以超快動力學研究FeSe0.912S0.088單晶之向列性

Abstract

近年來在鐵基超導體中，Nematic order是一個非常熱門的議題，在大部份的鐵基超導體，當Nematic order出現，通常都伴隨著反鐵磁序列的產生，因此一般認為Nematic order的來源與磁性相關，然而FeSe卻展現與眾不同的特性，在FeSe中並無量測到長程磁有序，卻仍然展現出Nematic order的特性，因此我們想藉由此研究了解鐵基超導中Nematic order機制的來源究竟是與磁性相關或是與電子軌域相關。 根據本團隊先前對FeSe的研究，FeSe單晶隨著溫度下降，在結構相變溫度前後，晶格結構的對稱性由四重對稱轉變為二重對稱，為了解四重對稱被破壞的來源與機制，我們透過飛秒雷射時間解析光譜，進行極化的激發-探測實驗，提供擁有角度解析以及時間解析的超快雷射光譜，進而研究隨著溫度的變化，晶體內部的載子在動態上之改變，在FeSe中Nematic order的來源與電子結構及自旋系統息息相關，並且我們觀測到在高於晶格結構相變溫度(Ts=90 K)，仍然有Nematic order存在，且持續到至少200 K。根據文獻中對FeSe的電子結構研究發現，在Γ point 及M point上的能帶隨溫度變化及費米面上的形變情形隨溫度變化也展現出Nematic order特性。 為延續FeSe中Nematic order的研究，我們選擇研究摻雜硫之FeSe，摻雜硫之FeSe單晶隨著硫的摻雜量越大，結構相變溫度(Ts)越低，而超導相變溫度(Tc)越高，直到硫的摻雜量為10%，超導相變溫度為最高，本論文所研究的樣品成分比例為FeSe0.912S0.088。 我們同樣透過飛秒雷射時間解析光譜，對FeSe0.912S0.088進行極化的激發-探測實驗，藉由研究晶體內部的載子隨著溫度在動態上之改變，定性的探討及推論Nematic order 機制作用的趨勢以及Nematic order的來源等，接著進行擬合，在定量上探討各向異性的大小，結果顯示我們同時觀測到了電子結構以及自旋系統的Nematicity，其中電子結構對Nematic order機制所造成的影響遠高於結構相變溫度。

Recently, nematic order has been one of the important topics in iron-based superconductors. In most of the iron-based superconductors, nematic order usually is accompanied by the antiferromagnetic order. It is generally believed that the origin of nematic order is related to the magnetic one. However, FeSe shows different characteristics. There is no long-range magnetic order in FeSe, but there still exists the nematic order. We would like to understand whether the origin of the nematic order in FeSe is magnetically related or associated with the electronic orbitals. According to the previous study of FeSe by our group, the symmetry of the lattice structure changes from four-fold symmetry to two-fold symmetry when the temperature drop below structural phase transition temperature(Ts = 90 K), which is called the nematic order. In order to understand the origin and mechanism of the nematic order, we measured the temperature-dependent and polarized transient reflectivity changes (ΔR/R) by dual-color pump probe spectroscopy with angle resolved and time resolved measurements. The origin of the nematic order in FeSe is related to the electronic structure and the spin sub-system. The nematic order is present above the lattice structure transition temperature (Ts = 90 K) to at least 200 K. From the ARPES study of FeSe in the literature, it is found that the Fermi surface and energy band at the Γ point and M point shows the nematic order characteristic below Ts. In order to continue the study of the nematic order, we have chosen to study the S-doped FeSe. The higher the concentration of sulfur is, the lower the structure phase transition temperature but the higher the superconducting phase transition temperature. When the doping amount of the sample is 10%, the superconducting temperature is the highest. The sample used in this thesis is FeSe0.912S0.088. In order to understand the origin and mechanism of the nematic order, we measured the temperature-dependent and polarized transient reflectivity changes (ΔR/R) by dual-color pump probe spectroscopy, which provides angle resolved and time resolved spectroscopy of FeSe0.912S0.088 single crystals. By realizing the ultrafast dynamics of quasi-particles in crystals, we observe the nematic signals of electronic structure and spin sub-subsystem. The nematic signals remain as the temperature is far above Ts.