Theoretical prediction of structural, electronic and optical properties of quaternary alloy Zn1-xBexSySe1-y

Within density functional theory based on the full potential-linearized augmented plane wave method, we carry out the first-principles calculation of the structural, electronic, and optical properties of the zinc blende quaternary alloy Zn_1-xBe_xS_ySe_1-y. The Perdew-Burke-Ernzerhof generalized gradient approximation based on the optimization of total energy and the Engel-Vosko generalized gradient approximation based on the optimization of the corresponding potential are used. Our investigation on the effect of the composition on lattice constants, bulk modulus, band gap, optical dielectric constant, and refractive index shows a non-linear dependence. The energy gap E_g(x, y) has been determined over the entire compositions x and y. In addition, the energy band gap of the technologically important quaternary alloy Zn_1-xBe_xS_ySe_1-y in conditions of being lattice matched to ZnS substrate has been investigated. It is noteworthy that the present work is the first theoretical study of the quaternary alloy of interest.     

(Fe1-xNix)2P电子结构与磁学特性的X射线近边吸收谱研究

利用X射线近边吸收谱对Fe₂P,Ni₂P及其掺杂物(Fe_1-xNi_x)₂P(x=01,025,05)中Fe,Ni,P的K边进行了研究.结合多重散射理论近边计算,讨论了金属原子不同位置格点3f,3g对近边谱特征的贡献,得出当Ni原子取代Fe原子时将优先占据Fe(3f)格点位置;根据第一性原理对能态的计算发现,不考虑磁性时不同格点P的pDOS未占据态电子结构与P-K近边吸收谱实验相符合;与考虑铁磁性Fe₂P 的DOS相比较后结果显示Fe₂P的磁性主要来源于Fe(3g)格点,铁磁性Ni₂P计算的Ni不同格点原子磁矩均接近于0,与它一般显顺磁性结论相一致. 关键词： X射线近边吸收谱 电子结构 多重散射理论 态密度     

退火对高Co含量Ti1-xCoxO2磁性半导体的影响

利用磁控溅射仪制备了高Co含量的Ti_1-xCo_xO₂磁性半导体样品,并对样品分别在200℃,300℃和400℃进行退火研究.使用透射电子显微镜(TEM)对退火前后样品的结构进行表征,并用X射线光电子能谱(XPS)对退火前后样品中Co元素的化学状态进行鉴定.结果表明高Co含量的Ti_1-xCo_xO₂磁性半导体处于一种亚稳状态,300℃以上的温度便使其结构与成分发生巨大变化.利用超导量子干涉磁强计(SQUID)测量退火前后样品的磁特性,结果表明样品的磁性有了明显的变化,这源于磁性产生的不同机理. 关键词： 磁性半导体 退火 磁性     

C20分子异构体的电子态

使用量子化学从头计算方法优化了C₂₀分子三种异构体（环形、碗形和笼形）的单重和三重电子态几何结构。从优化的几何结构出发，由二阶微扰（MP2）方法得到的单点能给出了一个不符合Hund法则的结果，也就是三种异构体的能量都是单重电子态的最低。使用UHF波函数给出了三种异构体的总电荷密度和静电势，结果显示在笼形和碗形结构的中心区域存在负电荷。最后，本文分析了环形结构的成键特征。     

First principles studies on the electronic structures of LiMxFe1-xPO4 （M = Co, Ni and Rh）

The local crystal structures and electronic structures of LiMxFe1-xPO4 （M = Co, Ni, Rh） are studied through first-principles calculations. The lattice constants and unit cell volumes are smaller for the Co and Ni doped materials than for pure LiFePO4, while larger than for the Rh doped material. The local structures around M atoms in the doped materials are studied in details. The total density of states （DOS） and atomic projected DOS （PDOS） are all calculated and analysed in detail. The results give a reasonable prediction to the improvement of electronic conductivity through Fe-site doping in LiFePO4 material.     

CoxCu1-x复相纳米线阵列的制备及其磁性的研究

利用电化学沉积方法在同一种富Co²⁺溶液Co²⁺/Cu²⁺=10∶1中,利用不同的沉积电位成功地制备了一系列不同成分(x=0.38—0.87)和复合相结构的Co_xCu_1-x纳米线阵列.发现随着纳米线中Cu含量的变化,Co_xCu_1-x纳米线的复相结构随之发生规律的变化,最终导致纳米线的磁性也随之规律的变化.随着纳米线中Cu含量的不断增加,一部分Cu与Co形成面心立方结构(fcc)的CoCu固溶体,减弱了磁晶各向异性与形状各向异性的竞争,从而提高样品的方形度;一部分Cu以fcc结构的Cu单质的形式存在于纳米线中,并随着Cu颗粒大小的不同分别起到破坏磁晶各向异性和钉扎畴壁的作用,从而增加纳米线的方形度和矫顽力.对比不同成分的样品,发现Co_xCu_1-x纳米线的方形度和矫顽力的最大值分别出现在Co₇₅Cu₂₅和Co₆₀Cu₄₀中,并且由于其特殊的复相结构致使它们的值要好于相同直径的单相结构的结果. 关键词： 纳米线 电化学沉积 磁性     

Microstructural and magnetic properties of Ax:Zn1-xO (A=Mn,Gd and Mn/Gd) nanocrystals

We report the microstructural and magnetic properties of transition (3d) and rare earth (4f) metal substituted into the A_x:Zn_1?xO (A=Mn, Gd and Mn/Gd) nanocrystal samples synthesized by solgel method. The structural properties and morphology of all samples have been analysed using X-ray diffraction (XRD) method and scanning electron microscopy. The impurity phase in the XRD patterns for all samples is not seen, except (Mn/Gd):ZnO sample where a very weak secondary phase of Gd₂O₃ is observed. Due to the large mismatch of the ionic radii between Mn²⁺ and Gd³⁺ ions, the strain inside the matrix increases, unlike the crystallite size decreases with the substitution of Mn and Gd into ZnO system. A couple of additional vibration modes due to the dopant have been observed in Raman spectrum. The magnetic properties have been studied by vibrating sample magnetometer. The magnetic hysteresis shows that Mn:ZnO and Gd:ZnO have soft ferromagnetic (FM) behaviour, whereas (Mn/Gd):ZnO has strong FM behaviour at room temperature (RT). The enhancement of ferromagnetism (FM) in (Mn/Gd):ZnO sample might be related to short-range FM coupling between Mn²⁺ and Gd³⁺ ions via defects potential and/or strain-induced FM coupling due to the expansion lattice by doping. The experimental results indicate that RTFM can be achieved by co-substitution of 3d and 4f metals in ZnO which can be used in spintronics applications.     

Zn1-xCoxO稀磁半导体薄膜的结构及其磁性研究

利用X射线吸收精细结构、X射线衍射和磁性测量等技术研究脉冲激光气相沉积法制备的Zn_1-xCo_xO (x＝0.01,0.02)稀磁半导体薄膜的结构和磁性.磁性测量结果表明Zn₁-xCo_xO样品都具有室温铁磁性.X射线衍射结果显示其薄膜样品具有结晶良好的纤锌矿结构.荧光X射线吸收精细结构测试结果表明,脉冲激光气相沉积法制备的样品中的Co离子全部进入ZnO晶格中替代了部分Zn的格点位置,生成单一相的Zn_1-xCo_xO 稀磁半导体.通过对X射线吸收近边结构谱的分析,确定Zn_1-xCo_xO薄膜中存在O空位,表明Co离子与O空位的相互作用是诱导Zn_1-xCo_xO产生室温铁磁性的主要原因. 关键词： 1-xCo_xO稀磁半导体')" href="#">Zn_1-xCo_xO稀磁半导体 X射线吸收精细结构谱 脉冲激光气相沉积法     

Sr掺杂对La_(1-x)Sr_xMnO_3/LaAlO_3/SrTiO_3界面电子结构的影响

采用基于密度泛函理论的第一性原理计算方法,系统地研究了La_(1-x)Sr_xMnO_3层中Sr的掺杂方式和掺杂量对4La_(1-x)Sr_xMnO_3/3LaAlO_3/4SrTiO_3(LSMO/LAO/STO)异质结构原子和电子结构的影响.结果表明:对于相同的Sr掺杂量,掺杂方式的差异对体系电子结构的影响微弱,不会导致体系发生金属-绝缘体转变;掺杂量的不同对体系电子结构有着显著的影响,当Sr的掺杂量较少时,LAO/STO界面处存在着准二维电子气,当Sr的掺杂量高于1/3时,LAO/STO界面处准二维电子气消失.我们相信,Sr的引入以及通过Sr掺杂量的改变可以对LSMO覆盖层极化进行调控,这也是导致体系LAO/STO界面处金属-绝缘体转变的可能原因,进一步为极化灾变机制导致的界面处电子重构提供了证据.     

Electronic structure and magnetic properties of SmCo7?xTix

Electronic structure of SmCo_7−x Ti_x alloy has been studied by means of the spin-polarized MS-Xα method. It is shown that a few of electrons are transferred to Sm(5d) orbital due to orbital hybridization between Sm and Co atoms. The exchange interaction between 5d-3d electrons is stronger, which is the main reason resulting in the long-range ferromagnetic order between Co and Sm atoms. The Curie temperature of SmCo_7−x Ti_x is generally lower than that of pure Co metal, which may be explained by the weaker average of coupling strength between Co sites due to some negative exchange couplings occurring mainly at 2e site. The calculated results for the Sm₅Co₂₈Ti₆ cluster may lead to a better understanding of why SmCo_7−x Ti_x is stable phase. Since the negative interaction of 2e sites weakens and the bonding at E _F strengthens with increasing Ti concentration, which result in the decrease in the free energy of the alloy, the stable ferromagnetic order forms inside SmCo_7−x Ti_x. Considering the localization of 4f states and 5d moment arising from the orbital hybridization, the calculated moment is 9.47 μ _B per formula unit that is in agreement with experiments.     

First principles study on the structural, electronic and optical properties of diluted magnetic semiconductors Zn1-xCoxX （X=S, Se, Te）

The electronic and optical properties of zincblende ZnX（X=S, Se, Te） and ZnX：Co are studied from density functional theory （DFT） based first principles calculations. The local crystal structure changes around the Co atoms in the lattice are studied after Co atoms are doped. It is shown that the Co-doped materials have smaller lattice constant （about 0.6%-0.9%）. This is mainly due to the shortened Co-X bond length. The （partial） density of states （DOS） is calculated and differences between the pure and doped materials are studied. Results show that for the Co-doped materials, the valence bands are moving upward due to the existence of Co 3d electron states while the conductance bands are moving downward due to the reduced lattice constants. This results in the narrowed band gap of the doped materials. The complex dielectric indices and the absorption coefficients are calculated to examine the influences of the Co atoms on the optical properties. Results show that for the Co-doped materials, the absorption peaks in the high wavelength region are not as sharp and distinct as the undoped materials, and the absorption ranges are extended to even higher wavelength region.     

MnxSi1-x磁性薄膜的结构研究

利用X射线衍射(XRD)和X射线吸收近边结构(XANES)方法研究了在Si(100)衬底上及600℃温度条件下用分子束外延(MBE)共蒸发方法生长的Mn_xSi_1-x磁性薄膜的结构.由XRD结果表明,只有在高Mn含量(8％和17％)样品中存在着Mn₄Si₇化合物物相.而XANES结果则显示,对于Mn浓度在0.7%到17%之间的Mn_xSi_1-x样品,其Mn原子的XANES谱表现出了一致的谱线特征.基于多重散射的XANES理论计算进一步表明,只有根据Mn₄Si₇模型计算出的理论XANES谱才能够很好的重构出Mn_xSi_1-x样品的实验XANES谱.这些研究结果说明在Mn_xSi_1-x样品中,Mn原子主要是以镶嵌式的Mn₄Si₇化合物纳米晶颗粒存在于Si薄膜介质中,几乎不存在间隙位和替代位的Mn原子. 关键词： xSi_1-x磁性薄膜')" href="#">Mn_xSi_1-x磁性薄膜 分子束外延 XRD XANES     

原子簇La8-xBaxCuO6的原子磁矩和自旋极化的电子结构研究

利用MS-Xα方法研究了化合物La_2-yBa_yCuO₄的原子磁矩和自旋极化的电子结构.理论计算得到母相氧化物La₂CuO₄的Cu原子磁矩为0.37μ_B，与实验值0.48±0.15μ_B基本一致. 研究结果显示, 由于Ba原子对部分La的替代，使构成化合物的基本原子簇La_8-xBa_xCuO₆的 关键词： 电子结构 自旋极化 磁矩 态密度 超导电性     

多铁材料Bi1-xCaxFeO3的介电、铁磁特性和高温磁相变

采用溶胶凝胶法制备Bi_1-xCa_xFeO₃ (x=0, 0.05, 0.1, 0.15, 0.2)陶瓷样品. X衍射图谱表明所有样品的主衍射峰均与纯相BiFeO₃相符合且具有良好的晶体结构. 随着x的增大, Bi_1-xCa_xFeO₃样品的主衍射峰由双峰(104)与(110) 逐渐重叠为单峰(110), 当x ≥0.15时, 样品呈现正方晶系结构; 扫描电镜形貌分析可知, 晶粒由原来的0.5 μm逐渐增大到2 μm. Bi_1-xCa_xFeO₃样品介电常数和介电损耗随着x 的增加先增大而后减小. 当f=1 kHz, Bi_0.9Ca_0.1FeO₃ 的介电常数达到最大值, 是BiFeO₃的7.5倍, 而Bi_0.8Ca_0.2FeO₃的介电常数达到最小值, 仅仅是BiFeO₃的十分之一. Bi_1-xCa_xFeO₃样品所呈现的介电特性是由偶极子取向极化和空间电荷限制电流两种极化机理共同作用的结果. 随着Ca²⁺ 的引入, BiFeO₃ 样品的铁磁性显著提高. X射线光电子能谱图表明Fe²⁺和Fe³⁺ 共存于Bi_1-xCa_xFeO₃ 样品中, Fe²⁺/Fe³⁺比例随着Ca²⁺ 掺杂量的增加而增大, 证明Ca²⁺掺杂增加了Fe²⁺的含量, 增强BiFeO₃的铁磁特性. 从M-T曲线观察到BiFeO₃样品在878 K附近发生铁磁相变, 示差扫描量热法测试再次证明BiFeO₃ 在878 K发生相变. Ca²⁺掺杂使BiFeO₃样品的T_N略有变化而T_M基本不变, 其主要原因是Fe-O-Fe反铁磁超交换作用的强弱和磁结构相对稳定.     

Electronic structures and magnetic properties in SmCo7-xMx

The electronic structures and magnetic properties of SmCo7 xMx (M=Ti, Si, Zr, Hf, Cu, B, Ag, Ga, Mn) compounds are investigated by using a spin-polarized MS-X.α method. The results show that the long-range ferromagnetic order is determined by a stronger 3d-5d interaction, rather than the traditional RKKY interaction, and the effects of doping element M on 3d-5d coupling are negligible in Sm-Co-based compounds. The nonmagnetic dopant Si atoms have a larger effect on the moments of 2e site although they preferably occupy the Co 3g sites, which results in the stronger uniaxial anisotropy of this compound. Analysis of the formation energies indicates that 5d-element doped compounds are more stable than other dopants, and furthermore, they have a higher Curie temperature above room temperature, which will be in favor of their potential application as high-temperature permanent magnets.     

Spin-polarized structural, electronic and magnetic properties of diluted magnetic semiconductors Cd1−xMnxTe in zinc blende phase

We have investigated the structural, electronic and magnetic properties of the diluted magnetic semiconductor (DMS) Cd_1−xMn_xTe (for x=0.75 and 1.0) in the zinc blende (B3) phase by employing the ab-initio method. Calculations were performed by using the full potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the frame work of spin-polarized density functional theory (SP-DFT). The electronic exchange-correlation energy is described by generalized gradient approximation (GGA). We have calculated the lattice parameters, bulk modulii and the first pressure derivatives of the bulk modulii, spin-polarized band structures, and total and local densities of states. We estimated the spin-exchange splitting energies Δ_x(d) and Δ_x(pd) produced by the Mn3d states, and we found that the effective potential for the minority spin is more attractive than that of the majority spin. We determine the s-d exchange constant N₀α (conduction band) and p-d exchange constant N₀β (valence band) and these somewhat agree with a typical magneto-optical experiment. The value of calculated magnetic moment per Mn impurity atom is found to be 4.08 μ_B for Cd_0.25Mn_0.75Te and 4.09 μ_B for Cd_0.0Mn_1.0Te. Moreover, we found that p-d hybridization reduces the local magnetic moment of Mn from its free space charge value of 5.0 μ_B and produces small local magnetic moments on the nonmagnetic Cd and Te sites.     

Structural and magnetic properties of Co1-xZnxFe2O4 nanoparticles

Co1-xZnxFe2O4 nanoparticles, prepared by the polyvinyl alcohol sol-gel method, have been investigated by x-ray diffraction and MSssbauer spectroscopy. These results are compared with those for the bulk material. The lattice parameters of CoZn ferrite nanoparticles are larger than those of the bulk material. Thermal scanning of MSssbauer measurement shows that the transition temperatures for nanoparticles are higher than those of the bulk material except for the sample CoFe2O4.     

锰位铁掺杂La0.67Sr0.33FexMn1-xO3薄膜的光诱导效应

分别采用溶胶-凝胶法和脉冲激光沉积的方法制备了La_067Sr_033Fe_xMn_1-xO₃(x=0, 005, 010, 015)系列块材和薄膜,研究了Fe部分替代对La_067Sr_033Fe_xMn_1-xO₃薄膜 关键词： 光诱导 Fｅ掺杂 晶格效应     

锰位铁掺杂La0.67Sr0.33FexMn1-xO3薄膜的光诱导效应

分别采用溶胶-凝胶法和脉冲激光沉积的方法制备了La_067Sr_033Fe_xMn_1-xO₃(x=0, 005, 010, 015)系列块材和薄膜，研究了Fe部分替代对La_067Sr_033Fe_xMn_1-xO₃薄膜     

Co50Fe50-xSix合金的结构相变和磁性

利用实验测量和理论计算相结合的方法,研究了介于B2结构CoFe低有序合金和L21结构Co2FeSi高有序合金之间的Co50Fe50-xSix合金的结构相变、磁相变、分子磁矩和居里温度.采用考虑Coulomb相互作用的广义梯度近似（GGA＋U）方法计算了合金的能带结构.研究发现,合金出现较强的原子有序倾向,表现出较强的共价成相作用.合金的晶格常数、磁矩、居里温度随Si含量的增加而线性地降低,极限成分Co2FeSi合金的分子磁矩和居里温度分别达到5.92μB和777℃.原子尺寸效应导致合金晶格发生变化,但并未成为居里温度和分子磁矩变化的主导因素.分子磁矩的变化符合Slater-Pauling原理,但发现原子磁矩的变化并非线性,据此提出了共价成相对磁性影响的观点.采用Stearns理论解释了居里温度的变化趋势,排除了原子间距对居里温度的主导影响作用.能带计算的结果还表明,Co2FeSi作为半金属材料并非十分完美,可能在实际应用中会出现自旋极化率降低的问题.发现该系列合金的结构相变和磁相变随着成分的变化聚集在窄小的成分和温度范围内.