弱無序銅鍺金薄膜之低溫電性傳輸行為研究

Abstract

電子的傳輸行為在凝態物理領域中，是一重要且基礎的議題。電子在週期性晶格中的傳輸行為，可由古典的波茲曼理論描述。隨著系統的無序程度增加，電子散射率也增加，使得量子干涉的效應逐漸重要。著名的弱局域效應和電子—電子相互作用，即為考慮此效應的結果。 我們製作一系列不同無序程度和不同厚度的CuGeAu 薄膜(原子百分比93︰4︰3)，測量其在低溫下電阻對溫度的關係、和電阻對磁場的關係。初步分析的結果顯示出樣品的無序度在k l ≈ 5 ~ 55 F 的範圍。將二維系統的量測結果，以上述兩個物理理論預測做擬合分析，得出不同於理論預測的T α 值( = 1.26 T α ，理論預測值則為≤ 1 T α )。此結果隱含著系統中，除以上兩個效應之外，尚存在一個造成低溫電導隨溫度變化有對數關係修正的其他機制，且此機制的大小不容被忽略。 同時，磁電阻在不同溫度下的變化情形，和弱局域理論所預測的結果幾乎相反，顯然有其他機制在主導磁電阻的變化。另外，我們從實驗結果歸納出，當k l F 接近或大於50 時，量子效應對低溫電導的修正即小於殘餘電阻的萬分之二倍。 本實驗室的黃旭明研究此系統的電子相位相干時間隨溫度的變化，得出此系統存在二能級系統的結論。而二能級系統對二維樣品低溫電導的修正正是對數關係，支持上述實驗結果= 1.26 > 1 T α 的合理性。另外磁電阻在不同溫度時的奇異行為，是否和二能級系統的存在相關，則待進一步的研究和討論。

The electrical-transport property is one of the most important and fundamental problems in condensed matter physics. In a periodic structure, the transport property can be well described by the Boltzmann’s transport equation. With increasing degree of disorder, the electron scattering rate will increase, and lead to novel quantum-interference effects between the conduction electron wavefunctions. The well known weak-localization and enhanced electron-electron interactions are the results of such quantum-interference effects. We have fabricated a series of CuGeAu (the atomic percentage is 93︰4︰3) films with different degree of disorder and with different thickness, and have measured the resistance versus temperature as well as the resistance versus magnetic field at low temperatures. The values of k l F for our samples are ≈ 5 − 55 . The resistance vs. T in the 2D samples obey an “a-b log T” dependence at low temperatures, which is consistent with the theoretical predictions of the weak-localization and electron-electron interaction effects. Quantitatively, we fitteded the data in terms of these two theories and obtained a resistance slope = 1.26 T α , which was significantly larger than the predicted value of ≤ 1 T α . This result implies that some extra mechanism must exit in our samples, which also caused a -log T dependent resistance. On the other hand, the behavior of the magnetoresistivity at different temperatures is opposite to the theoretical predictions of the ii i weak-localization effect. This observation again implies that other mechanism must exist to influence the magnetoresistivity. In addition, we found that, when the value of k l F approaches 50 or larger, the quantum-interference correction is less than 2 × 10-4 of the residual resistance. Previously, Huang et al. have studied the electron phase-coherent time as a function of temperature in disordered CuGeAu thin films. They concluded that there existed two-levels tunneling systems in their samples. Since two-levels systems can also cause a “–logT” dependent resistance, our result of a large slope of T α (= 1.26) is in line with their observation. How the anomalies in magnetoresistivities at different temperatures might be explained in terms of two-level systems requires further theoretical and experimental studies.