【作者】 刘娟；

【导师】 姚凯伦；

【作者基本信息】 华中科技大学 ， 凝聚态物理， 2011， 博士

【摘要】 高温超导是指材料在某个相对较高的临界温度Tc>30K(-243.2℃),电阻突降至零的现象。从上世纪60年代到80年代,人们一直认为超导的相变温度不会超过理论预言的30K。直到1986年,IBM的研究者Karl Muller和Johannes Bednorz发现了第一种高温超导材料——镧钡铜氧化物。次年,他们因此获得诺贝尔奖。2008年,日本科学家发现了铁基超导体。物理学界认为这是高温超导研究领域的一个“重大进展”。铜基超导的机制目前尚不明确,需要进一步的讨论和研究,但到目前为止能够达成共识的有几方面：未掺杂的母体材料具有反铁磁基态,超导出现在掺杂以后。导电性主要来自Cu2+离子的dx2-y2轨道,这意味着在铜基超导体中电电相互作用比电声相互作用更重要,它与传统的超导材料不同。对反铁磁基态费米表面的研究发现在布里渊区中四点产生了费米表面的嵌套现象,这几点超导能隙增大,证实自旋密度波的存在。与传统的符合BCS理论的超导材料相比在铜基超导中发现了弱的同位素效应。与铜基超导材料相似,铁基超导材料在低温下也出现反铁磁磁序,并且伴随着四方相到斜立方相的结构相变。但是,低温下它们不是Mott绝缘体而是导电性差的坏金属。其超导相变温度Tc对Fe-As-Fe键角很敏感,实验证明最理想的Tc值出现在FeAs4四方晶格没有被扭曲的环境下。对其波函数的宇称对称性存在争议,主流的观点是扩展的s波图景。通过对铁基超导母体材料进行电子空穴掺杂和利用高压合成技术,其超导相变的临界温度已提高至56K。根据中子衍射研究表明,多数母体材料具有反铁磁性,并且其中有不少具有较为复杂的非线性磁结构。通过化学掺杂或者高压实现的超导往往抑制了材料的磁序,因此,这些材料的磁性与超导关系密切,搞清楚这些超导材料磁性的性质和来源显得尤为重要。本文采用基于全势线性缀加平面波方法的WIEN2k程序包和计算非线性磁螺旋磁结构的WIENNCM程序包计算了铁基超导母体材料的磁结构和电子能带结构。具体研究了铁基超导母体PrFeAsO, CeFeAsO, NdFeAsO的结构、磁性和超导电性。另外,本文还研究了两个混合价铁硫化合物的结构,磁性和电子性质。具体内容如下：1.通过密度泛函理论研究了CeFeAsO的非线性磁基态。铁基超导材料自发现以来引起了广泛的关注,到目前为止国内外很多课题组对其进行了密度泛函理论计算和电子结构的研究,但这些研究都集中关注铁的磁性,对稀土元素的磁性探讨很少。有大量的课题组对LaFeAsO进行电子结构的计算,而对于其它的铁基母体材料CeFeAsO, PrFeAsO, NdFeAsO, SmFeAsO, GdFeAsO等的理论方面的研究极少。实验研究表明在这一系列的“1111”型铁基超导材料中,LaFeAsO1-xFx具有最低的超导相变温度26K,这些材料的超导相变温度随稀土元素离子半径的增大而增加。因此,提供包含稀土元素磁矩母体材料的电子和能带结构以供实验对比是非常有意义的事情。在对CeFeAsO的研究中我们得到Ce的计算磁矩为0.87μB,与实验值0.83(2)μB一致。而在加入自旋轨道耦合作用时Cc的轨道磁矩会随内部磁场的不同而改变方向,所以如果考虑自旋轨道耦合作用,CeFeAsO内Ce的磁矩出现两个不同的值。一个是与实验值很相近的0.909μB，另一个是0.488μB,情况实验上还未曾观察到。为了跟CeFeAsO的研究结果进行对比,我们还计算了PrFeAsO和NdFeAsO的磁矩,Nd和Pr的磁矩都为实验值的两倍。这说明CeFeAsO与PrFeAsO、NdFeAsO的磁性有很大差异。在所计算的这三个材料中,铁的磁矩都大于2.0μB,远高于实验值。Fe磁矩较小的实验观测值被认为是源自费米表而的嵌套和自旋密度波的存在。另外,能带结构和态密度结果表明Ce—4f电子和Fe—3d电子对费米而作主要贡献。在Γ(0,0,0)点有四条能带穿越费米而,它们在费米表面形成四个空穴型口1袋。费米表而的形状会随Ce的自旋方向改变而发生改变,这表明Ce的自旋方向对其导电性有直接影响。特别是,当Ce的自旋方向转到与FeAs平面垂直时,CeFeAsO的电场梯度会从负值变为正值。2.单独对PrFeAsO的基态磁结构作了研究。研究包括总能的计算,结构的优化和态密度分析。实验上,有三个课题小组对PrFeAsO的磁结构进行中子衍射和μ子自旋驰豫研究和对称性分析,分别得出了三个不同的磁结构：i)Kimber小组得出PrFeAsO的磁结构为二维的线性磁结构,其中Pr的磁序与Fe平面的相似ii)戴鹏程等人的小组同样用中子衍射实验得出PrFeAsO的磁结构为一个非线性的三维磁结构,Pr的磁矩方向垂直于FeAs平而。iii)Maeter等人用μ子自旋驰豫研究和对称性分析说明PrFeAsO为一个三维的非线性磁结构,但是他们的研究结果与戴等人的不同,他们认为Fe的磁矩方向与xy平面呈一个夹角,Pr的磁矩方向在xy平而内。我们基于这三种实验结构构建了8种磁结构模型,总能的计算结果和结构优化结果都表明戴鹏程小组测得的磁结构有最低的能量,PrFeAsO基态具有一个非线性的三维磁结构。态密度的结果表明PrFeAsO的导电性受电子的强关联影响。3.采用GGA方法我们研究了混合价化合物Ba2F2Fe1.5S3,分析了态密度、电子能带结构和自旋磁矩。这些计算结果表明通过S离子键桥连接的FeⅡ和FeⅢ离子在基态下具有反铁磁相互作用,这个结论证实了实验上对Ba2F2Fe1.5S3材料存在低维反铁磁作用的假设。化合物的自旋磁矩主要来自FeⅡ和FeⅢ离子,也有少部分来自S离子。能带结构分析显示化合物Ba2F2Fe1.5S3具有半金属性。4.采用GGA和GGA+U方法,我们研究了Ba4Fe2I5S4的磁结构和电性质。我们计算了它的非磁、铁磁和三种反铁磁相。结果表明,在基态下它有链间的铁磁相互作用和链内的反铁磁相互作用。由于链间铁磁相互作用远小于链内反铁磁相互作用,穆斯堡尔谱实验在低温下显示长程反铁磁序。另外,电子结构研究结果表示在基态下Ba4Fe2I5S4具有半金属性。5.用密度泛函理论研究了氰桥MnⅡCrⅢ聚合物晶体,从能量和态密度结果我们发现[Mn(NNdmenH)(H2O)][Cr(CN)6]·H2O和脱水的[Mn(NNdmenH)] [Cr(CN)6]都具有金属性的铁磁基态。实验测得这两种聚合物分别在35.2K和60.4K具有亚铁磁序。能量的计算结果表明它们应该在低于12K时发生了结构相变。这两种聚合物的磁性主要集中在MnⅡ、CrⅢ上,还有部分在氧原子,氮原子和碳原子上更多还原

【Abstract】 High-temperature superconductors (abbreviated high-Tc or HTS) are materials that have a superconducting transition temperature (Tc) above 30 K (-243.2℃). From 1960 to 1980,30 K was thought to be the highest theoretically possible Tc. The first high-Tc superconductor was discovered in 1986 by IBM Researchers Karl Muller and Johannes Bednorz, for which they were awarded the Nobel Prize in Physics in 1987. Fe-based superconductors were discovered in 2008. Physicist believe that it is a "significant progress" in high-temperature superconductivity.Possible mechanisms for superconductivity in the cuprates are still the subject of considerable debate and further research. Certain aspects common to all materials have been identified. Similarities between the antiferromagnetic low-temperature state of the undoped materials and the superconducting state that emerges upon doping, primarily the dx2-y2 orbital state of the Cu2+ions, suggest that electron-electron interactions are more significant than electron-phonon interactions in cuprates-making the superconductivity unconventional. Recent work on the Fermi surface has shown that nesting occurs at four points in the antiferromagnetic Brillouin zone where spin waves exist and that the superconducting energy gap is larger at these points. The weak isotope effects observed for most cuprates contrast with conventional superconductors that are well described by BCS theory. Most undoped iron-based superconductors show a tetragonal-orthorhombic structural phase transition followed at lower temperature by magnetic ordering, similar to the cuprate superconductors. However, they are poor metals rather than Mott insulators. Strong evidence that the Tc value varies with the As-Fe-As bond angles has already emerged and shows that the optimal Tc value is obtained with undistorted FeAs4 tetrahedra. The symmetry of the pairing wavefunction is still widely debated, but an extended s-wave scenario is currently favoured.Doping by electron or hole, the critical temperature of iron-based superconductors has been increased to 55K. The parent compounds show antiferromagnetic orderings in low temperature and more complex noncollinear magnetic structures according to neutron diffraction experiments. The superconducting realized by chemical doping or high-pressure suppress magnetic ordering; therefore, understanding the origin of magnetism of these compounds is key to reveal the mechanisms for superconductivity. In this paper, using WIEN2k package based on full-potential linearized augmented plane wave method and its noncollinear magnetic version WIENNCM package, we calculated the noncollinear magnetic structure and electronic structure of the Fe-based parent superconductors. We report the magnetic structure, magnetism and superconductivity of PrFeAsO, CeFeAsO and NdFeAsO. Moreover, the paper investigates the magnetism and electronic structure of two mixed iron-sulfur compounds. Details as follows:1. The noncollinear magnetic ground state in CeFeAsO has been investigated by density-functional theory. Many groups discussed the electronic structure of LaFeAsO, but a few calculations were performed on the others. One of the reasons is that LaFeAsO has the simplest magetic structure and it is not necessary to consider the La spin. However, the superconducting critical temperature Tc in these compounds increase with the increasement of the rare-earth ionic size and LaFeAsO1-xFx has the lowest transition temperature. Giving the electronic structure of LnFeAsO including the rare-earth spin is significant and essential in this domain. It is found that the magnitude of Ce magnetic moment, which is independent of its spin direction, is 0.87μB, in accord with experimental value 0.83(2)μB. Whereas the spin-orbit coupling is considered, the Ce orbital moments change with the different internal magnetic field and affect its total magnetic moment. One type of Ce ions has the magnetic moment 0.909μB, which is very close to the experimental value 0.83(2)μB. Another type of Ce ions has the magnetic moment 0.488μB, which has not been reported yet in experiments. It is also found that the rare earth magnetic moments in NdFeAsO and PrFeAsO are two times over than experimental observations. The different rare earth errors between theory and experiment imply that magnetism is related to the onset of superconducting critical temperature. However, the calculated Fe magnetic moment is over 2.0μB in all solutions. From the bandstructure and density of states one can find that Ce-4f and Fe-3d orbits are the major contribution to the Fermi level. In CeFeAsO, there are four bands crossing the Fermi level atΓ(0,0,0) point, wherein four hole-like pockets are formed. Furthermore, we find the Fermi surface shape is varies with the Ce spin direction, indicating the electroconductibility affected by the Ce spin direction directly. In particular, if the Ce spin is perpendicular to the FeAs plane, the electronic field gradient will change from the negative value into positive one.2. Noncollinear magnetic investigations of the ground state in PrFeAsO have been performed by density-functional theory. We calculated the total energy and made structure optimization, and the electronic density of states of PrFeAsO was analyzed. There are three different magnetic structures in PrFeAsO defined by experiments. Based on these magnetic structures we studied four collinear and four noncollinear cases. The ground state is found to take the ordering proposed by Zhao, in which FeAs plane is of stripe antiferromagnetism and Pr spins are perpendicular to Fe spins. The electronic density of states indicates that for PrFeAsO the increase of the electron Coulomb interaction leads to the decrease of the conductivity.3. The first principles within the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation GGA approach were applied to study the new mixed valence compound Ba2F2Fe1.5S3. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has an antiferromagnetic interaction between the Felll and Fell ions arising from the bridging S atoms, which validate the experimental assumptions that there is low dimensional antiferromagnetic interaction in Ba2F2Fe1.5S3. The spin magnetic moment mainly comes from the FeⅢand Fell ions with smaller contribution from S anion. By analysis of the band structure, we find that the compound has semiconductor property.4. The first principles within the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) approach were applied to study the new mixed valence compound Ba2F2Fe1.5S3. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has an antiferromagnetic interaction between the FeⅢand Fell ions arising from the bridging S atoms, which validate the experimental assumptions that there is a low-dimensional antiferromagnetic interaction in Ba2F2Fe1.5S3. The spin magnetic moment mainly comes from the FeⅢand Fell ions with smaller contribution from S anion. By analysis of the bandstructure, we find that the compound has half-metallic property.5. First-principles calculations have been performed to study the electronic structure and the magnetic properties for the cyanide-bridged MnⅡCrⅢcoordination polymer crystal. The calculations were based on density-functional theory and the full potential linearized augmented plane wave. From the calculated energy and the density of states, it is found that [Mn(NNdmenH)(H2O)][Cr(CN)6]H2O and dehydrated [Mn(NNdmenH)][Cr(CN)6] have metallic ferromagnetic ground state. Experiment result exhibited these two compounds have ferrimagnetic ordering at 35.2 and 60.4 K, respectively. We believe they take phase transition below 12 K. The spin magnetic moments of these two compounds are mainly assembled at the Mn", CrⅢatom, with a few contributions from the oxygen, nitrogen, carbon atoms.更多还原