| Summary: | 博士 === 國立清華大學 === 物理系 === 104 === This dissertation is composed of two parts. For the first part, we examine the existence of sinusoidal charge density wave (shear-type distortion) of La0.5Sr1.5MnO4 under its semiconductor-insulator phase transition. We use (h/4 h/4 0)_t superstructure reflection with h is an odd number to enhance the sensitivity of the asymmetric intensity distribution of multi-beam resonant X-ray diffraction to the distortion modes. Our results confirmed the existence of a sinusoidal charge density wave, and suggested a mixed-type distortion for La0.5Sr1.5MnO4 under the phase transition temperature, TCO = 220 K. We also use (h/2 h/2 0)_t superstructure reflection with h is an odd number to investigate the charge disproportion between different Mn sites in its charge ordering phase. The difference between the simulated multi-beam diffraction patterns at resonant energy reconfirm the 0.15 charge disproportion model.
The second part is about the study of the crystal symmetry of YBaCuFeO5. To distinguish if YBaCuFeO5 is centrosymmetric or non-centrosymmetric, we study the two-beam resonant X-ray diffraction and non-resonant multi-beam X-ray diffraction with both +G and –G geometry. Compared to the measured data together with the dynamical diffraction theory simulations suggest a centrosymmetric nature of YBaCuFeO5 crystal structure. To further clarify if Fe and Cu share the same site positions, we conduct resonant multi-beam X-ray diffraction on both Fe and Cu K-edges. The different asymmetry of the measured data suggest the bond length of Cu and its apical O is actually longer than the one between Fe and its apical O.
|
|---|
