| Summary: | 碩士 === 國立交通大學 === 電子物理系所 === 95 === Coherent acoustic phonon oscillations (CAPO) have been observed in many materials, such as superconductor, semiconductor, and magnetic materials using ultrafast pump-probe spectroscopy. Especially the colossal magnetoresistance manganates La1-xCaxMnO3 , one of interesting materials in our lab, exhibited strongly CAPO at various doping concentrations (x=0, x=0.3 and x=0.58). However, several mechanisms about the generation and detection of CAPO have been proposed and disputed each other. In this thesis, we have prepared La0.42Ca0.58MnO3 thin films with various thicknesses by pulsed laser deposition and have measured the transient reflectivity
(ΔR/R) as a function of a pump-probe delay at various temperatures. We found that a slower oscillation with a period of hundreds of ps in ΔR/R is further modulated with a faster oscillation with a period of tens of ps. The characterization of these oscillations can be elucidated by the ultrafast generated strain pulse model, which was proposed by Thomsen [8]. Moreover, the ultrasound velocity at various temperatures also has been calculated in this material and is consistent with that generated by a conventional pulse-echo-overlap technique with a frequency of several megahertz [16][17]. The anomaly of sound velocity near the charge ordering phase transition has been observed.
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