Ultrasonic Studies Of Charge Ordering And Phase Separation In Perovskite-type Manganites

The perovskite type transition-metal oxides are the typical strongly correlated systems due to their strong coupling among charge, spin, orbit and lattice degree of freedom. Many related physical phenomena such as super-conducting, metal-insulator transition, charge and orbital ordering, and phase separation have been found in these compounds. The divalent alkaline-earth ions doped perovskite type manganites have recently received much attention due to their 'colossal magnetoresistance' (CMR) effect. Recent studies for manganites have revealed that the CMR effect is attributed not only to the double-exchange interaction but also to other interactions and competitions, such as charge-orbital ordering, electron-phonon coupling, and phase coexistence. However, the microscopic origin of these intriguing phenomena in manganites is still a widely studied and yet unresolved problem. So it is necessary to further investigate the correlations between the charge, spin, and orbit in these compounds.In this dissertation, we systematically measured the ultrasonic sound velocity and attenuation properties near electronic, magnetic and structural phase transitions in Nd_1-xCa_xMnO₃, Nd_0.5Ca_0.5Mn_1-xM_xO₃ (M=Co,Al) and La_0.5-xNd_xCa_0.5MnO₃. A series of dramatic ultrasonic anomalies were observed in these compounds. Combined with the transport and magnetic measurements, we found that the charge ordering in Nd_1-xCa_xMnO₃ is dominated by Jahn-Teller effect. In addition, we made detailed analyses of the correlated interactions in layered cobalt oxides YBaCo₂O_5+δ and gave experimental evidence of the electron-phonon coupling the compound. 1. Studies of the charge ordering and Jahn-Teller effect in Nd_1-xCa_xMnO₃The stability of CO state and Jahn-Teller coupling energy Ejt as a function of Ca²⁺ concentration have been investigated by measuring the ultrasonic velocity,magnetization and resistivity for Nd_1-xCa_xMnO₃(x=0.4⁰.8). It was found that the increases with the increasing of Ca²⁺ concentration x, and reaches the maximum at around x=0.6, which is similar in characteristics to the Tco. The results imply that the...