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     

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Mg1-xAlxCNi3(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Mg1-xNaxCNi3,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.     

Bi2Te3-xSex(x≤3)同晶化合物电子结构的第一性原理研究

本文采用基于密度泛函理论的第一性原理全势能线性缀加平面波方法(FLAPW),分析了Bi₂Te_3-xSe_x体系中各原子自旋轨道耦合(SOI)的p_1/2修正对体系性质的影响,并对Bi₂Te_3-xSe_x(x≤3)同晶化合物的电子特性进行系统的理论研究,首次计算出Bi₂S 关键词： 2Te_3-xSe_x(x≤3)同晶化合物')" href="#">Bi₂Te_3-xSe_x(x≤3)同晶化合物 第一性原理 电子结构 自旋轨道耦合     

La0.1Bi0.9-xEuxFeO3化合物的结构与性能研究

利用固相反应烧结技术制备La_0.1Bi_0.9-xEu_xFeO₃系列化合物. 利用X射线粉末衍射进行物相鉴定和结构分析,确定了材料的相关系：x≤0.05,材料为R3c结构相;0.08≤x≤0.12,材料为赝R3c结构相;x≥0.15是Pbnm相,其中0.15≤x≤0.20区域Pbnm相存在畸变. 磁测量结果表明,材料具有弱铁磁性,对于x≤0.20材料,磁矩在x=0.12成分存在极值. 利用阻抗分析仪测量了室温介电常数随成分的变化关系.讨论了材料的结构与弱铁磁性和室温介电常数间的关系. 关键词： 0.1Bi_0.9-xEu_xFeO₃')" href="#">La_0.1Bi_0.9-xEu_xFeO₃ X射线衍射 磁性 介电常数     

La0.7Sr0.3-x□xCoO3(0≤x≤0.2)钴氧化物中锶空位诱导的自旋态转变效应

研究了锶空位对La_0.7Sr_0.3-x_xCoO₃ (0≤x≤0.2)多晶钴氧化物结构、磁性和输运性质的影响.结果表明：随着锶空位浓度x的增大,A位阳离子无序度增大,导致铁磁双交换作用减弱及反铁磁超交换作用增强,两者相互竞争,出现团簇自旋玻璃态;空位浓度超过10%后,Co—O键长迅速减小,导致晶体场劈裂能加大,大部分三价钴离子以低自旋态出现,系统基态为类超顺磁态, 关键词： 空位掺杂 钴氧化物 自旋转变     

Y掺杂SrTiO3晶体材料的电子结构计算

采用基于第一性原理的密度泛函理论平面波超软赝势法, 研究了Y掺杂SrTiO₃体系的空间结构和电子结构性质, 得到了优化后体系的结构参数, 掺杂形成能, 能带结构和电子态密度. 对比掺杂浓度为0125, 025, 033时,Sr_1-xY_xTiO₃和SrTi_1-xY_xO₃的掺杂形成能,发现Y替代Sr能形成更稳定的结构. 对Sr_1-xY_xTiO₃（x=0, 0125, 025, 033） 的结构进行了优化,结果表明Y替代Sr后, 随着掺杂浓度增大, 体系的晶格常数逐渐减小, 稳定性逐渐增强. 对不同掺杂浓度的Sr_1-xY_xTiO₃能带结构的计算结果表明：纯净的SrTiO₃是绝缘体, 价带顶在R点, 导带底在Γ点, 费米能级处于价带顶; 掺杂Y后, 费米能级进入到导带底中, 体系呈金属性;掺杂浓度越大,费米能级进入导带的位置越深,禁带宽度也近似变宽. 关键词： 3')" href="#">SrTiO₃ 电子结构 掺杂 VASP     

关于Zn1-xBexO电子结构和光学性质的研究

采用基于密度泛函理论和平面波赝势技术的CASTEP程序对Zn_1-xBe_xO合金电子结构和光学性质进行了计算.当0≤x≤1,其带隙从0.963 eV变化到7.293 eV.分析了晶格畸变和能带间排斥效应对带隙的影响.当Be含量x=0.125,0.25,0.375,0.5,0.625,0.75时,a/b轴压应变控制着带隙变化;当x=0.875,1时,c轴压应变控制着带隙变化.能带间的p-d排斥影响价带顶变动,Γ_1v与Γ_1c之间排斥影响导带底变动.这些能带间的排斥效应被用来分析Zn_1-xBe_xO带隙变动.另外,也分析了Zn_1-xBe_xO介电函数虚部ε₂. 关键词： 带结构 光学性质 应变 排斥     

Tb$lt;sub$gt;0.3$lt;/sub$gt;Dy$lt;sub$gt;0.6$lt;/sub$gt;Pr$lt;sub$gt;0.1$lt;/sub$gt;(Fe$lt;sub$gt;1-$lt;i$gt;x$lt;/i$gt;$lt;/sub$gt;Al$lt;sub$gt;$lt;i$gt;x$lt;/i$gt;$lt;/sub$gt;)$lt;sub$gt;1.95$lt;/sub$gt;合金的磁性、磁致伸缩和穆斯堡尔谱研究

系统研究了室温下Tb_0.3Dy_0.6Pr_0.1（Fe_1-xAl_x）_1.95 （x=0.05,0.1,0.15,0.2,0.25,0.3）合金中元素Al替代Fe对结构、磁性、磁致伸缩性能和自旋重取向的影响.测量结果发现,x<0.2时Tb_0.3Dy_0.6Pr_0.1（Fe_1-xAl_x）_1.95合金基本上是纯的单相,x=0.2时出现其他杂相,杂相随Al替代量的增加不断增多.随Al替代量x的增加,点阵常数a接近于线性增大,Curie温度T_C逐渐下降,而矫顽力H_c急剧下降.振动样品磁强计（VSM）测量发现,磁化强度M随Al替代量x的变化较为复杂.VSM计和磁致伸缩效应测量共同表明,少量Al的替代有利于降低磁晶各向异性,而且随着Al替代量x的增多磁致伸缩系数快速减小,x>0.15时巨磁致伸缩效应消失.穆斯堡尔效应研究发现,随Al含量的增加Tb_0.3Dy_0.6Pr_0.1（Fe_1-xAl_x）_1.95合金中易磁化轴可能在{110}面逐渐偏离了立方晶体的主对称轴,发生自旋重取向,从而引起合金宏观磁性、磁致伸缩性能的变化. 关键词： 磁致伸缩 立方Laves相 自旋重取向 穆斯堡尔谱     

NaZn13型结构LaFe13－xAlxCy化合物的磁熵变与磁相变的研究

研究了NaZn₁₃型结构LaFe_13-xAl_xC_0.1(x=1.6，1.8)间隙化合物的磁制冷能力和磁相变.利用麦克斯韦关系式计算得到，高Al含量LaFe_13-xAl_x碳化物的最大磁熵变值|ΔS|_m低于低Al含量碳化物的最大磁熵变值.随Al含量的增加，化合物的磁熵变峰展宽，但由于磁熵变大幅降低，衡量磁制冷能力的q值随之降低.基于朗道相变原理，考虑到自旋涨落的影响，磁自由能可以展开到磁化强度的6次方项，材料的相变类型由磁化强度的4次方项系数a₃(T)的符号来进行判断.随着Al含量的增加，研究的碳化物相变由弱的一级相变转为二级相变. 关键词： 13-xAl_x碳化物')" href="#">LaFe_13-xAl_x碳化物 磁制冷能力 磁相变     

Pr掺杂DyFeO3体系的自旋重取向相变、晶格畸变与Raman光谱研究

利用固相反应法制备了Dy_1-xPr_xFeO₃系列化合物. X射线粉末衍射晶体结构分析表明, 随着Pr掺杂量x的增加, 样品晶胞体积逐渐增大, 晶格畸变减弱. Raman光谱测量表明稀土离子有效质量[m_eff=xm_Pr+(1-x)m_Dy] 与晶格结构的变化共同导致该体系Raman光谱的变化. 随Pr掺杂量的增加, 波数小于200 cm^-1的振动模式基本保持不变, 而波数大于200 cm^-1的振动模式(除420 cm^-1处的B_3u模式外)向低频移动. 磁测量结果表明, 由Dzyaloshinsky-Moriya 相互作用导致的宏观磁性随Pr掺杂量增加逐渐减弱. 稀土离子与铁离子磁晶格的耦合作用以及晶格结构畸变的变化共同导致该体系自旋重取向相变温度在一定的掺杂量 (x=0.3)前后先升高后降低. 关键词： 稀土铁氧体 自旋重取向 晶体结构 Raman光谱     

Interplay between the magnetic fluctuations and superconductivity in lanthanum cuprates

We report an analysis of the magnetic fluctuations in superconducting La_2?xSr_xCuO₄ and related lanthanum cuprates that have different symmetry of the low-temperature structure. NMR and ESR investigations revealed a dynamical coexistence of superconductivity and the antiferromagnetic correlations in most of the superconductivity region of the phase diagram. We show that, for all compounds, regardless of their low-temperature symmetry and their superconducting properties, the enhancement of the spin stiffness near 1/8 doping takes place.     

Coexistence of superconductivity and magnetism in the La0.87−xLnxSr0.13FeAsO (Ln=Sm, Gd, Dy) system

The interplay between the superconducting phase and spin density wave order phase was studied. We report the magnetic and superconducting properties of the hole-doped FeAs-based superconducting compound La_0.87−xLn_xSr_0.13FeAsO (Ln=Sm, Gd, Dy; 0≤x≤0.06). Both resistivity and magnetic susceptibility measurements show that the superconducting transition temperature decreases with increase in composition of magnetic ions. The hysteresis loop of the La_0.87−xLn_xSr_0.13FeAsO sample shows a superconducting hysteresis in addition to a paramagnetic background. The experiment demonstrates that the magnetism and superconductivity coexist in hole-doped FeAs-based superconducting compounds. Among these three magnetic rare-earth elements, the influence of Dy³⁺ doping on superconductivity is more evident than that of Gd³⁺ doping, while the influence of Sm³⁺ doping is the weakest. The trend is consistent with the variation of the lattice parameter along c-axis.     

Magnetism on Mg1−xZnxCyNi3

The magnetic phase diagram for Mg_1−xZn_xC_yNi₃ has been tentatively constructed based on magnetization and muon spin relaxation (μSR) measurements. The superconducting phase was observed to fade as x (y) increases (decreases). The low y samples show early stages of long-range ferromagnetism, or complete long-range ferromagnetism. In the phase diagram, the ferromagnetic phase exists in addition to the superconducting phase, suggesting that there is some correlation between superconductivity and ferromagnetism, even though the coexistence of ferromagnetism and superconductivity is not observed from the μSR measurements down to 20 mK for the superconducting sample (T_c=2.5 K, (x, y)=(0, 0.9)).     

Magnetism and Superconductivity inCeCu2(Si1−xGex)2 Probed by Cu NQR

We report Cu-NQR results on Ge-doped heavy-fermion superconductor CeCu₂(Si_1–x Ge _x )₂ (0<x0.2) and undoped Ce_0.99Cu_2.02Si₂. The main effect of the Ge doping is considered to be a negative pressure, since the strength of hybridization decreases by the Ge doping. With increasing x, the dynamical characteristics of the magnetic order at x=0 change to more static ones which suggests a localized regime above x0.25. From the derived T–x phase diagram, it is suggested that the magnetic and the superconducting phases are almost degenerate in undoped CeCu₂Si₂. An exotic interplay between the magnetism characterized by the slow fluctuations and the superconductivity is implied in the region of small x.     

Structure and superconductivity of Mg1−xPbxB2

A series of polycrystalline samples of Mg_1−xPb_xB₂ (0≤x≤0.10) were prepared by a solid state reaction method and their structure, superconducting transition temperature and transport properties were investigated by means of X-ray diffraction (XRD) and resistivity measurements. Mg_1−xPb_xB₂ compounds were shown to adopt an isostructural AlB2-type hexagonal structure in a relatively small range of lead concentration, x≤0.01. The crystalline lattice constants were evaluated and were found to exhibit slight length compression as x increases. The superconducting transition temperature (T_c) steadily decreases with Pb doping. It is suggested that the mechanism of superconductivity reduction by lead doping can be attributed to the chemical pressure effect.     

The 3-band Hubbard-model <Emphasis Type="Italic">versus</Emphasis> the 1-band model for the high-<Emphasis Type="Bold">T</Emphasis><Subscript>c</Subscript> cuprates: Pairing dynamics,superconductivity and the ground-state phase diagram

One central challenge in high-T _c superconductivity (SC) is to derive a detailed understanding for the specific role of the Cu-d _x2-y2 and O-p _x,y orbital degrees of freedom. In most theoretical studies an effective one-band Hubbard (1BH) or t-J model has been used. Here, the physics is that of doping into a Mott-insulator, whereas the actual high-T _c cuprates are doped charge-transfer insulators. To shed light on the related question, where the material-dependent physics enters, we compare the competing magnetic and superconducting phases in the ground state, the single- and two-particle excitations and, in particular, the pairing interaction and its dynamics in the three-band Hubbard (3BH) and 1BH-models. Using a cluster embedding scheme, i.e. the variational cluster approach (VCA), we find which frequencies are relevant for pairing in the two models as a function of interaction strength and doping: in the 3BH-models the interaction in the low- to optimal-doping regime is dominated by retarded pairing due to low-energy spin fluctuations with surprisingly little influence of inter-band (p-d charge) fluctuations. On the other hand, in the 1BH-model, in addition a part comes from “high-energy” excited states (Hubbard band), which may be identified with a non-retarded contribution. We find these differences between a charge-transfer and a Mott insulator to be renormalized away for the ground-state phase diagram of the 3BH- and 1BH-models, which are in close overall agreement, i.e. are “universal”. On the other hand, we expect the differences - and thus, the material dependence to show up in the “non-universal” finite-T phase diagram (T _c-values).     

Competing energy scales in high‐temperature superconductors: Ultrafast pump–probe studies

We present a review of photoexcited quasiparticle dynamics of cuprate and pnictide high‐temperature superconductors in regimes (temperature, doping) where different phases such as superconductivity, spin‐density‐wave (SDW) and pseudogap phases coexist or compete with one another. We start with the overdoped cuprate superconductor Y_1–xCa_x Ba₂Cu₃O_7–δ, where the superconducting gap and pseudogap coexist in the superconducting state. In another cuprate Tl₂Ba₂Ca₂Cu₃O_y, we ob‐ serve a competition between SDW and superconducting orders deep in the superconducting state. Finally, in the underdoped iron pnictide superconductor (Ba,K)Fe₂As₂, SDW order forms at 85 K, followed by superconductivity at 28 K. We also find the emergence of a normal‐state order that suppresses SDW at a temperature T * ～ 60 K and argue that this normal‐state order is a precursor to superconductivity. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Co掺杂对ZnO薄膜结构和性能的影响

采用PVA溶胶-凝胶方法，在玻璃衬底上制备了Zn_1-xCo_xO薄膜，利用X射线衍射仪(XRD)研究了不同Co含量对其微结构的影响.采用振动样品磁强计(VSM)测量了Zn_0.88Co_0.12O样品室温下的磁性.采用荧光光谱仪研究了Zn_1-xCo_xO样品室温下的发光特性，分析掺杂含量对其发光性能的影响，发现随着掺杂含量的增加，蓝光发光峰有一定的红移现象. 关键词： PVA方法 ZnO 掺杂