無序金鈀合金厚膜之熱電勢研究

Abstract

電子在固態裡的傳輸行為一直是重要且有趣的問題。在傳輸過程中電子會與聲子、雜質或無序散射，這些作用決定了電子的傳輸特性，而這些特性除了表現在電阻率上，也會反應於熱電勢。當無序程度增加，古典的波茲曼方程不再可以完全描述電子的傳輸，因此本文想要藉由實驗探討結構無序對電子-聲子作用所造成的影響。 我們使用濺鍍的方法製備了一系列不同無序程度的Au50Pd50厚膜(厚度約1000~4000埃)，利用低溫的He4冷卻系統與自製的樣品座，測量低溫下電阻率對溫度、熱電勢對溫度的關係。發現隨著無序增加，熱電勢的量值變小，這可能與能帶尖峰結構的改變有關。隨著溫度降低，熱電勢對溫度關係逐漸偏離線性且漸漸變大，此增強效應反映了電子質量的增強。進一步對電阻率與熱電勢分別使用類似Bloch-Gruneisen form與兩種簡化模型的電子-聲子偶合函數(Eliashberg function)進行擬合，發現系統無序程度越大，電子-聲子作用強度越大，而德拜溫度會稍為變小。另外電子質量增強係數也會隨無序增加而稍微變大，最後趨向飽和，這是因為係數A與德拜溫度對無序關係有某種程度的補償。

The electrical transport behavior in condensed matter physics is an important and interesting problem. Electrons scatter with phonons, impurities and disorder in the solid, and electrical-transport properties are decided by the interactions (scattering processes) which affect not only on resistivities but also thermoelectric powers. With increasing the degree of disorder, the transport properties can not be well described by the Boltzmann equation. In this thesis, we would experimentally investigate the effect of structure disorder on the electron-phonon interactions. We have fabricated a series of AuPd (atomic percentage is 50:50) thick films (thickness：1000~4000 angstrom) with different degree of disorder and measured the resistance versus temperature as well as the thermoelectric power versus temperature from 300K down to liquid-helium temperatures. The smaller magnitude of thermoelectric power with increasing disorder is observed, and this may be due to the disordered effect which makes the band structure become flatter and the Fermi level blurs gradually. Moreover, the thermoelectric power is not simply proportional to temperature below 200K and the result can be understood as a result of enhanced electron-phonon coupling or electron-phonon mass enhancement. In addition, we use Bloch-Gruneisen form to fit resistivity data as well as use two simple models of Eliashberg function to fit thermoelectric power enhancement data. The stronger electron-phonon interactions and smaller Debye temperature with larger degree of disorder are indicated. We found that, first the parameter of electron mass enhancement increases and finally saturates with increasing disorder. This is due to some compensation of coefficient A and Debye temperature in relation to disorder.