高溫超導體/錳氧化物異質結構之合成與特性研究

Synthesis and Characterization of High-Tc Superconductor/Manganite Heterostructure

鄭淑齡(Su Ling Cheng)

指導教授 ： 莊東漢

共同指導教授 ： 林昭吟(Jauyn Grace Lin)

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2018年)

https://doi.org/10.6342/NTU201800936

摘要

本論文主要利用雷射濺鍍法製作了高溫超導體YBa2Cu3O7 (YBCO)及鐵磁錳氧化物Sm0.67Sr0.33MnO3(SS0.33MO)之異質結構。用來觀察磁極化粒子在YBCO之自旋擴散長度(spin diffusion length，FM)以及磁極化粒子和庫柏對在YBCO/SS0.33MO界面之交互作用。希望能夠提供一些信息給後續有興趣致力於研究SS0.33MO之基礎物理性質或者是進一步想將其應用在YBCO/SS0.33MO自旋閥元件的人。
本研究主要有三部分的工作。其一，我製作了高溫超導體YBa2Cu3O7 (YBCO)及鐵磁錳氧化物(La,Sm,Sr,Ca)MnO3的單層薄膜。並且以電性、磁性、微結構、結晶結構、表面形貌及電子結構等分析結果，加以探討。首先我將不同厚度的YBCO及(La,Sm,Sr,Ca)MnO3之單層膜成長在(001)鈦酸鍶基板上。我發現隨著厚度的減少，在LS0.33MO和LC0.33MO系列之電跟磁轉換溫度隨之降低;SS0.33MO之磁轉換溫度降低，但電傳導溫度卻隨之增加。電子結構之分析指出，SS0.33MO與其他系列相比，具有較大的電荷波動能(charge fluctuation energy, Ecf)和交換能量(exchange energy，Eex)。且SS0.33MO的Ecf值與d-d庫倫排斥力(d-d Coulomb repulsion energy，U)的數值相近。我推測這可能是造成金屬-半導體轉換溫度不同之可能因素。
其二，我製作了YBCO/(La,Sm,Sr,Ca)MnO3雙層膜，探討界面的逆鄰近效應(Inverse proximity effect)，並利用實驗結果和理論模擬定義出磁極化粒子在YBCO層之自旋擴散長度(spin diffusion length，FM)。我發現影響FM之大小主要與Eex相關， LS0.33MO，LC0.33MO， SS0.33MO和 SS0.45MO之FM分別為10 nm，9.7 nm，5.0 nm和3.0 nm。
其三，我製作了[YBCO/ SS0.33MO]4之超晶格結構，並利用X光吸收能譜(X-ray absorption spectra，XAS)和中子反射光譜(Polarized neutron reflectivity，PNR)，分析並模擬YBCO和SS0.33MO界面之磁特性。從PNR之模擬結果得知，在SS0.33MO層之界面形成了一沒有磁性的耗損層(depletion layer)。從XAS之實驗結果顯示，全電子產率(Total electron yield，TEY)量測到之Cu L3-和Mn L3-吸收峰的能量與螢光產率(Fluorescence yield，FY)量測到之能譜相比，其峰值往低能量偏移了0.4 eV，意指著其電荷可能由Cu的位置傳遞到Mn的位置。

關鍵字

高溫超導體/錳氧化物異質結構 ； 自旋擴散長度 ； 中子反射率

並列摘要

Heterostructures of high-critical temperature (Tc) superconductor YBa2Cu3O7 (YBCO) and ferromagnetic (FM) manganite Sm0.67Sr0.33MnO3 (SS0.33MO) are synthesized by using a pulsed laser deposition system to investigate the spin diffusion length of spin-polarized quasiparticles in YBCO and the magnetic interaction at the interface of YBCO/SS0.33MO. Hopefully, results of this thesis can provide the information to those interested in the fundamental characteristics and practical applications of YBCO/SSxMO in the future.
I include three parts of the work in this study. In the first part, series of high-Tc superconductor YBCO and FM (La,Sm,Sr,Ca)MnO3 single layers are grown on a (001) SrTiO3 substrate with various thicknesses. The crystalline structure, electrical transport, magnetic properties, microstructure, and electronic structure are analyzed. With decreasing the film thicknesses, the magnetic and electrical transition temperatures decrease in La0.67Sr0.33MnO3 (LS0.33MO) and La0.67Ca0.33MnO3 (LC0.33MO) systems. However, for SSxMO (x = 0.33 and 0.45) systems, an increase in the electrical transition temperature is observed. Furthermore, the analysis of the electronic structure shows that the energy gap, charge fluctuation energy (Ecf), and exchange energy (Eex) of SS0.33MO are much larger than those of LS0.33MO and LC0.33MO. Particularly, the Ecf value of SS0.33MO is comparable to the reported value of the d–d Coulomb repulsion energy (U). This may be caused by different electrical transport mechanisms of the FM layers.
In the second part, the inverse proximity effect is investigated in YBCO (d nm)/(La,Sm,Sr,Ca)MnO3 (10 nm) bilayers to determine the spin diffusion length (FM) value of spin-polarized quasiparticles in the YBCO layers. The value of FM is 10.0 nm for LS0.33MO, 9.7 nm for LC0.33MO, 5.0 nm for SS0.33MO, and 3.0 nm for Sm0.55Sr0.45MnO3, which shows a strong correlation with the △Eex value.
In the third part, [YBCO (17 nm)/SS0.33MO(10 nm)]4 superlattices on (001) (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) are synthesized to investigate the interfacial magnetic properties of YBCO and SS0.33MO using probes of X-ray absorption spectra (XAS) and polarized neutron reflectivities (PNRs). Simulation results of the PNR curves show that a depletion layer is formed at both sides of the SS0.33MO interface with a suppressed magnetization. The XAS show that the positions of the Cu L3 and Mn L3 edges shift to a lower energy by a difference of 0.4 eV, suggesting the charge transfer from the Cu sites into the Mn sites.

並列關鍵字

High-Tc Superconductor/Manganite Heterostructure ； spin diffusion length ； Polarized neutron reflectivity

參考文獻

References

[1] S. Soltan, (2005). Interaction of Superconductivity and Ferromagnetism in YBCO/LCMO Heterostructures. (Doctoral dissertation) MAX-PLANCK-INSTITUT FÜR FESTKÖRPERFORSCHUNG STUTTGART, Germany.

[2] H. Kamerlingh Onnes, Further experiments with liquid helium. C. On the change of electric resistance of pure metals at very low temperatures etc. IV. The resistance of pure mercury at helium temperatures: Springer Netherlands, 1991.

[3] W. Meissner and R. Ochsenfeld, Naturwissenschaften 21, 787 (1933).

[4] L. N. Cooper, Phys. Rev. 104, 1189 (1956).