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     

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.     

MgCNi3空穴(Na)掺杂的磁性和声子谱研究

应用全势线性响应线性糕模轨道方法,对MgCNi3镁位进行钠掺杂的虚原子近似,计算Mg1-xNaxCNi3(x=0.0,0.06,0.11,0.115,0.12)的电子态密度和晶格振动(x=0.0,0.06),并讨论磁性和声子谱.计算结果表明,随着掺杂量x的增加,原胞磁矩迅速增加,在x=0.115时,费米能级处总态密度几乎落在范霍夫奇异峰峰顶上且有较大的自旋涨落,预示MgCNi3处在铁磁不稳定的边缘.x=0.12的费米面处总态密度与x=0.115相比有较大的下降,使得超导库珀对配对数减少,会带来超导电性的降低.x为0.06的钠掺杂,使得布里渊区沿三个主要高对称性方向(Γ-X、Γ-M、Γ-R)虚频范围扩大,容易使MgCNi3结构不稳定而产生结构相变。     

超导体YB6和LaB6的电子结构研究

本文使用准确的第一原理方法，对超导体YB6和LaB6进行了电子结构比较研究，为进一步的实验研究提供理论参考．     

一硼化物和二硼化物超导体电子结构研究

本文使用准确的第一原理方法,对一硼化物和二硼化物超导体进行了电子结构研究,从而发现硼化物超导体所具有特殊的电子结构属性.     

超导体YB6和LaB6的电子结构研究

本文使用准确的第一原理方法,对超导体YB6和LaB6进行了电子结构比较研究,为进一步的实验研究提供理论参考.     

超导体YB6和LaB6的电子结构研究

本文使用准确的第一原理方法,对超导体YB6和LaB6进行了电子结构比较研究,为进一步的实验研究提供理论参考.     

Cr掺杂浓度对AlN半金属性影响的第一性原理研究

本文基于密度泛函理论的第一性原理方法, 在广义梯度近似(GGA)下计算了掺杂过渡金属Cr原子后AlN晶体自旋极化的能带结构、分态密度等性质. 结果表明, 半金属能隙随着掺杂浓度的增大而减小. 文中以掺杂浓度为12.5%的Cr-AlN(2×2×1)为例, 分析了其自旋极化的能带结构、分态密度和磁矩等性质, 发现Cr-3d电子对自旋向下子带导带底的能量位置起决定作用. 随着掺杂浓度的增大, Cr原子间相互作用增强, Cr-3d能带向两边展宽, 导致自旋向下子带导带底的能量位置下降, 从而半金属能隙变窄.     

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(E_F)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 关键词： 电子结构 态密度 超导电性     

In掺杂ZnO超晶格光学性质的研究

采用基于第一性原理的线性缀加平面波方法, 计算了超晶格ZnO掺In的光学性质. 计算结果表明, 掺入In元素后Fermi能级进入导带, 介电函数、吸收系数、折射率、反射率都在Fermi面附近产生新的跃迁峰. 随着掺杂层数的增多, 跃迁峰发生红移, 当掺杂两层In时跃迁峰峰值最大, 同时吸收系数的吸收边随掺杂层数的增多而逐渐减小. 与In掺杂ZnO超晶胞相比, ZnO超晶格在可见光范围内具有高透过率的特点.     

新型超导体MgCNi3的输运性质研究

测量了新型超导体MgCNi3(Tc=8K)的电阻率、正常态Hall效应和热电势等输运性质.电阻率温度曲线表明,在高于70K的温区可以用电声子散射的BlochGr櫣ｎｅｉｓｅｎ公式拟合.Hall系数RH和热电势S在Tc以上的整个温度范围都为负值,强烈表明MgCNi3的载流子类型为电子型.RH在从Tc到140K的温区内基本不随温度变化,但在140K到室温范围,RH随温度升高其绝对值减小,对RH的这种温度依赖关系进行了讨论.S在150K以上近似与温度成线性关系,而在150K以下显示非线性,用电声子相互作用的重整 关键词： 电阻率 热电势 Hall效应     

Electronic structure of the new MgCNi3 superconductor and related intermetallic compounds

The band structure of the new perovskite-like MgCNi₃ superconductor is studied by the self-consistent FP-LMTO method. The superconducting properties of MgCNi₃ are associated with the occurrence of an intense peak in the density of Ni3d states at the Fermi level. The absence of superconductivity for nonstoichiometric MgC_1?x Ni₃ compositions is caused by the transition of the system to a magnetic state. The possibilities of finding superconductivity for ScBNi₃, InBNi₃, MgCCo₃, and MgCCu₃ isostructural with MgCNi₃ are discussed.     

新型超导体MgB2和MgCNi3热、电输运性质研究

报道了两种新型超导体MgB2,MgCNi3和氧化物高温超导体Bi2Sr2Ca0.9Ce0.1Cu2O8+y的热导率-温度关系和电阻率-温度关系.实验发现氧化物高温超导体在进入超导态后热导有所上升,出现极大值后再下降,而MgB2和MgCNi3则单调下降.由Wiedemann-Franz定律分别计算了它们在正常态的电子热导和声子热导.由于共有化电子的非局域性,在MgB2和MgCNi3中电子热导的贡献占据相当大的成分.对电子热导的分析结果表明,在MgB2和MgCNi3的电子热导中静态缺陷对电子的散射占主导地位.     

Effects of copper doping in MgB2 superconductor

Magnesium diboride, recently discovered as superconductor, is extremely interesting in respect to pairing mechanisms and phonon-mediated superconductivity, but its transition temperature (39 K) is decidedly too low for practical applications. A rush for substitutions started, intended to raise T_c and possibly maintain acceptable critical current density. The difficult introduction of Cu was attempted, through a complex procedure employing CuB₂₄ instead of metallic Cu: owing to the tendency of Cu to form inter-metallic compounds with Mg, only a maximum of 3 mol% of Cu was safely substituted to Mg in MgB₂ lattice and a small parameters contraction proportional to Cu content was recorded. The results are a decrease in T_c of about 0.5 K, not far from that found with Al addition, whose reasons are discussed and interpreted in terms of holes doping.     

Influence of Pb doping on properties of BSCCO superconductor

Summary We report here about measurements of resistancevs. temperature, alternating current (a.c.) susceptibility and X-ray diffraction performed on Ba₂Sr₂Ca₂Cu₃O _x superconductor doped with various amounts of Pb. The results could be explained in terms of Pb contribution in increasing both the amount of the highestT _{C 0} phase and connectivity between superconductive regions. The Pb substitution on Bi sites is also discussed.     

Homogeneous NMR line shift induced by magnetic doping in a superconductor

It is shown theoretically that a uniform conduction electron spin polarization, induced by magnetic doping, arises in a superconducting state. Measurements of the corresponding Knight shift enable one to determine the sign of the s-f exchange constant.     

Stabilization of ErFeAsO-based superconductor by hydrogen doping under high pressure

Here we report on the successful synthesis of ErFeAsO-based superconductor by hydrogen doping using high-pressure (HP) synthesis technique. The ErFeAsO_1?y target pellets were sandwiched between LaFeAsO_0.8H_0.8 pellets, which act as a hydrogen source. Hydrogen released from the hydrogen source soaks into the target pellets during HP synthesis and the target pellets crystallizes with the tetragonal crystal structure with the lattice constants a = 3.8219 Å and c = 8.2807 Å. The hydrogen doped sample ErFeAsO_1?y(H) thus obtained shows superconducting critical temperatures (T_c) of 44.5 K and 42.5 K determined by onsets of resistive and diamagnetic transitions, respectively.     

MgCNi3超导体在静水压下的弹性性质

 在0.1 MPa到0.5 GPa压力范围内，采用脉冲回波重合法（Pulse-Echo Overlap Method）测量了MgCNi₃超导体的横波及纵波声速，计算了体弹模量B、剪切模量G等弹性参数以及德拜温度、比热等热力学参数随压力的变化。常压下的θ_D与c_V的值分别为454.82 K和111.30 J/(mol·K)。对B-p曲线作线性拟合，得到体弹模量B₀在零压附近的导数B′₀，由此建立了MgCNi₃的状态方程。     

Abrupt transition in quasiparticle dynamics at optimal doping in a cuprate superconductor system

We report time-resolved measurements of the photoinduced change in reflectivity, DeltaR, in the Bi2Sr2Ca(1-y)Dy(y)Cu2O8+delta (BSCCO) system of cuprate superconductors as a function of hole concentration. We find that the kinetics of quasiparticle decay and the sign of DeltaR both change abruptly where the superconducting transition temperature T(c) is maximal. These coincident changes suggest that a sharp transition in quasiparticle dynamics takes place precisely at optimal doping in the BSCCO system.