Studies of work function, electron-phonon coupling constant, and superconductivity transition temperature of Pb thin films on Ge(111)

鉛薄膜在鍺(111)基底上的功函數,電子聲子耦合參數,及超導溫度的量測

劉若亞(Ro-Ya Liu)

指導教授 ： 唐述中

國立清華大學/原子科學院/先進光源科技學位學程/碩士(2013年)

https://doi.org/10.6843/NTHU.2013.00260

摘要

在第一部分的研究中，我們利用高解析度的角解析光電子能譜儀(Angle -resolved Photoemission Spectroscopy, ARPES)量測了在n型高摻雜的Ge(111)上，1到13層的鉛薄膜功函數，我們發現功函數隨薄膜層數有高低震盪的現象(Bilayer Oscillation)，並且發現當最高被佔據的量子井態(Highest Occupied Quantum Well State, HOQWS)的能量越高，功函數也越高，在11層附近找到了震盪的反轉點。與前人所做量測的，3到16層的鉛薄膜熱穩定度一起比較，我們得到功函數與熱穩定度的相位差大約是1.62，和侷限在薄膜的自由電子模型所預測的1.57，也就是1/4週期，非常接近。
第二部份的研究中，我們利用微觀四點探針方法(Microscopic four-point probe, m4pp)量測3 ML,4 ML, 及8 ML的鉛薄膜低溫的導電性質，得到超導溫度分別是，1.60 K，3.65 K，及5.21 K。同時利用角解析光電子能譜(ARPES)，量測6 ML, 8 ML的量子井態的電子聲子耦合常數(Electron Phonon Coupling, EPC)，分別是1.09，1.27。從上述兩種實驗可以發現薄膜的超導溫度和電子聲子耦合參數皆隨著層數增加而變大，此現象和目前許多類似系統上的研究結果頗一致，最後，我們以此電聲耦合參數所預測的超導溫度與實際量測值做了比較和詳細的討論。

關鍵字

薄膜 ； 量子井態 ； 功函數 ； 表面能 ； 角解析光電子能譜 ； 微觀四探針電性量測 ； 超導溫度 ； 電子聲子耦合參數

並列摘要

We measured work function of Pb thin films/Ge(111) from 1 to 13 ML. The work function shows a Bilayer oscillation with thin film thickness. Besides, the higher highest occupied quantum well state (HOQWS), the higher work function. A turning point of the bilayer oscillation was found near 11ML. Compare the work function with thermal stability of thin films, we found that the phase difference between work function and thermal stability is 1.62, which is close to the phase difference, 1/4 of periodicity, 1.57 predicted by free electron model.
Another topic is the microscopic four-point probes (micro 4PP) conductivity measurement on Pb thin films. We measured the superconductivity transition temperature (Tc) of 3 ML, 4 ML, and 8 ML is 1.60 K, 3.65 K, and 5.21 K. We also measured the electron phonon coupling (EPC) of 6 ML and 8 ML Pb thin film by ARPES. The EPC of 6 ML and 8 ML QWS of Pb thin film is 1.09 and 1.27. From above results, we found that both the Tc and EPC of the Pb films are increasing with film thickness. It is concordant with many results found in similar systems. Finally, we made a comparison and discussion of superconductivity transition temperature by micro 4PP measurement and by EPC prediction.

並列關鍵字

thin film ； quantum well state ； work function ； surface energy ； angle-resolved photoemission spectroscopy ； microscopic four point probe conductivity measurement ； superconductivity transition temperature ； electron-phonon coupling constant

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