銅氧化物高溫超導薄膜在臨界溫度附近的漲落增益之導電率及高溫超倒薄膜共振器之微波性質

Abstract

本論文中，我們研究層狀銅氧化物高溫超導薄膜的電性及微波性質。本 研究共分為兩部分：(1) 在第一部份中藉由熱擾動增強之電導性，我們在 同一片釔鋇銅氧(YBCO)薄膜樣品研究氧含量對電阻率的影響，並且與單晶 粒鉈(Tl)-2223、多晶鉈-2223及多晶鉈-1223等樣品性質比較。利用經過 適當數據分析所得之結果並藉由適當模型驗證，吾人可求出參數-相干長 度進而得知樣品電性傳輸性質之特性。在YBCO系統其氧含量在大於6.75條 件下，在接近臨界溫度附近我們找到一個新的層間耦合尺度。關於這個新 的結果可經由吾人所提出之修正之Lawrence-Doniach模型來加以說明。所 有的分析結果顯示，當氧含量減少時，YBCO 會逐漸呈現二維行為。其平 面傳導(in-plane)延伸至晶格之外之能力會逐漸減弱，最後會被限制在一 個晶格中，如同Tl-2223系統之結果。然而，具有一個絕緣之鉈-氧(Tl-O) 層之Tl-1223系統其in-plane 傳導亦可延伸至一個晶格外，雖然其傳導特 徵與Tl-2223之結果是一致的。這結果顯示，Tl-1223的傳導行為是界乎二 維之 Tl-2223與三維之滿氧YBCO系統之間。(2) 在第二部分中，我們研 究由銅氧化物超導體所製作之薄膜共振器的微波性質。利用在微帶線型共 振器所測量之與溫度相關之共振頻率可求得與氧相關之有效磁場穿透深度 。所得之滿氧約為 4900A，這結果與利用Anderson的層間偶合理論所計算 之c方向穿透深度結果 ~ 4200A非常相近。在氧含量x=6.8的條件下穿透深 度約為滿氧時之1.6倍。而c方向等效質量比 。這說明了當氧含量減少時 ，YBCO系統的層間偶合強度減弱、磁場容易穿透超導體表面，使得超導載 子在c軸方向傳導更不容易。這結果和在第一部份的研究結論是一致的。 此外，我們利用預先退火處理氧化鎂基板 (MgO) 方法，發現超導晶粒在 未經過退火處理之MgO 基板呈任意分布的成長，然而在經退火處理之基板 上則其超導晶粒呈方向性排列之規則成長。如此之結構造成較少的微波損 耗。另一方面，吾人發現微帶與基板間之表面粗造度和薄膜微帶之顯微結 構對穿透深度皆具重要影響。 The electric and microwave properties of the cuprate layered high- superconducting thin film have been study. The study has divided into two main parts:(1) In the part I, by the thermal fluctuation-enhanced conductivity we have studied the oxygen effect of the YBCO system on the electric resistivity using the same film, and compared with the results of the Tl-2223 revealed on a single grain as well as of the granular thin films of Tl-2223 and Tl-1223 systems. Using the observation from the proper data manipulation and comparing with the proper model, the parameter of the coherence length can be extracted and than the characteristic of the electrical transport property can be obviously revealed. Besides, we have found a new length scale in YBCO system near for the case of x>6.75. The modified Lawrence- Doniach model proposed by us can realize this unconventional behavior. All of the evidences show that the YBCO system is more two-dimensional feature as decreasing the oxygen content. And the capability of the in-plane conductivity extending to several unit cells is weaken and finally is limited within an unit cell as the result of Tl-2223 system in which two insulting Tl-O planes intervene between the adjacent groups of CuO2 planes. However, the system of Tl-1223 with one Tl-O layer between the group of CuO2 planes, we found that the in-plane may also extend to several unit cells even though the feature of is the same as that of the Tl-2223 rather than of the YBCO (x>6.75) system. It indicates that the characteristic of of the Tl-1223 thin film intervenes between the 2-D behavior of the Tl-2223 and the quasi-3-D one of the nominally fully-oxygen case of the YBCO system. (2) In part II, we have studied the microwave properties of the thin film resonator, which is composed of the cuprate superconductors. By using the temperature dependent resonate frequencies measured from the microstrip line resonator, the oxygen function of the effective penetration depth can be determined. We found that for the fully-oxygen case, A, which is consistent with the result A calculated by the Anderson's interlayer coupling theory. For the case of x=6.8, the is extended to about 1.6 time of the fully oxygenated case. And the mass ratio can be determined to be 1.3. The fact indicates that the oxygen deficiency leads to the weaken interlayer coupling and causes to the magnetic field is easier to penetrate into the superconductor, which give rise to the carriers transporting in the c-direction more difficult. The result is consistent with the conclusion in the study on the fluctuation-enhanced conductivity of this dissertation in Part I.By pre-annealing the as-polished MgO substrates, finally, we found that the grains of the YBCO thin film will change from random distribution to the directional alignment, which lead to the less microwave loss. We also found that the penetration depth is sensitive to the roughness of the interface between the strip and the substrate as well as the microstructure of the strip.