利用時間解析飛秒光譜研究摻碲硒化鐵超導體之超快動力學

Abstract

在本篇論文裡，我們利用時間解析飛秒光譜來研究不同碲摻雜的硒化鐵(FeSexTe1-x)單晶－即所謂的11型鐵基超導體的超快動力學。在400 nm的光激發及800 nm的光探測下，我們可以量測FeSe0.760，FeSe0.166Te0.666，和FeTe0.875單晶在不同溫之瞬時反射率變化(ΔR/R)。整體來說，11型鐵基超導的ΔR/R光譜中包括兩個訊號，一個是準粒子的弛緩過程，另一個是聲頻聲子的振盪信號。不過這些特徵會隨著Te的摻雜而改變，以具有超導特性的FeSe0.76，FeSe0.166Te0.666樣品而言，ΔR/R的大小在Tc及100 K附近(結構相變發生處)有不尋常的增加。而且透過曲線擬合，我們可以獲得準粒子的弛緩時間同時算出電子聲子(A1g mode)的耦合強度λ，在20 K時，FeSe0.760單晶的λ=0.129，而FeSe0.166Te0.666單晶的λ=0.080。此外，ΔR/R中振盪信號所對應的聲頻能量亦有不尋常的減小，稱之為聲子軟化(phonon softening)現象。然而上述之變化並未在不具超導特性的樣品FeTe0.875單晶上觀察到。因此，我們推測11型鐵基超導體(FeSexTe1-x)在正常態(約100 K)所出現的聲子軟化現象與形成超導有密切的關係。

In this dissertation, we used time-resolved femtosecond spectroscopy to study the ultrafast dynamics of FeSexTe1-x single crystals, i.e. 11 type iron-based superconductors. The transient reflectivity (ΔR/R) of FeSe0.760, FeSe0.166Te0.666, and FeTe0.875 single crystals has been performed by 400 nm pump-800 nm probe at various temperatures. In general, the typical ΔR/R of 11 type iron-based superconductors includes two significant components. One is the relaxation of photoinduced quasiparticles. The other is the oscillation component due to the acoustic phonon. Interestingly, these characteristics in ΔR/R are strongly changed by the content of Te in FeSe single crystals. For FeSe0.760 and FeSe0.166Te0.666 single crystals, the abnormal changes in the amplitude of ΔR/R are clearly observed at Tc and around 100 K where the structure-phase transition occurred. Also, the electron-phonon(A1g mode) coupling constant can be estimated to be 0.129 and 0.080, respectively, from the relaxation time ΔR/R. Moreover, the acoustic phonon energy estimated from the period of oscillation in ΔR/R markedly shrinks at Tc and around 100 K, which is the so-called phonon softening. However, above phenomena are not observed in the non-superconducting FeTe0.875 single crystals. Therefore, these results imply that the phonon softening occurred in normal state may be intimately correlated to the superconductivity of 11 type iron-based superconductors.