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类型 基础研究 预答辩日期 2017-12-03
开始(开题)日期 2014-05-28 论文结束日期 2017-08-26
地点 东南大学四牌楼校区五五楼305 论文选题来源 973、863项目     论文字数 4.3 (万字)
题目 Fe(Se,Te)薄膜的生长与超导性能的研究
主题词 铁基超导体,超导薄膜,上临界磁场,临界电流密度
摘要 自2008年新型铁基超导材料LaFeAs(O,F)被发现以来,新型铁基超导体吸引了科学界的广泛关注。作为铁基超导体家族中的一员,铁硫族超导体Fe(Se,Te)由于其简单的晶体结构易于超导机理的研究而备受青睐。相比于铁砷超导体来说,铁硫族超导体的毒性更低,更适合工业化应用。由于几何特性的优势,薄膜相比于单晶来说更有利于传输特性研究。在此论文中,我们利用脉冲激光沉积系统PLD方法在不同的衬底上制备Fe(Se,Te)薄膜样品,对比研究了不同衬底对于薄膜性能的影响,找出了最佳的衬底类型,并且研究了利用缓冲层改善薄膜样品的性能,此外我们还对不同衬底上的薄膜样品的上临界磁场进行研究。另外,我们还对O2对于Fe(Se,Te)薄膜超导电性的影响进行了研究。 首先系统地对比研究了不同的衬底对FeSe0.5Te0.5薄膜样品结构和超导性能的影响。在不同类型,包括氧化镁(MgO), 铝酸镧(LaAlO3)以及氟化钙(CaF2)单晶衬底上沉积薄膜样品,对薄膜样品结构和超导性能进行研究。结果发现衬底与薄膜样品的ab面内的晶格常数不匹配度不会影响薄膜的晶格常数,而衬底的类型影响薄膜样品的结晶质量,进而影响超导特性。研究结果发现沉积在MgO衬底上的FeSe0.5Te0.5薄膜的结晶度较差,并且薄膜含有沿着晶格c轴方向的缺陷,薄膜的超导转变温度Tc和临界电流密度Jc很低。而沉积在CaF2衬底上的FeSe0.5Te0.5薄膜具有最高的Tc和Jc值。 其次,在 (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT)单晶衬底、表面沉积有CaF2缓冲层的LSAT单晶衬底以及CaF2单晶衬底,三种晶格常数相似的单晶衬底上利用PLD方法沉积FeSe0.7Te0.3薄膜,研究结果发现,尽管衬底的面内晶格常数相似,但是不同衬底上的FeSe0.7Te0.3薄膜的面内晶格常数有着很大的不同。并且FeSe0.7Te0.3薄膜的超导转变温度Tc与薄膜的a轴的晶格常数息息相关。由于衬底和薄膜的热膨胀系数存在差异,在薄膜沉积之后冷却的过程中,衬底会对薄膜产生面内的收缩应力,改变薄膜的a轴的晶格常数,使得沉积在CaF2衬底上的薄膜具有最高的Tc,19 K。 理解铁基超导体的标度行为对于阐明超导微观理论至关重要。本论文研究了不同收缩应力下的FeSe0.7Te0.3薄膜的上临界磁场Hc2。通过对FeSe0.7Te0.3薄膜的上临界磁场的拟合,结果发现FeSe0.7Te0.3薄膜在Tc处的上临界磁场的斜率可以利用-dHc2/dT与Tc成正比标度,证实FeSe1-xTex与其他铁基超导体存在相同的标度行为。 此外,将Fe(Se,Te)薄膜置于空气中,由于空气中氧气的作用,薄膜的超导性能有所提高。随着接触空气时间的推移,沉积在CaF2衬底上的Fe(Se,Te)薄膜的Tc从最初的14.4 K提高到了19.3 K,且真空退火能够破坏氧气导致的超导电性。进一步的研究发现,尽管超导电性得到了显著的改善,但是置于空气中的薄膜的晶格结构却没有变化。证实了从空气中并入的氧会占据填隙位置,改善超导电性,Tc升高,并且这是一个可逆的过程。
英文题目 Preparation and Superconducting properties of Fe(Se,Te) thin films
英文主题词 iron-based superconductors, superconducting thin film, upper critical field, critical current density
英文摘要 After the discovery of superconductivity at 26 K in LaFeAs(O, F), extensive research on iron-based superconductors has been carried out. As a member of the iron-based high temperature superconductors, iron chalcogenide Fe(Se,Te) attracted much attention due to its simple structure, which is favorable for probing the superconducting mechanism. Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based superconductors. Compared to single crystals, thin films are suitable for investigating transport properties and superconducting electronics applications thanks to their geometry. In this dissertation, we investigated the influence of substrate type on transport properties of superconducting Fe(Se,Te) thin films to find the the suitable substrate. Furthermore, we found that CaF2, as the buffer layer, improved the structural and superconducting properties of Fe(Se,Te) thin films. Moreover, the influence of excess Fe and the mechanism of the improvement of superconducting properties were proformed. FeSe0.5Te0.5 thin films were grown by pulsed laser deposition on CaF2, LaAlO3 and MgO substrates and structurally and electro-magnetically characterized in order to study the influence of the substrate on their transport properties. The in-plane lattice mismatch between FeSe0.5Te0.5 bulk and the substrate shows no influence on the lattice parameters of the films, whereas the type of substrate affects the crystalline quality of the films and, therefore, the superconducting properties. The crystalline quality of the film on MgO is inferior to other films. Defects parallel to the c axis are found in the film on MgO. The lowest Tc and Jc are measured for the film on MgO with extended defects and the poorest degree of texture. The film on CaF2 exhibits the highest Tc and Jc . Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate. Understanding of the universal behaviors found for iron-based superconductors is important for elucidation of the microscopic theory superconductivity. In this work, we investigate FeSe0.7Te0.3 thin films in different in-plane strain states. The strain allow as varying superconducting transition temperature Tc in a broad range maintaining the composition and impurity scattering rates. We show that the slopes of upper critical field Hc2 are scaled by the relation -dHc2/dT ~ Tc, indicating that FeSe1-xTex follows the same universal behavior as other iron-based superconductors. The superconductivity was improved in Fe(Se,Te) thin films by oxygen incorporation via exposing to the air. The transition temperature Tc increased from 14.4 K to 19.3 K with increasing exposure time. Furthermore, no shift of (00l) peaks was observed between the as-grown sample and the exposed sample, in spite of the dramatic improvement of superconducting property. These results suggested that the incorporated oxygen partially occupied the interstitial site and improved superconductivity.
学术讨论
主办单位时间地点报告人报告主题
德国莱布尼茨研究所IFW 2015.06.11 德国莱布尼茨研究所IFW,D1E13 袁菲菲 Influence of oxygen on superconducting properties of Fe(Se,Te) thin film
德国莱布尼茨研究所IFW 2017.03.30 德国莱布尼茨研究所IFW, D1E13 袁菲菲 Universal scaling behavior of the upper critical field in strained FeSe0.7Te0.3 thin films
德国莱布尼茨研究所IFW 2016.11.21 德国莱布尼茨研究所IFW,D1E13 袁菲菲 The influence of the in-plane lattice constant on the superconducting transition temperature of FeSe0.7Te0.3 thin films
德国莱布尼茨研究所IFW 2016.04.21 德国莱布尼茨研究所IFW,D1E13 袁菲菲 Influence of substrate type on transport properties of superconducting FeSe0.5Te0.5 thin film
德国莱布尼茨研究所IFW 2015.12.10 德国莱布尼茨研究所IFW,D1E13 袁菲菲 Critical current density scaling of FeSe0.5Te0.5 thin films on different substrates
德国德累斯顿工业大学物理学院 2015.11.27 德累斯顿工业大学物理学院207 Kazumasa Iida Hall-plot of the phase diagram for Ba(Fe1?xCox)2As2
德国莱布尼茨研究所IFW 2014.09.15 德国莱布尼茨研究所IFW,D2E27 Firtz Kurth High Magnetic Field Properties of Fe - Pnictide Thin Films
德国卡尔斯鲁厄研究所KIT 2014.06.24 德国卡尔斯鲁厄研究所KIT107 Marco Langer Superconductivity in Ru-substituted BaFe2As2 thin films
     
学术会议
会议名称时间地点本人报告本人报告题目
德国物理学年会DPG 2016.03.14-18 德国雷根斯堡 Influence of substrate type on transport properties of superconducting FeSe0.5Te0.5 thin films
欧洲应用物理会议EUCAS 2015.09.14-17 法国里昂
德国物理学年会DPG 2015.03.09-13 德国柏林
德国莱布尼茨研究所学术交流会议 2015.11.09-12 德国沃利茨 Influence of CaF2 buffer on structural and superconducting properties of FeSe0.5Te0.5 thin films
     
代表作
论文名称
Effects of oxide precursors on superconducting properties of polycrystalline SmFeAsO1-xFx
Influence of substrate type on transport properties of superconducting FeSe0.5Te0.5 thin films
Anisotropy of iron-platinum-arsenide Ca-10(PtnAs8)(Fe2-xPtxAs2)5 single crystals
The influence of the in-plane lattice constant on the superconducting transition temperature of FeSe
 
答辩委员会组成信息
姓名职称导师类别工作单位是否主席备注
闻海虎 正高 长江特聘教授 博导 南京大学
王智河 正高 教授 博导 南京大学
刘楣 正高 教授 博导 东南大学
王继刚 正高 教授 博导 东南大学
徐明祥 正高 教授 博导 东南大学
      
答辩秘书信息
姓名职称工作单位备注
赵海军 其他 讲师 东南大学