positions show redshift at the charge-orbital order-disorder phase transition temperature (TCO1, 370 K). X-ray diffraction data show J-T distortions relax, resulting in a redshift of d-d transitions be lower. Room temperature Raman scattering spectrum of SmBaMn2O6 exhibits four phonon modes at about 196 cm-1, 330 cm-1, 485 cm-1, and 614 cm-1 attributed to the rotationlike mode 1, rotationlike mode 2, Jahn-Teller mode and breathing mode. With increasing temperature, rotationlike mode 1 and breathing mode disappear above 330 K. Jahn-Teller mode and breathing mode show a splitting and many vibrational modes appear below charge-orbital order phase transitiontemperature (TCO2, 200 K), suggesting the strong charge-orbital coupling and the appearance of superstructure.
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