新型超导材料MgCNi3的电子结构与超导电性研究

用MS-Xα方法研究了非氧化物超导材料MgCNi3的电子结构.研究结果显示,态密度分布曲线的主峰靠近Fermi面,主要来自于Ni的d电子的贡献.用T(T=Co,Mn,Cu)替代MgCNi3中的部分Ni形成化合物MgCNi2T,替代使Ni的价电子数减小,价态发生变化,Fermi面处态密度N(EF)减小.计算结果表明:无论是电子掺杂(Cu)还是空穴掺杂(Co,Mn),MgCNi3的超导电性都被抑制;Mn掺杂比Co掺杂更快地抑制超导电性,Co是作为空穴掺杂而不是作为磁性杂质掺杂去抑制超导.     

掺杂MgCNi3超导电性和磁性的第一性原理研究

从第一性原理出发，计算了MgCNi₃的电子能带结构.MgCNi₃中C 2p与Ni 3d轨道杂化使穿梭费米面上的Ni 3d能带表现出平面性，费米面落在态密度范霍夫奇异(vHs)峰的右坡上.vHs峰上大的电子态密度和铁磁相变点附近的自旋涨落是决定MgCNi₃超导电性的重要因素.研究了三种替代式掺杂对其超导电性和磁性的影响，发现电子掺杂使费米能级下滑到态密度较低的位置，导致体系转变为无超导电性的顺磁相；同构等价电子数的金属间化合物的轨道杂化，引起费米面上态密度的减少，降低了超导电性；而空穴掺杂使费米面向vHs峰值方向移动，虽然费米面上电子态密度增大可能提高超导电性，但增强了的Ni原子磁交换作用产生铁磁序，破坏了超导电性. 关键词： 电子结构 超导电性 磁性 掺杂     

Mg12O12纳米线掺杂3d过渡金属元素的第一性原理计算研究

基于密度泛函第一性原理计算,系统研究了Mg₁₂O₁₂笼状团簇组装一维纳米线及其掺杂3d族元素体系的几何结构与电子结构.结果表明：Mg₁₂O₁₂团簇组装一维纳米线为非磁性半导体,带隙值为3.16 eV;掺杂Sc和V后,体系由半导体转变为金属;掺杂Ti、Cr、Mn、Fe、Co、Ni、Cu后体系仍然保持半导体特性、但带隙值明显减小,而掺杂Zn时带隙值变化不大;掺杂V、Cr、Mn、Fe、Co、Ni、Cu后纳米线具有磁性.     

新超导体MgCNi3的虚晶掺杂研究

用全势线性缀加平面波方法,考虑局域自旋密度近似研究虚晶掺杂MgCNi₃的超导电性和磁性.计算了自旋极化能带结构、体弹性模量和它对压力的导数、原子磁矩m及其变化率.计算结果表明，对于电子掺杂的Mg_1-xAl_xCNi₃(0≤x≤0.5)，超导电性和磁涨落随掺杂量的增加逐渐减小.空穴掺杂的Mg_1-xNa_xCNi₃,在x＝0.12处出现铁磁相变，超导电性消失.在MgCNi₃少量空穴掺杂区域(0≤x<0.12)，表现为超导与磁涨落共存的不稳定状态. 关键词： 超导电性 能带结构 态密度 磁性     

3d过渡金属元素掺杂Al12N12纳米线几何结构和电子性能的第一性原理研究

基于密度泛函理论的第一性原理计算方法,系统地研究了不同3d过渡金属元素(Sc、Ti、V、Cr、Mn、Fe、Co和Ni)掺杂Al₁₂N₁₂纳米线的几何结构、稳定性和电子结构.结果表明：所有掺杂体系均是热力学稳定的;掺杂Ni时体系保留了原有的非磁性间接带隙半导体特性;当掺杂其它原子(Sc、Ti、V、Cr、Mn、Fe、Co)时体系仍然保持为半导体,但带隙明显减小.掺杂过渡金属原子对于Al₁₂N₁₂纳米线的电子结构具有明显的调控作用,在能带调控和光电方面有潜在的应用前景.     

F掺杂SnO2电子结构的模拟计算

采用基于密度泛函理论的平面波赝势方法对SnO₂:F体系的电子结构进行了第一性原理模拟计算.用广义梯度近似方法优化SnO₂:F体系的晶胞结构，计算了体系基态总能.通过确定F掺杂对O的优先替代位置，计算了SnO₂:F的能带结构、态密度、分波态密度.分析了F掺杂对SnO₂晶体的电子结构和晶体性质及光学吸收边的影响，从理论上得出光学吸收边发生蓝移.对不同掺杂量的体系电子结构进行了分析. 关键词： F掺杂 2')" href="#">SnO₂ 电子结构 态密度     

YBa2(Cu0.95M0.05)3O7-δ的代换效应及其中子衍射研究

X射线衍射实验表明YBa₂(Cu_0.95M_0.05)₃O_7-δ(M=Ti,V,Cr,Mn,Fe,Co,Ni,Cu和Zn)均为单相结构。Fe,Co,Ni和Zn对Cu的替代使超导临界温度T_c显著下降,而同样含量的Ti,V,Cr,Mn对Cu的替代并未对超导性能产生显著影响。并利用中子衍射分析了Ti,Mn,Fe和Co对Cu原子的取代,发现代换原子对Cu的两个晶位各自存在不同的择优占据 关键词：     

Mg,Al掺杂对LiCoO2体系电子结构影响的第一原理研究

为了研究Mg, Al掺杂对锂二次电池正极材料LiCoO₂体系的电子结构的影响，进而揭示Mg掺杂的LiCoO₂具有高电导率的机理，对Li(Co, Al)O₂和Li(Co, Mg)O₂进行了基于密度泛函理论的第一原理研究. 通过对能带及态密度的分析，发现在Mg掺杂后价带出现电子态空穴，提高了电导，并且通过歧化效应（disproportionation）改变了Co-3d电子在各能级的分布，而Al掺杂则没有这些作用. O关键词： 2')" href="#">LiCoO₂ 电子结构 第一原理 电导     

Co掺杂MgF2电子结构和光学特性的第一性原理研究

基于密度泛函理论（DFT）的第一性原理平面波超软赝势方法,计算了Co掺杂MgF₂晶体的几何结构、电子结构和光学性质.结果表明,Co掺杂导致MgF₂晶体结构畸变,可能发生一种类四方和斜方型结构相变.由于Co原子的加入,体系的禁带宽度减小,可观察到半导体—金属性转变.计算也表明,Co掺杂对静态介电常数和光吸收系数有重要调制作用,所得结果与最近实验测量很好相符,揭示了Co:MgF₂体系在光学元器件方面的潜在应用. 关键词： 密度泛函理论（DFT） 第一性原理 超软赝势 2')" href="#">Co掺杂MgF₂     

Mn掺杂(ZnSe)12团簇物性研究

采用密度泛函理论研究Mn原子单掺杂和双掺杂（ZnSe）₁₂团簇的结构、电子性质和磁性质.考虑三种掺杂方式：替代掺杂,外掺杂和内掺杂.比较掺杂团簇的稳定性.结果表明：无论是单掺杂还是双掺杂,替代掺杂团簇是最稳定结构.在结构优化的基础上对掺杂团簇进行磁性计算.团簇磁矩主要来自Mn原子3d态的贡献,4s和4p态贡献了一小部分磁矩.由于轨道杂化,相邻的Zn和Se原子上产生少量自旋.研究发现：内双掺杂团簇是铁磁耦合,在纳米量子器件领域有潜在的应用价值.     

YBa2Cu1-xCox(Cu1-yZny)2Oz体系的Co,Zn共同掺杂效应

本文报道通过对YBa₂Cu_1-xCo_x(Cu_1-yZn_y)₂O_z(0≤x,y≤0.1)体系晶体结构、氧含量、正常态电阻-温度关系、Hall效应以及超导临界温度等的综合测量,发现随着Co和Zn含量的增加,体系经历了从正交结构的超导金属向四方结构的非超导半导体的转变,超导临界温度T_c和载流子浓度n_h均迅速下降,Co 关键词：     

Electronic structures of doped BaFe<Subscript>2</Subscript>As<Subscript>2</Subscript> materials: virtual crystal approximation versus super-cell approach

Using virtual crystal approximation and super-cell methods for doping, a detailedcomparative study of electronic structures of various doped BaFe₂As₂ materials by first principlessimulations is presented. Electronic structures remain unaltered for both the methods incase of passive site doping but in case of active site doping, the electronic structurefor virtual crystal approximation method differ from that of the super-cell methodspecially in the higher doping concentrations. For example, both of these methods giverise to a similar density of states and band structures in case of hole doping (replacingK in place of Ba) and isovalent P doping on As site. But in case of electron doped (Co inplace of Fe) systems with higher doping concentration, electronic structures calculatedusing virtual crystal approximation approach deviates from that of the super-cell method.On the other hand, in case of low isovalent Ru doping at the Fe site implemented byvirtual crystal approximation, one acquires an extra shift in the chemical potential incomparison to that for the super-cell method. This shift may be utilized to predict thecorrect electronic structure as well as the calculated Fermi surfaces within virtualcrystal approximation. But for higher Ru (that has different electronic configuration thanFe) doping concentration, simple shifting of chemical potential fails, the calculatedelectronic structure via virtual crystal approximation approach is very different fromthat by the super-cell formalism.     

磁性离子Fe和Ni替代YBCO体系的结构特征和载流子局域化

为阐明磁性离子在不同替代位置对YBCO体系超导电性的影响机制，利用正电子湮没及相关实验手段结合数值模拟，系统研究了Fe和Ni掺杂的YBa₂Cu₃O_7-δ体系. 结果表明，Fe和Ni离子在替代过程中均以离子团簇的形式进入晶格. 当离子进入CuO₂面时，由于团簇改变了周围的电子结构，造成电子的局域化，并直接影响了电子对的配对和输运，因而强烈抑制了体系的超导电性.而当掺杂离子进入Cu-O链区时，它们同样通过团簇的形式改变周围 关键词： YBCO超导体 磁性离子替代 正电子湮没 数值模拟     

Synthesis of superconductor MgCNi3 with carbon nanotubes

MgCNi3, an intermetallic compound superconductor with a cubic perovskite crystal structure, has been synthesized using fine Mg and Ni powders and carbon nanotubes （CNTs） as starting materials by the conventional powder metallurgy method. The composition, microstructure and superconductivity are characterized using x-ray diffraction （XRD）, energy dispersive x-ray （EDX） analysis, scanning electron microscopy （SEM）, and superconducting quantum interference device （SQUID） magnetometer. The results indicate that the phases of the synthesized samples are MgCNi3 （major phase） and traces of C and MgO. The MgCNi3 particle sizes range from several hundreds of nanometres to several micrometres. The onset superconducting transition temperature Tc of the MgCNi3 sample is about 7.2 K. The critical current density Jc is about 3.44 × 10^4 A/cm^2 calculated according to the Bean model from the magnetization hysteresis loop of the slab MgCNi3 sample at 5 K and zero applied field.     

Study of the influence of hole concentration on high-Tc superconductivity of cuprates Bi-2212 by X-ray photoelectron spectroscopy

In this paper, we report the observation of X-ray photoelectron spectra (XPS), which were obtained from high-T_c superconducting compounds Bi-2212 with some percentage of 3d elements (Fe, Co, and Ni) substituted for copper. The changes in characteristics of XPS spectra Sr 3d and Cu 2p_3/2 with the doping content were explained in the relation to the hole concentration on CuO₂ planes, which resulted in high-T_c superconductivity.     

Magnetic and electronic structure in new iron-based layered SrFe1 − xCoxAsF systems

By using the first-principles calculation, we study the structural, magnetic, and electronic properties of the SrFeAsF compound and its Co-doped counterpart SrFe_1 − xCo_xAsF (x=0.125). It is shown that the competition of the nearest and next-nearest neighbor exchange coupling J₁ and J₂ between Fe ions gives rise to a frustrated striped antiferromagnetic order in SrFeAsF and an accompanied lattice distortion, while for the Co-doped case, both J₁ and J₂ decrease significantly as well as the lattice distortion, and thus the antiferromagnetic order is suppressed greatly. This is further confirmed by the electronic structure calculation that the density of states at the Fermi level increases with Co doping as well as the itinerancy of Fe d electron.     

The spin-dependent transport properties of endohedral transition-metal-fullerene X@C66H4 (X=Fe,Co, Mn,Ni)

Inspired by recent experiments on the successful synthesis of hydrofullerene C₆₆H₄ in Tian et al. (2019) [12] with two negatively curved heptagons. Based on the density functional theory and nonequilibrium Green's function method, we report the spin-dependent transport through transition-metal-atom-encapsulated C₆₆H₄ hydrofullerene, i.e., X@C₆₆H₄(X=Fe, Co, Mn, Ni), contacted by single gold atoms via semi-infinite non-magnetic Au electrodes. It is found that, Mn- and Fe-doped systems show highly spin-polarized transmission as well as considerable magnetic moments whereas Ni-doped systems show completely spin-unpolarized transmission and nonmagnetic. Interestingly, Co-doped systems show two spin states, i.e., spin-polarized and spin-unpolarized ones. Further analysis shows that, for Mn-, Fe- and Ni-doped systems, the spin-polarized/unpolarized state is caused by the finite/(nearly-)zero magnetism of the encapsulated metal atom. While the magnetism in Co-doped systems is quenched for the top hexagonal doping case, but not for the side heptagonal doping one, which induces the spin-unpolarized/spin-polarized state. And the screening effect of electrodes on the magnetism of Co is the underlying physical mechanism. Our findings would be beneficial to the design of spintronics devices.     

Band structure and properties of superconducting MgB<Subscript>2</Subscript> and related compounds (A review)

This paper presents an overview of the current state of the art in research into the electronic structure and properties of a new superconductor, namely, MgB₂, and a large number of related compounds by computational methods of the band theory. Consideration is given to the specific features of the surface states of magnesium diboride, the electron and hole doping effects in this compound, and the concentration dependences of the band structure and the properties of Mg_1?xMe_xB₂ and MgB_2?yX_y solid solutions and a number of superstructures. The electronic properties of AlB₂-like phases, boron, higher borides, a series of ternary layered boron-containing phases, and compounds with structures of the antiperovskite type (MgCNi₃ and others) are discussed in terms of their superconducting characteristics. The results obtained in modeling nanotubes and fullerene-like nanoparticles based on MgB₂ and related borides are analyzed.     

Renormalization group analysis of competing orders and the pairing symmetry in Fe-based superconductors

We analyze antiferromagnetism and superconductivity in novel Fe-based superconductors within the weak-coupling, itinerant model of electron and hole pockets near (0, 0) and (π, π) in the folded Brillouin zone. We discuss the interaction Hamiltonian, the nesting, the RG flow of the couplings at energies above and below the Fermi energy, and the interplay between SDW magnetism, superconductivity and charge orbital order. We argue that SDW antiferromagnetism wins at zero doping but looses to superconductivity upon doping. We show that the most likely symmetry of the superconducting gap is A_1g in the folded zone. This gap has no nodes on the Fermi surface but changes sign between hole and electron pockets. We also argue that at weak coupling, this pairing predominantly comes not from spin fluctuation exchange but from a direct pair hopping between hole and electron pockets.