Abstract
本論文的內容分成兩大部份,分別是A15結構化合物超導體薄膜研製及高溫超導薄膜在超導溫度下的微波表面電阻量測.在A15超導薄膜的研究上,我們以矽為基板濺渡Nb3Ge化合物,改變快速退火溫度,並藉X-ray, TEM及Tc量測以判斷退火溫度對薄膜品質的影響.我們的實驗發現RTA(rapidthermal annealing) 900度的樣品得到較佳結果.伴隨A15結構Nb3Ge的形成,Nb5Ge3相也隨之產生.要得到單一相很困難. 我們試著了解正常態Nb3Ge的傳輸性質,我們發現Nb3Ge薄膜的金屬性滿足Bloch-Gruneisen法則.對於低溫下電阻上升的樣品,則以弱侷限效應(weak localization effect)可以作很好的解釋.從擬合的參數來看,晶粒邊界散射(grain boundaryscattering)理論無法滿足實際的實驗情況. A15化合物的另一研究中,我們以很高的速率在Si基板上濺鍍Nb,藉著Nb原子的快速轟擊,Si表面產生Nb3Si化合物.在這實驗中,我們得到文獻所未記載的高超導溫度(從13.3K起降,在11.9K時零電阻).輔以其它量測儀器的實驗結果,我們證明這個溫度的正確性,而且確認是Nb3Si化合物.從穿透式電子顯微鏡(TEM)的照片比對,我們認為高Tc的原因是我們生長出針狀Nb3Si,藉由尖端累積電荷的結果,迫使費米面上的能態密度提高,故提升Nb3Si的超導溫度.BCS公式提供了這個論點的理論基礎. 對於高溫超導薄膜的微波表面電阻研究上,我們設計了一介質共振腔,並使用於變溫系統上.從Maxwell方程式的推導,共振頻率和Q值可以計算.我們觀察到Rs(T)在低溫時與T成正比,且低溫下Tl-2212的Rs值比YBCO薄膜小,這實驗與二流體模型的預測吻合. 高溫超導穿透深度的溫度相依性(lambda(T))可以從腔體共振頻率隨溫度改變的漂移推得.我們實驗發現change of lambda(T)在低溫時是T^2關係(d-wave),而溫度升高時卻呈指數關係(s-wave).我們也研究了YBCO薄膜衰減造成的影響.從這實驗中我們獲得一些寶貴的證據顯示雜質在HTSC薄膜內角色使得溫度升高時高溫超導體內電子波函數的對稱性有急遽的變化.In an attempt to study the transport properties of Nb3Ge thinfilms on silicon substrates, a series of samples are measuredfor the resistance versus temperature curves forcharacterization. We used the X-ray diffractionand TEM tocharacterize the film quality. In our analysis, the sample whichis post annealed by RTA at 900C yields better result. Thecoexisted Nb5Ge3phases also grows up as the RTA temperature wasincreased, it was hard to prevent their formation. To realizethe electron scattering mechanism in Nb3Ge film, soemtheoriesare quoted by which we can explain the anomalousresistivity phenomenon. Wefind that the metallic behavior of thesamples can be explained by Bloch-Gruneisen's law while thenonmetallic conductivity can be regarded as due toweaklocalization. Dendritic textured Nb3Si thin films, depositedby sputtering pure niobiumonto silicon substrates, exhibit arather high offset critical temperature, Tc.X-ray and TEMmeasurements reveal the films to be polycrystalline with the A15phase. The resistance begins to drop sharply at temperature of13.3K anddown to zero at 11.9 K without residual resistance.From BCS theory, we infer that the high Tc property may resultfrom a high density of states modulatedby this dendritictexture. A temperature controlled dielectric resonator isemployed as a convenientmethod to measure the microwave surfaceresistance Rs(T). As derived fromMaxwell's equations, theresonant frequencies and Q-value are calculated. We haveobserved that Rs(T) is linear at low temperature; in addition,thelow temperature Rs value of Tl-2212 film is smaller than thatof YBCO which is in agreement with the result of two-fluidmodel. A formula which extracts the temparature dependence ofpenetration depthlambda(T) is also calculated, the delta-lambda(T) for both HTSC films elucidates a T^2 dependence at lowtemperatures and an exponential dependence at high temperatures.An impurity scattered mechanism changes a pure d-wave to the s-wave by thermal agitation is proposed fot the pairing states ofthese high-Tc cuperconducting films.