三維無序鈦-鋁-鉻超導合金之上臨界磁場

Abstract

我們製作了一系列三維 Ti-Al-Cr 超導合金 (將 Ti 的莫耳百分比固定，然後逐漸增 加 Cr 的含量，以此來取代 Al 的含量)，並測量了這一系列樣品的臨界溫度以及上臨界 磁場。根據以往的經驗，我們參雜 Al 以增加樣品的無序程度，使樣品的電阻率提高， 增加量測時的解析程度。 從我們的實驗結果中發現，當 Cr 的含量增加到 3% 以後，alpha-Ti 合金的超導 臨界溫度從 0.4 K被提昇至 4 K，與其它實驗結果類似。然而，在上臨界磁場方面則發 現，與傳統無序超導體理論中用來描述上臨界磁場的 Werthamer-Helfand-Hohenberg 理論比較，我們的結果在較高溫度時 (約 0.8 倍臨界溫度) 即明顯向下偏離傳統理論的 預測，且其偏離量隨著溫度的降低而逐漸增加。另外，若以將電子局域效應列入考慮 的理論 (Fukuyama-Ebisawa-Maekawa 理論、Coffey-Muttalib-Levin 理論)來解釋，亦同 樣無法說明我們的結果。

A series of three-dimensional (3-D) polycrystalline disordered superconducting Ti-Al-Cr alloys have been made (with a total doping level of 7.2 at % for Ti, Cr atoms are gradually added into Ti host). The superconducting transition temperatures and the temperature variation of the upper critical fields are measured. Previously, we have found that the residual resistivities of 3-D polycrystalline disordered alpha-Ti alloys can be raised by controlling the doping level of Al. Therefore, in this work Al atoms are introduced into alpha-Ti to make the alloys disordered enough so that the accuracy of measurements can be improved. The transition temperatures are found to be considerably changed from about 0.4 K for pure Ti to about 4 K for Ti-Al-Cr samples with the doping level of 3-6 at % of Cr. This has a resemblance to other earlier experimental results. Howerer, comparison of the upper critical fields with the traditional Werthamer-Helfand-Hohenberg theory that describes the behavior of the upper critical field in a dirty superconductor shows a negative deviation from the classical variation at rather high temperatures (above the half of the transiton temperature). Such a deviation increases as temperature decreases. Moreover, we find that this unusual behavior of the upper critical fields cannot be qualitatively explained in terms of Anderson localization effects through the models developed by Fukuyama-Ebisawa-Maekawa and Coffey-Muttalib-Levin, respectively.