無序銅鍺合金之量子磁傳輸特性

Abstract

在凝態物理中，電子的傳輸特性一直是個重要的課題。在無序系統中，一般較廣為大家所接受的是弱局域效應與無序增強電子-電子交互作用力在其中扮演重要的角色。許多實驗的結果（例如溫度相關傳輸特性、弱局域範圍中在費米能面的能態庫侖異常現象以及磁電阻量測），都可以用上述兩個理論來解釋。然而當系統的無序程度持續增強，將發生所謂的金屬-絕緣相變，目前尚未有明確的理論可以連接弱無序和強無序理論來描述此相變的演變。因此，我們進行一系列從弱無序過渡到強無序的三維銅鍺樣品來研究相變的轉換機制。 在我們的銅鍺系統（Cu(x)Ge(100-x) ）中，可以藉由改變銅鍺間的相對莫耳比例（11<X<40）來控制樣品的無序程度，且樣品的無序程度隨著銅含量的減少而增加。我們觀察到從弱無序地帶過渡到強無序地帶，磁電阻變化率從正值變為負值，這結果與理論預期針對無序強自旋-軌道散射系統考量電子波函數從延展態轉變為局限態所預期的磁電阻異號結果相符合。另外，我們以弱無序磁電阻修正理論來擬合過渡地帶銅較多（弱無序）的樣品，我們發現非彈性散射率 ，其中 隨著無序程度的增加有減少的趨勢，但原則上介於1.5∼2之間。但是在強無序範圍，弱無序磁電阻修正理論不再適用，而我們發現磁電阻變化與外加磁場的平方成正比，與NSS模型所預期的結果相符合（Zhang，1991、1992；Tremblay，1989）。

The electronic transport property is a very important topic in condensed physics. It is generally accepted that both localization and disorder-enhanced electron-electron interaction play important roles in disordered systems. Many experimental results of temperature dependent resistivity, magnetoresistance, Coulomb anomalous behavior of the density of states in weakly disordered regime can be interpreted by theories very well. Increase in the degree of the disorder will lead to a Metal-Insulator transition, however, a description of complete transition incorporating both theories is still unclear. Thus, we performed transport measurements in a series of three-dimensional CuxGe100-x samples spanning from weakly to strongly disordered regime to study the evolution of mechanisms through the transition. In this alloy system, the degree of disorder can be easily controlled by adjusting the relative molar concentration between Cu and Ge (11<x<40). We observed a sign change in the magnetoresistance through this transition in our CuxGe100-x systems. The results are consistent with theoretical predictions that the magnetoresistance be opposite in these two regimes in systems with spin-orbital interaction due to the change in electronic states from extended states to localized states. In addition, by comparing data for Cu-rich samples with weak-localization theory to extract the (the inelastic scattering time), we found that and the value of p is in between 1.5 and 2. In the insulating regime, the weak-localization theory is no more applicable, we found that the magnetoresistance varies proportionally to the square of applied magnetic field as expected by NSS model .