【作者】 张奕；

【导师】 谭明秋；

【作者基本信息】 浙江大学 ， 凝聚态物理， 2017， 博士

【摘要】 自超导体被发现以来,就一直是凝聚态物理研究中的一个重要方向。科学家依据各种超导体相变过程、超导温度、磁学性质、相互作用强度等对其进行了各式分类以系统地研究其内在超导原理。2004年科学家们发现了一种重费米子超导体CePt3Si,它的晶体结构没有反演对称中心。随着研究的逐渐深入,人们发现它了更多的奇异性质并将这些性质和其晶体结构特性联系在了一起。许多其他已经被发现的超导体被重新展开研究,新发现的非中心对称超导体也被针对性地研究。除了实验工作归纳了一些比热、相变等方面的特殊现象,各式理论也被提出。密度泛函理论在凝聚态物理中对材料的电子结构计算中一直发挥不可替代的重要作用,由于这类晶格结构会在哈密顿量中引入一种不同于一般自旋轨道耦合效应的被称为“反对称自旋轨道耦合”的相互作用项,第一性原理计算应该也可以描述这种相互作用。本论文正是利用第一性原理计算软件针对三种不同结构的超导体电子结构进行研究,研究内容和主要结论可以分为三部分:1.基于全势局域轨道和线性缀加平面波方法我们计算了新近发现的Laves相超导体Calr2的电子结构和光学性质。费米面附近态密度主要由铱的d轨道的dxy+dz2占据态贡献,在全相对论条件下费米面因能带劈裂发生分裂,费米面的拓扑形状发生了变化;我们利用基于德哈斯-范阿尔芬效应原理的软件扫描了费米面,自主开发了 一段用于从费米面轨道截面的散乱极值数据中自动筛选dHvA信号的C语言程序。作为其后分析一些费米面时的理论依据。光学性质计算结果显示,当入射光强从零开始逐渐提高到1.45eV时,体系光吸收模式由带内吸收转向带间吸收。2.计算了非中心对称超导体Re6Hf的电子结构和光学性质。能带和态密度的结果显示费米能级附近以d-d轨道相互作用为主。Re6Hf的反对称自旋轨道耦合效应比Re6Zr中的更强,能级分裂约40meV。ASOC导致了费米面的拓扑转变,首次发现自旋极化计算结果显示自旋朝上电子的态密度极大值比自旋朝下电子高出不少,且差值费米能级以下1.OeV和0.6eV处达到最大的现象。这可能是两种自旋朝向电子互相耦合、叠加分布的结果,从而维持了总磁距为零;光学性质计算结果显示,等离子频率为1.84eV,光吸收模式转换出现在1.4eV左右。3.计算了结构同样无反演对称中心的两能隙超导体LaNiC2电子结构。耦合效应引起的穿过费米能级的能带劈裂值为62.9meV和27.5meV。我们引入自旋极化下的态密度差值来描述类似Re6Hf中出现的差异,并发现这种差值伴随能量的不同竟然显示出一定的周期性。不考虑耦合效应时体系有两个费米面,且第二费米面极小。独特之处在于LaNiC2费米面的拓扑差别不仅存在于耦合效应影响前后,还出现在自旋极化导致的劈裂的费米面。我们分析了费米面德哈斯-范阿尔芬效应模拟结果,并发现频率极值和能带有一一对应关系,这在以前的相关研究中从未被提及。更多还原

【Abstract】 The topical of superconductor has been a significant subject in condensed matter physics since its discovery decades ago. The scientists classified the materials according to their phase transition types, superconductive temperatures, magnetic properties and interaction strength in order to study the theories systematically. A new heavy-fermion superconductor CePt3Si was discovered in 2004 whose crystalline structure doesn’t contain a space-reversal-invariant center. By the intensive study of it,more unusual physical properties related to its special structure have been exhibited .Many traditional superconductors were restudied and novel ones are being researched in this new aspect. Various theorems are proposed based on the experimental reports on specific heat, phase transition, etc. The density-functional theory has always been an useful method in calculations of electronic structures. The ab-initio calculation process can describe the particular interaction called "antisymmetric spin-orbital coupling" effect which is different from the term in normal Hamiltonian. The main contents and conclusions of this thesis are as follows:1. We calculate the electronic structure and optical properties of newly discovered superconductor CaIr2 based on full-potential local orbital and linear augmented plane waves methods. The densities of states (DOS) near Fermi level are contributed by d-electrons of Ir. The number of Fermi surfaces is four including two hole-pockets and two electron-pockets without including SOC which jump to fourteen in the situation of spin-polarization. The SOC effect changes the energy levels and adds another three pairs of Fermi surfaces in this system, causing a topological transition of Fermi surfaces (TTFS). In this thesis I have preformed a series of de Haas-van Alphen(dHvA) effect simulations on the calculated band structures as guidance for the research on the following materials. The optical properties data shows that the intra-band light absorption turns inter-band type when the energy of incident light reaches 1.45eV.2. The same properties of noncentrosymmetric superconductor (NCS) Re6Hf have been calculated as well. The d-d interaction dominates near Fermi level according to the results of bands and DOS. The electronic structures affected by ASOC indicate that even the magnetic moment of system yields zero, the polarization DOS are asymmetric. The number of Fermi surfaces is five without ASOC which increases to 8 after considering of it, suggesting the ASOC effect only splits two surfaces instead of lifting or declining the bands near Fermi level. The energy splitting due to ASOC has been estimated to be 34 meV and 44 meV,respectively. The plasma frequency is 1.84 eV and the optical absorption transition point locates near 1.4 eV.3. At last the electronic structures of NCS LaNiC2 considered as a two-gap superconductor has been calculated and discussed. The splitting in two bands crossing Fermi level turn out to be 62.9 meV and 27. 5meV. By Introducing the difference of polarization DOS like that in Re6Hf, we find some regular transitions between them following the increase of energy. There exist two Fermi surfaces without ASOC and the second one is particularly small. They turns only four after adding the ASOC effect. Two reasons may be responsible to it: Firstly the relatively simple crystalline structure makes the DOS locate in clearly discrete energy ranges. New surfaces wouldn’t appear even the bands were lifted. Secondly the weight of heavy atoms is not high, resulting in a comparatively weak interaction which may not change the bands to a great extend. We also make the dHvA simulation on it and get some interesting results.更多还原