芳香烃化合物的结构与超导等物性的理论研究

发布时间：2018-04-15 12:51

  本文选题：有机超导体 + 第一性原理　； 参考：《中国科学技术大学》2017年博士论文

【摘要】：金属掺杂的稠环芳香烃有机超导体是最近这几年超导领域最重要的发现之一,以实现室温超导为目的,研究人员们致力于此类新型超导材料和超导现象的探索和观察。有机超导体具有很轻的重量及低维特性,一方面使其成为潜在的高温超导体应用材料,如芳香烃材料被广泛地应用于制造有机发光二级管(OLED),另一方面也使得有机超导体成为人们研究低维体系的电子-电子相互作用和电子-声子相互作用的独特材料。但目前有机超导体的研究仍然存在一些问题,包括掺杂结构及原子位置难以确定、对超导机理的认识还不够清晰、许多材料的研究仍不够深入等。针对这些存在的科学问题,本论文采用第一性原理计算的方法系统深入地研究了芳香烃超导体的晶体结构、掺杂原子位置、电子性质、磁性等物性,以及电子关联效应与超导机理间的关系。本论文的研究内容主要分为以下几个方面:1、K掺杂的四环体系的晶格结构、热力学稳定性、电子性质和超导预测。通过考虑不同的空间群对称性和掺杂浓度,发现最稳定的掺杂结构是P2_1对称性的K_2掺杂的结构,并给出了掺杂的K原子位置;通过计算给出了不同掺杂浓度下的电子能带结构,发现尽管K_2掺杂是半导体,但是给予少量微扰电荷将会使体系发生绝缘体-金属转变,并且分析晶体的热力学稳定性后发现K_2掺杂带微扰电荷后的体系是稳定的;通过差分电荷密度的分析证实了体系内K原子转移电子到有机分子上的超导机制;估算了带微扰电荷后K_2掺杂体系的超导转变温度。2、Sm掺杂的四环体系的晶格结构、电子性质和磁性研究。针对不同的空间群和初始磁组态,确立了掺杂Sm原子后的晶格结构和Sm原子位置,发现Sm原子掺杂四环体系最稳定的结构是C2/c对称性下的反铁磁结构;分析了电子结构随Hubbard U参数的变化;计算了 GGA和GGA+U两种方法下的态密度后发现,GGA泛函下,Sm的4f电子在费米能级处有一个非常尖锐的DOS峰,GGA+U泛函下,Sm的4f电子的DOS被推开,费米能级上的电子态主要由C的2p电子贡献;Sm掺杂时体系表现为反铁磁金属性,Sm原子自旋磁矩为5.9 μB,轨道磁矩为-1.6μB,总磁矩为4.3μB,C原子上的极化磁矩为0.1μB;Sm在这类材料中是+2价的,两个电子转移到有机分子上;同时也证实在此类体系中电子关联效应不可忽略。3、K掺杂的对三联苯体系的晶体结构、电子性质、掺杂导致的成键情况演化和对超导现象的推测。经过计算确定了 K掺杂的对三联苯体系最稳定的结构是P2_1对称性下的K_2C_(18)H_(14),K原子掺杂在了相应的对三联苯分子的C-C单键相连的桥键处;通过电子性质的计算发现三种掺杂浓度下都有两条能带穿过费米面而呈现金属态,其中最稳定的K_2C_(18)H_(14)的费米面处的态密度值为每分子1.28states.eV-1;由差分电荷密度和电荷局域密度的分析证实K原子转移电子到有机分子上;根据计算推测K_2C_(18)H_(14)对应于最近实验上发现的7.2K的超导相。4、K掺杂三环体系的表面生长构型下的掺杂难度、掺杂结构、电子性质及电荷转移。对于K掺杂的三环分子体系,建立了表面吸附模型,从晶体生长的角度分析了各种掺杂浓度下最可能产生的晶格结构和掺杂位置,发现K吸附在表面的情况是总能最高、最不稳定的结构,推测K原子是可以掺杂进体块内的,而不是吸附在表面;给出了各掺杂浓度下的态密度分布及与块体材料态密度的对比,发现表面生长情况下的K原子掺杂三环分子体系也可以形成金属态;发现K原子转移电子到有机分子上,表面吸附的情况下也可能成为超导体。芳香烃化合物体系自发现超导现象以来就引起了人们极大的兴趣和广泛的关注,本论文对芳香烃化合物超导体的结构、磁性、电子性质、晶体生长以及电子关联效应等超导机制方面给出了清晰的物理图像,对以后超导现象在宏观和微观表象的研究都具有重要的指导作用,有助于人们寻找高温超导甚至室温超导的探索,也对有机超导体的制备和应用有指引意义。
[Abstract]:Metal doped polycyclic aromatic hydrocarbon organic superconductors is in recent years the superconducting field one of the most important discoveries, in order to achieve the purpose of room temperature superconductivity, researchers are committed to this new type of superconducting material and the phenomenon of the exploration and observation of organic superconductors. It has light weight and low dimensional characteristics, on the one hand their the potential application of high temperature superconductors materials, such as aromatic hydrocarbon material is widely used in the manufacture of organic light emitting diode (two OLED), on the other hand also makes organic superconductor materials become a unique low dimensional system of electron electron interactions and electron phonon interactions. But the research of organic superconductors there are still some problems, including doping structure and atomic positions are difficult to determine, understanding of the superconducting mechanism is still not clear enough, many research materials is still not deep enough. Aiming at the existing scientific knowledge The crystal structure of this paper, in-depth study of the aromatic hydrocarbon superconductor system using the first principle calculation method, the doping position, electronic properties, magnetic properties, and the relationship with the electron correlation effect between the superconducting mechanism. The main contents of this thesis are as follows: 1. The thermodynamic stability of lattice the structure, four K doped system, electronic properties and superconducting prediction. By considering different space group symmetry and doping concentration, doping found that the most stable structure is the structure symmetry of P2_1 doped K_2, and gives the K atoms doped; calculate the electronic band under different doping concentration the structure, found that although the K_2 doped semiconductor, but given a small amount of perturbation charge will make the system have the insulator metal transition and thermodynamic analysis of stability of the crystals found in K_2 doped with perturbation power After the charge system is stable; through the analyzing of the differential charge density confirmed system K atom transfer electrons to the superconducting mechanism of organic molecules; estimate the superconducting transition with perturbation charge after doping K_2 temperature.2, the lattice structure of Sm doped ring system, electronic and magnetic properties for research. The space group and initial magnetic configuration different, established after the doping of Sm atom lattice structure and Sm atoms and Sm atoms doped ring structure found that the most stable system is antiferromagnetic structure C2/c symmetry; analyzes the changes in the electronic structure of Hubbard with the U parameter; density of states of GGA and GGA+U two methods after calculation, GGA functional, 4f Sm has a very sharp DOS peak at the Fermi level, GGA+U functional, Sm 4f electronic DOS is open, the electronic states on the Fermi level is mainly composed of 2p electronic contribution of C; Sm doping 浣撶郴琛ㄧ幇涓哄弽閾佺�侀噾灞炴�，

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