高溫超導釔鋇銅氧薄膜之反鄰近效應研究與碲化鉍/釔鋇銅氧之拓撲/超導異質界面特性之研究

在近期的研究文獻中，反鄰近效應(Inverse Proximity Effect, IPE)被發現並探討；實驗結果指出，過量參雜(overdoped, OD)的超導材料與少量參雜(underdoped, UD)的超導材料交疊形成的雙層膜，會提高薄膜之臨界溫度(critical temperature, T_c)，這種現象被認為是結合了少量參雜層中較高的庫柏電子(cooper pairs)配對規模(pairing scale)，與過量參雜層中較強的相剛度(phase stiffness)。本研究雙層膜之上層膜選用OD材料釔鈣鋇銅氧(Y_0.85 Ca_0.15 Ba_2 Cu_3 O_y,YCBCO)，下層膜則為UD材料釔鑭鋇銅氧(Y_1 La_0.15 Ba_1.85 Cu_3 O_y,YLBCO)；實驗中先以射頻式濺鍍腔體濺鍍成長高品質c軸取向YBCO薄膜，其臨界溫度(critical temperature, T_c)約為90.5 K，並以此濺鍍條件成長YCBCO與YLBCO，T_c分別為77 K與87 K，結果與文獻紀錄吻合；接著以同樣條件成長不同厚度比例之雙層膜，發現厚度比於20 nm:90 nm時，雙層膜具有最高的T_c，並以此作為樣品，與上述三種材料進行電性、磁性之量測。電性上我們量測了樣品的霍爾效應(Hall effect)，在縱向電阻上得到樣品的釘扎能(pinning energy)、上臨界磁場(upper critical field, H_c2)與相干長度(coherent length, ξ)，橫向電阻上我們量測了樣品在一般態時的載子濃度(hole concentration, p)與霍爾角(Hall angle)，並進行了定量上之探討，且提出Anderson理論解釋之。最後以不同電流密度量測雙層膜之電阻與溫度關係，觀察到近似IPE之現象，我們歸因於YBCO較短的相干長度與其不明確的理論模型。除了超導/超導界面的反鄰近效應外，近期的研究顯示，拓撲絕緣體(Topological Insulator, TI)與超導體(superconductor, SC)之異結構亦表現出有趣的特性，因此本研究成長TI/SC雙層膜以研究其界面特性。我們選用的上層膜TI材料為碲化鉍(Bi2Te3)，下層膜SC材料為YBCO與YCBCO，期能藉由SC相異的載子濃度，調節TI之表面電子態；實驗中我們先以蒸鍍法(evaporate deposition)成長碲化鉍(Bi2Te3)薄膜，並以X射線衍射儀(X-ray diffractometer, XRD)測定薄膜之晶軸取向；接著進行電性上之量測，估計其費米能級(Fermi energy, E_F)為1.11 eV，並以Hikami-Larkin-Nagaoka公式進行擬合，確認Bi_2 Te_3中之電子軌道與自旋耦合機制。最後，我們成長了TI/SC雙層膜，並量測電流穿過界面之電阻與溫度關係、電流與電壓關係，然而因為界面之電阻過大，其雜訊大於SC本身與TI表面電子態之貢獻；我們將其歸因於材料間的晶格不匹配，並提出了未來解決此問題的方法。

關鍵字

超導；釔鋇銅氧；反鄰近效應；拓撲絕緣體

並列摘要

Inverse Proximity Effect (IPE) has been discovered and discussed in recent researches and which indicates that the ctitical temperature of bilayers formed by the overdoped (OD) superconductor and the underdoped (UD) superconductor can be increased, in which a combination of the high cooper pairing scale in UD layer with the strong phase stiffness in OD layer is considered. In this thesis, Y_(1-x) Ca_x Ba_2 Cu_3 O_y (YCBCO) is selected to be the OD material, and Y_1 La_0.85 Ba_1.85 Cu_3 O_y (YLBCO) be the UD material. High-quality c–axis oriented YBCO thin film is sputtered by RF sputtering at first, while the critical temperature(T_c) is about 90.5 K. YCBCO and YLBCO films are grown refering to this condition, and T_c values of 77 K and 87 K are obtained respectively. Then, the bilayer samples with different thickness ratios are grown under the same conditions. It is found that T_c performs the best superconductivity when thickness ratio of YCBCO: YLBCO is 20 nm: 90 nm. The electrical and magnetic properties of the above three materials are measured and studied. We measure the Hall effect and calculate the pinning energy, upper critical field and coherent length of the samples on the longitudinal resistivity ρ_xy (T). On the transverse resistivity ρ_xx (T), we calculate the hole concentration (p) and Hall angle in the normal state, apply quantitative investigations and propose the Anderson’s theory for explanation. Finally, the ρ_xx (T) curve of the bilayer is measured at different current densities. An IPE-like phenomenon is observed and can be attributed it to the short coherence length of YBCO and its unclear theoretical model. In addition to the IPE of SC/SC interfaces, recent studies have shown that the heterostructure of topological insulator (TI) and superconductor (SC) also exhibit interesting characteristics. In this thesis, bilayers composed of TI and SC are studied. The TI material used is Bi_2 Te_3, and the SC material of the is YBCO and YCBCO. The surface electron state of TI is expected to be adjusted by the different carrier concentration of SC. Bi_2 Te_3 thin film is grown by the thermal evaporation, and the crystal orientation is determined by X-ray diffractometer (XRD). The electrical properties are measured and the Fermi energy (E_F) is calculated to be 1.11 eV. The Δσ_xx-H curve is fitted by the Hikami-Larkin-Nagaoka formula, to clarify the electron orbital-spin coupling mechanism. Finally, TI/SC bilayer is grown and its physical properties are measured, such as ρ_xx (T) and I-V characters through the interface. However, the resistance of the interface is too large, causing that the noise is much larger than the signal of SC combined with TI. We attribute the result to the lattice mismatch between the materials and propose a way to solve this problem in the future.