利用飛秒光譜研究六方晶系鐿錳氧單晶之超快動力學

Abstract

在本論文中，我們利用波長可調的時間解析激發探測實驗量測六方晶系鐿錳氧單晶的超快動力學，研究此材料在磁化，極化，以及應力三個自由度之間的耦合行為。在實驗中，我們依不同波長的探測光子能量去找出錳d-d能帶間隙(Edd)隨溫度的變化，並且發現在TN處能帶間隙會有異常的藍移現象。透過分析量測到的瞬時反射率變化ΔR/R曲線，發現其中包含有”負分量”以及”同調聲子振盪”。其中負分量與ab平面上的熱彈效應有關，並且直接關係到這類h-YbMnO3材料在TN附近的磁彈耦合。此外，由沿c軸傳遞的形變脈衝產生在ΔR/R曲線的振盪分量，我們發現振盪週期會隨降溫而衰減，特別是在溫度降至TN以下，推測可能是在TN附近的反鐵磁排序造成樣品鐵電性質改變造成的。這結果也暗示了此一樣品中在TN附近的鐵電效應主要由磁彈效應來控制。

In this paper, the ultrafast dynamics probed by wavelength-tunable femtosecond pump-probe spectroscopy was performed to disclosed the coupling among the magnetization, polarization, and strain degree of freedom simultaneously in the hexagonal YbMnO3 single crystals. In the experiment , we found the Mn d-d transition energy (Edd) at different temperature by wavelength-tunable pump energy, and we also observed the extra-large Edd blueshift behavior near TN. Analyses based on the measured transient-reflectivity-change(ΔR/R) curves reveal that the evolvement of the negative component and the coherent acoustic phonon. The negative component in ΔR/R is intimately correlated to the thermoelastic effect on the ab plane of h-YbMnO3 associated with the magnetoelastic coupling around TN. Moreover , the variation of the oscillation period in ΔR/R caused by a strain pulse propagating along c-axis decrease with decreasing temperature , and exhibits an abrupt drop around TN. We presumed that the abrupt drop around TN due to antiferromagnetic ordering-induced changes in the ferroelectricity. This result implies that the magnetoelectric effect near TN are controlled by the magnetoelastic effect.