压力下122系铁基超导体的第一性原理研究

发布时间：2018-09-19 12:55

【摘要】：超导材料由于其广阔的运用前景和新奇的量子效应一直备受关注,人们希望通过不断的探索来理解超导机制。2008年以来发现的铁基超导体,是继铜基高温超导之后的第二类高温超导体,这一发现为研究超导机理提供了新的视野和思路。现阶段研究表明,通过对超导母体进行掺杂或加压抑制结构相变和磁性相变,就会出现超导相,并且超导电性会出现在靠近反铁磁的边界上,由此可见结构相变和磁性对高温超导电性起到非常重要的作用,研究清楚结构相变和磁性变化对解决高温超导机理具有非常重要的意义。我们的研究重点就是压力和掺杂对122系铁基超导体的晶体结构,磁性和金属性的影响。得到的结论如下：(1)反铁磁结构的AFe2As2(A=Ca、Si、Ba)分别在2.8GPa、8GPa和13GPa处发生结构相变,都由正交相转变为四方相,相变同时Fe的局域磁矩变为零,金属性增加。压力抑制了AFe2As2(A=Ca、Sr、Ba)中电荷在各元素之间的转移,在相变点处,抑制增强。对于AFe2As2(A=Ca、Sr、Ba)来说,Fe的局域磁矩对晶格常数a,b变化比较敏感,电荷转移对品格常数c的变化比较敏感。(2)通过对BaMn2As2和BaMn2P2的加压研究表明,BaMn2As2和BaMn2P2在大约8GPa和7.5GPa处金属化,压力继续增加,分别在59GPa和41GPa附近发生结构相变,都是由四方相转变为“塌缩四方相”,相变同时Mn的磁矩存在突变,金属性增加很快。(3)找到了不同掺杂量的BaFe2-xMnxAs2的稳定掺杂位。分别计算了BaFe2-xMnxAs2的形成能与结合能,说明BaFe2-xMnxAs2很可能在热力学上是稳定的。BaFeMnAs2是具有Ba-FeAs-Ba-MnAs-Ba层状排列特征的金属费米面处的电子主要由Fe的3d电子贡献,Mn的3d电子几乎没有贡献。Fe和Mn的3d电子几乎没有相互作用。(4)比较各种可能的磁结构表明,CaCr2As2和SrCr2As2具有G型的稳定磁结构。计算了XCr2As2(X=Ca、Sr、Ba)的弹性常数,体积弹性模量和剪切模量,且都满足波恩判据,说明他们具有力学稳定性,他们都具备良好的韧性,其大小可以按CaCr2As2, SrCr2As2, BaCr2As2排列。通过压力研究发现,XCr2As2(X=Ca、Sr、Ba)分别在16GPa、 32GPa、24GPa附近发生结构相变,均由常压下的四方相转变为“塌缩四方相”,相变点处Cr局域磁矩均有不同程度突变。
[Abstract]:Superconducting materials have attracted much attention because of their wide application prospects and novel quantum effects, and people hope to understand the superconducting mechanism through constant exploration. Iron based superconductors have been discovered since 2008. It is the second kind of high temperature superconductors after copper base high temperature superconductors. This discovery provides a new vision and train of thought for the study of superconducting mechanism. At present, it is shown that the superconducting phase will appear by doping or pressurizing the superconducting matrix to suppress the structural and magnetic phase transition, and the superconductivity will appear near the boundary of antiferromagnetism. It can be seen that structural phase transition and magnetic properties play a very important role in the superconductivity at high temperature. It is very important to study the structural phase transition and magnetic changes in order to solve the mechanism of high temperature superconductivity. Our research focuses on the effects of pressure and doping on the crystal structure, magnetic properties and gold properties of 122 iron based superconductors. The results obtained are as follows: (1) the antiferromagnetic structure AFe2As2 (AFe2As2) takes place at 2.8GPa8GPa and 13GPa, respectively. The phase transition takes place from the normal phase to the tetragonal phase, and the local magnetic moment of the Fe changes to zero, and the gold properties increase. The pressure inhibits the transfer of the charge between the elements in AFe2As2 (Aca-CaOS-Sr-Ba), and at the transition point, it inhibits the enhancement of the charge. For AFe2As2 (AFe2As2), the local magnetic moment of Fe is more sensitive to the change of lattice constant Ab, and the charge transfer is more sensitive to the change of character constant c. (2) the pressure of BaMn2As2 and BaMn2P2 shows that BaMn2As2 and BaMn2P2 are metallized at about 8GPa and 7.5GPa, and the pressure continues to increase. The structural phase transitions occurred in the vicinity of 59GPa and 41GPa, respectively, from tetragonal phase to collapsed tetragonal phase. At the same time, the magnetic moment of Mn changed and the gold properties increased rapidly. (3) the stable doping sites of BaFe2-xMnxAs2 with different doping amounts were found. The formation and binding energies of BaFe2-xMnxAs2 were calculated, respectively. It shows that BaFe2-xMnxAs2 is probably thermodynamically stable. BaFeMnAs2 is the electron at the Fermi surface of metal with the characteristic of Ba-FeAs-Ba-MnAs-Ba layered arrangement. It is mainly contributed by the 3D electron of Fe. The 3D electron of mn has almost no contribution. Fe and the 3D electron of Mn have almost no mutual contribution. (4) the comparison of various possible magnetic structures shows that CaCr2As2 and SrCr2As2 have G-type stable magnetic structures. The elastic constants, bulk modulus of elasticity and shear modulus of XCr2As2 are calculated, and all of them satisfy the Bonn criterion. It shows that they have mechanical stability, they all have good toughness, and their size can be arranged according to CaCr2As2, SrCr2As2, BaCr2As2. It is found that the structural phase transition occurs in the vicinity of 16GPA and 32GPa24Ga, respectively, from the tetragonal phase to the "collapsed tetragonal phase" at normal pressure, and the local magnetic moments at the phase transition point are all mutated to different degrees.
【学位授予单位】：华南理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TM26

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