無序銅鍺合金之穿隧電子能態密度研究

Abstract

摘要 在真實的世界當中，非有序固體之電子傳輸特性非常的重要。一般普遍認為局域效應及無序增強電子-電子交互作用對無序系統皆有重要的影響。許多實驗的結果（例如溫度相關傳輸特性、磁電阻、以及弱無序地帶中在電子能面上展現庫侖異常的現象），都可用上述兩個理論來解釋。但是，當無序性持續增強，可以導致系統發生金屬-絕緣體相變，目前尚未有一個明確的物理機制可以描述這個現象。因此，我們製作了一系列從弱無序過渡到強無序的三維銅鍺樣品，並以低溫穿隧實驗來探討電子-電子交互作用在這轉換間所扮演的角色。 我們的樣品（CuxGe100-x）可以藉著改變銅鍺間相對之莫耳比例(30>x>11)來控制樣品的無序程度，此無序程度隨著銅的相對莫耳比例（x）減少而增加。我們的數據顯示：無論是在弱無序到強無序的過渡地帶，或是在強無序地帶，因為庫侖異常，都使得電子能態密度在費米能面附近出現減小的現象。這個異常的現象隨系統趨於弱無序迅速地減弱。除此之外，我們還發現在過渡地帶能態密度與能量成對數關係（Nn(E)μ ln(E)）；而在強無序地帶更接近絕緣態處，能態密度與能量成指數關係（Nn(E)μEm）），此處m在低能量，約等於1.45；在高能量，近似於0.5。這些數據顯示，無序增強電子-電子交互作用，確實是導致弱無序轉變至強無序的一個重要機制，而且，在這兩個區域，對傳輸特性以及穿隧電子能態密度有不同的影響。

ABSTRACT The electronic properties of disordered solids are very important in real systems. It is generally accepted that both localization and disordered enhanced electron-electron interaction play important roles. Many experimental results of temperature dependent transport, magnetoresistance, Coulomb anomalous behavior of the density of states in weakly disordered regime, can be interpreted by theories. However, when the degree of the disorder is increased leading to a Metal-Insulator transition, a description of complete transition incorporating both theories is unclear. Thus, we made a series of three dimensional CuxGe100-x samples spanning from weakly to strongly disordered regime and performed tunneling and transport measurements at low temperatures to study electron-electron interaction effects in both regimes. For our samples, the degree of disorder can be easily controlled by adjusting the relative molar concentration between Cu and Ge (30>x>11). Disorder increases with decreasing the relative concentration of Cu, x. Our data show that Coulomb anomaly cause a strong depression in density of states Nn(E) near the Fermi energy in the crossover and strongly disordered regime. This anomaly weakens rapidly when CuGe film is near the weakly disordered regime. In addition, our result that Nn(E) is proportional to ln(E) in crossover regime can not be described by the current theory. Moreover, in more strongly disordered regime, Nn(E) seems to be proportional to Em, where m is roughly 1.45 in low energy and roughly 0.5 in high energy. These data indicate that the disorder enhanced electron-electron interaction effects can drive the crossover form weak disorder to strong disorder, and has different influences in transport and tunneling density of state of the samples in these two regimes.