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基于密度泛函理论的第一性原理计算, 研究了二维应变作用下LiFeAs超导薄膜的磁性结构、电子能带和态密度变化, 分析了应变对其超导电性的作用. 结果显示, 对体系施加1%—6%的二维平面张、压应变均不改变其基态条形反铁磁性结构, 费米面附近的电子态密度主要来自于Fe-3d轨道电子以及少量的As-4p电子. 研究发现, 与无应变情形相比, 当施加压应变时, 体系中Fe离子的反平行的电子自旋局域磁矩减小, 薄膜反铁磁性受到抑制, 费米面上电子态密度增加, 超导电性来自于以反铁磁超交换耦合作用为媒介的空穴型费米面和电子型费米面间嵌套的Cooper电子对. 而在张应变作用时, 局域反铁磁性增强, 费米面上电子态密度减小, 金属性减弱, 特别是张应变时费米面上空穴型能带消失, Cooper电子对出现概率显著降低, 将抑制超导相变.The magnetism, band properties and electronic density of states of LiFeAs superconducting thin film with two-dimensional strain are investigated by using the first principles calculations based on density functional theory, and the influences of different strains on the characteristics of superconducting films are analyzed in detail. The results show that the magnetic ground configuration is the striped antiferromagnetic state of nostrained LiFeAs thin film, and the ground structure of this system is unchanged in the range of applied 1%−6% compressive and tensile strain. The density of states near the Fermi level is mainly from the contribution of Fe-3d orbital and a few As-4p electrons. The electron spin exchange coupling between Fe ions is realized by As ions. Furthermore, unlike the case of the nostrain and the tensile strain, with increasing the compressive strain, the localized antiparallel electron spin magnetic moments of Fe ion decrease, the density of states at the Fermi surface improves, and the itinerant electron magnetism of Fe ions increases, which all greatly suppress the antiferromagnetic properties of thin film and enhance the superconducting phase transition temperature. The superconductivity of LiFeAs thin film originates from the Cooper pairs of electrons between the hole-type and electronic-type bands near the Fermi surface through the antiferromagnetic superexchange coupling effect. Instead, the LiFeAs thin film with the tensile strain presents completely opposite properties, that is to say, the decrease of the electronic density of states in the Fermi level brings about the weakening of the metal properties and the increasing of the antiferromagnetic exchange coupling. Particularly, the band structure of hole-type near the Fermi surface disappears, and the occurrence of Cooper pairs of electrons becomes significantly reduced, resulting in the suppressed superconducting phase transition when the LiFeAs thin film is subjected to tensile strain. In addition, the change of antiferromagnetic exchange coupling and magnetic moments of Fe ions are also explained according to the variation of electronic density of states of the Fe-3d energy levels during the distortion of FeAs tetrahedrons due to compressive strain. In brief, our researches provide an effective way to improve the superconducting properties of LiFeAs thin film and may promote the relevant practical applications of iron-based superconductors in the future.
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Keywords:
- superconducting thin film /
- strain /
- magnetism /
- electronic structure
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图 2 LFA薄膜中Fe离子可能的四种磁性结构 (a) 条形铁磁; (b)条形反铁磁; (c) 棋盘形铁磁; (d) 棋盘形反铁磁; 箭头表示自旋方向
Fig. 2. Four possible kinds of magnetic structures of Fe ion in LFA thin films: (a) Striped-type ferromagnetic order; (b) striped-type antiferromagnetic order; (c) checkerboard-type ferromagnetic order; (d) checkerboard-type antiferromagnetic order. The arrows represent the directions of electronic spins.
图 3 LiFeAs薄膜中四种磁性结构的相对能量随晶格常数的变化
Fig. 3. Relative energies of different magnetic states varying with lattice constant of LFA thin film.
图 4 LFA超导薄膜AFM-2a磁性结构在不同应变条件下的能带结构 (a)无应变; (b) 压应变(−3%); (c) 张应变(3%)
Fig. 4. The band structure of LFA superconductor thin film (AFM-2a magnetic states) under different strains: (a) Nostrained effect; (b) compressive strain (−3%); (c) tensile strain (3%).
图 5 LFA超导薄膜在不同应变条件时各离子的电子态密度 (a) 无应变; (b) 压应变(−3%); (c) 张应变(3%)
Fig. 5. The density of electronic states of different ions in LFA superconductor thin film under different strains: (a) Nostrained effect; (b) compressive strain (−3%); (c) tensile strain (3%).
图 6 以Fe离子为中心的As四面体结构畸变与3d轨道能级分裂 (a) 无应变时扁平的四面体; (b) 压应变作用下畸变后的正四面体
Fig. 6. The tetrahedral structure distortion of As centered on Fe ions and 3d orbital energy splitting of Fe ions: (a) Tabular tetrahedron without strain effect; (b) the distorted regular tetrahedron under compressive strain effect.
表 1 无应变时铁基超导薄膜LFA不同磁性结构下的单胞能量
Table 1. Energy of unit cell of the iron-based superconductor thin film LFA in different magnetic structures.
磁性结构 条形铁磁 条形反铁磁 棋盘形铁磁 棋盘形反铁磁 单胞能量/eV −131.2446 −131.2508 −130.8171 −130.8156 
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表 2 不同应变作用下LFA薄膜中Fe离子的自旋磁矩
Table 2. Magnetic moments of Fe ions of LFA thin films under different strains.
应变 Fe 离子总磁矩
${m_{\rm{t}}}$/${\mu _{\rm{B}}}$巡游磁矩
${m_{\rm{i}}}$/${\mu _{\rm{B}}}$局域磁矩
${m_{\rm{l}}}$/${\mu _{\rm{B}}}$−3% 1.615 0.291 1.324 0% 1.532 0.187 1.345 3% 1.414 0.026 1.388
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[1] Nomura T, Kim S W, Kamihara Y, Hirano M, Sushko P V, Kato K, Takata M, Shluger A L, Hosono H 2008 Supercond. Sci. Technol. 21 125028
Google Scholar
[2] Dai P C 2015 Rev. Mod. Phys. 87 855
Google Scholar
[3] 杜增义, 方德龙, 王震宇, 杜冠, 杨雄, 杨欢, 顾根大, 闻海虎 2015 64 097401
Google Scholar
Du Z Y, Fang D L, Wang Z Y, Du G, Yang X, Yang H, Gu G D, Wen H H 2015 Acta Phys. Sin. 64 097401
Google Scholar
[4] Dubroka A, Kim K W, Rossle M, Malik V K, Drew A J, Liu R H, Wu G, Chen X H, Bernhard C 2008 Phys. Rev. Lett. 101 097011
Google Scholar
[5] Ma L, Zhang J, Chen G F, Yu W Q 2010 Phys. Rev. B 82 180501
Google Scholar
[6] Qureshi N, Steffens P, Drees Y, Komarek A C, Lamago D, Sidis Y, Harnagea L, Grafe H J, Wurmehl S, Buchner B, Braden M 2012 Phys. Rev. Lett. 108 117001
Google Scholar
[7] Wang M, Wang M Y, Miao H, Carr S V, Abernathy D L, Stone M B, Wang X C, Xing L Y, Jin C Q, Zhang X T, Hu J P, Xiang T, Ding H, Dai P C 2012 Phys. Rev. B 86 144511
Google Scholar
[8] Umezawa K, Li Y, Miao H, Nakayama K, Liu Z H, Richard P, Sato T, He J B, Wang D M, Chen G F, Ding H, Takahashi T, Wang S C 2012 Phys. Rev. Lett. 108 037002
Google Scholar
[9] Qureshi N, Steffens P, Lamago D, Sidis Y, Sobolev O, Ewings R A, Harnagea L, Wurmehl S, Buchner B, Braden M 2014 Phys. Rev. B 90 144503
Google Scholar
[10] Zhang S J, Wang X C, Sammynaiken R, Tse J S,Yang L X, Li Z, Liu Q Q, Desgreniers S, Yao Y, Liu H Z, Jin C Q 2009 Phys. Rev. B 80 014506
Google Scholar
[11] Zeng B, Watanabe D, Zhang Q R, Li G, Besara T, Siegrist T, Xing L Y, Wang X C, Jin C Q, Goswami P, Johannes M D, Balicas L 2013 Phys. Rev. B 88 144518
Google Scholar
[12] 靳常青, 刘青清, 邓正, 张思佳, 邢令义, 朱金龙, 孔盼盼, 望贤成 2013 高压 27 473
Google Scholar
Jin C Q, Liu Q Q, Deng Z, Zhang S J, Xing L Y, Zhu J L, Kong P P, Wang X C 2013 Chinese Journal of High Pressure Physics 27 473
Google Scholar
[13] Li Y, Yin Z P, Wang X C, Tam D W, Abernathy D L, Podlesnyak A, Zhang C L, Wang M, Xing L Y, Jin C Q, Haule K, Kotliar G, Maier T A, Dai P C 2016 Phys. Rev. Lett. 116 247001
Google Scholar
[14] Miao H, Qian T, Shi X, Richard P, Kim T K, Hoesch M, Xing L Y, Wang X C, Jin C Q, Hu J P, Ding H 2015 Nat. Commun. 6 6056
Google Scholar
[15] Pitcher M J, Parker D R, Adamson P, Herkelrath S J C, Boothroyd A T, Ibberson R M, Brunell M, Clarke S J 2008 Chem. Commun. 45 5918
[16] 李世超, 甘远, 王靖辉, 冉柯静, 温锦生 2015 64 097503
Google Scholar
Li S C, Gan Y, Wang J H, Ran K J, Wen J S 2015 Acta Phys. Sin. 64 097503
Google Scholar
[17] Tapp J H, Tang Z J, Lv B, Sasmal K, Lorenz B, Chu P C W, Guloy A M 2008 Phys. Rev. B 78 060505
Google Scholar
[18] Kawasaki S, Mabuchi T, Maeda S, Adachi T, Mizukami T, Kudo K, Nohara M, Zheng G Q 2015 Phys. Rev. B 92 180508
Google Scholar
[19] Wang H D, Dong C H, Li Z J, Mao Q H, Zhu S S, Feng C M, Yuan H Q, Fang M H 2011 Europhys. Lett. 93 47004
Google Scholar
[20] Tafti F F, Ouellet A, Juneau-Fecteau A, Faucher S, Lapointe-Major M, Doiron-Leyraud N, Wang A F, Luo X G, Chen X H, Taillefer L 2015 Phys. Rev. B 91 054511
Google Scholar
[21] Krüger E, Strunk H P 2014 J. Supercond. Nov. Magn. 27 601
Google Scholar
[22] Mollah S 2004 J. Phys.: Condens. Matter 16 R1237
Google Scholar
[23] 张加宏, 马荣, 刘甦, 刘楣 2006 55 4816
Google Scholar
Zhang J H, Ma R, Liu S, Liu M 2006 Acta Phys. Sin. 55 4816
Google Scholar
[24] 俞榕 2015 64 217102
Google Scholar
Yu R 2015 Acta Phys. Sin. 64 217102
Google Scholar
[25] Chen Z J, Xu G B, Yan J G, Kuang Z, Chen T H, Li D H 2016 J. Appl. Phys. 120 235103
Google Scholar
[26] Yu R, Zhu J X, Si Q M 2011 Phys. Rev. Lett. 106 186401
Google Scholar
[27] 衣玮, 吴奇, 孙力玲 2017 66 037402
Google Scholar
Yi W, Wu Q, Sun L L 2017 Acta Phys. Sin. 66 037402
Google Scholar
[28] Lankau A, Koepernik K, Borisenko S, Zabolotnyy V, Büchner B, Brink J V D, Eschrig H 2010 Phys. Rev. B 82 184518
Google Scholar
[29] Li B, Xing Z W, Liu M 2011 Acta Phys. Sin. 60 077402 (in Chinese)
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