稀土(La/Ce/Pr/Nd)掺杂锐钛矿相TiO_2磁性及光催化活性的第一性原理研究

采用密度泛函理论下的平面波赝势方法,建立了未掺杂锐钛矿TiO_2和La/Ce/Pr/Nd单掺杂的锐钛矿TiO_2模型.几何优化后,通过计算形成能分析了掺杂结构的稳定性;通过自旋电子态密度的计算分析了各个掺杂模型的磁性状态,并采用比较磁性基态能量的方法对分析结果加以验证;讨论了各稀土元素掺杂对锐钛矿能带结构和吸收光谱的影响.结果表明:La/Pr掺杂的锐钛矿TiO_2具有亚铁磁性,Nd掺杂的锐钛矿具有反铁磁性,Ce掺杂锐钛矿为顺磁体;Ce掺杂对锐钛矿能带结构影响较小,吸收光谱红移不明显,而La/Nd掺杂则能有效提升锐钛矿对可见光的吸收系数,Pr掺杂能使锐钛矿TiO_2在红外光区出现吸收峰.     

第一性原理研究B掺杂Mn_4Si_7的电子结构和光学性质

采用基于密度泛函理论的第一性原理计算方法,对未掺杂及B掺杂Mn_4Si_7的电子结构和光学性质进行理论计算.研究结果表明,未掺杂Mn_4Si_7是间接带隙半导体,其禁带宽度为0.786 eV,B掺杂后其禁带宽度下降为0.723 eV. B掺杂Mn_4Si_7是p型半导体材料.未掺杂Mn_4Si_7在近红外区的吸收系数达到10~5 cm~(-1),B掺杂引起Mn_4Si_7的折射率、吸收系数、反射系数及光电导率增加.     

稀土元素(Ce,Pr)掺杂GaN的电子结构和光学性质的理论研究

采用基于密度泛函理论的第一性原理平面波赝势法计算了稀土元素Ce,Pr掺杂GaN的晶格参数、能带、电子态密度和光学性质,使用LSDA +U的方法处理具有强关联作用的稀土4f态,并分析比较计算结果.计算表明:掺入Ce和Pr后的体系相比未掺杂的GaN晶格常数增大,带隙变窄,并分别在禁带中和价带顶附近引入了局域的杂质能级,该能级主要由Ce和Pr的4f电子提供;掺杂后都发生了磁有序化并产生磁矩;最后定性分析了掺杂前后介电函数和光吸收系数的变化,掺Ce的体系在介电函数和吸收系数的低能区出现了峰值,该峰的出现和禁带中的杂质能级到导带底的跃迁有关,而掺Pr的体系由于带隙变窄使介电峰和吸收边发生红移.     

铁磁性高锰硅化物Mn_4Si_7电子特性的第一性原理计算

本文采用基于第一性原理的密度泛函理论超软雁势平面波方法,对铁磁性半导体高锰硅化合物Mn_4Si_7进行了理论计算.结果表明块体Mn_4Si_7是准直接带隙半导体材料,其价带主要是由Mn的3d轨道电子构成,导带主要是由Mn的3d及Si的3p轨道电子构成.相同自旋轨道下,自旋向下态的电子更容易占据较高的能级.而自旋向上态的电子对Mn_4Si_7的禁带宽度起主导作用. Mn_4Si_7的费米能级附近各轨道未被电子占满,且自旋向上态与自旋向下态电子的不对称分布使其具有了磁性.为Mn_4Si_7磁学特性提供主要贡献的是Mn的3d轨道电子,而Si的3p和3s轨道电子提供了一个小的贡献.     

La,Ce,Nd掺杂对单层MoS_2电子结构的影响

为了研究稀土掺杂对单层MoS2电子结构的影响,文章基于密度泛函理论框架下的第一性原理,采用平面波赝势方法分别计算了本征及La,Ce,Nd掺杂单层MoS2的晶格参数、能带结构、态密度和差分电荷密度.计算发现,稀土掺杂所引起的晶格畸变与杂质原子的共价半径大小有关,La杂质附近的键长变化最大,Nd杂质附近的键长变化最小.能带结构分析表明,La掺杂可以在MoS2的禁带中引入3个能级,Ce掺杂可以形成6个新能级,Nd掺杂可以形成4个能级,并对杂质能级属性进行了初步分析.差分电荷密度分布显示,稀土掺杂可以使单层MoS2中的电子分布发生改变,尤其是f电子的存在会使差分电荷密度呈现出反差极大的物理图象.     

稀土掺杂对LiFePO_4性能影响的第一性原理研究

掺杂是提高LiFePO_4体相电子电导率,优化其电化学性能的重要方法之一.稀土元素因具有高的电子电荷、大的离子半径以及强的自极化能力,成为掺杂改性的重要选择.本文利用基于密度泛函理论的第一性原理方法研究了稀土元素(La,Ce,Pr)掺杂的锂离子电池正极材料LiFePO_4的性质.计算结果表明,稀土元素掺杂均不同程度地增加了 LiFePO_4的晶格常数和晶胞体积.在脱锂过程中,稀土掺杂后材料体积变化率明显减小,材料的循环性能提升,但电池能量密度下降.稀土掺杂使LiFePO_4由原来的半导体特性转变为金属特性,增加了材料的电子电导率.力学特性的计算表明稀土显著增加了 LiFePO_4材料的延展性.另外,La和Ce掺杂后的LiFePO_4在Li离子迁移过程中表现出复杂的能垒变化,在远离稀土离子处迁移势垒呈现出不同程度的减小,而在靠近稀土离子处迁移势垒起伏较大.与Ce掺杂相比,La掺杂造成的离子迁移势垒的变化程度更大,表明稀土离子掺杂对体系局域结构产生较大的影响.     

稀土X (X=La、Ce、Pr、Nd)修饰单层h-MoS2的NO气敏性

为提高h-MoS₂对NO的气敏性,采用第一性原理研究了X(X=La、Ce、Pr和Nd)对h-MoS₂的稳定性、吸附特性、功函数和伏安特性的影响.研究结果表明：X(X=La、Ce、Pr和Nd)取代Mo原子后得到的形成能均为负,说明掺杂体系容易形成且稳定存在.同时X掺杂后的h-MoS₂布居数相比于未掺杂前的大,也说明掺杂有利于体系稳定.NO吸附在La、Ce原子顶部位置的吸附能为-1.215 eV、-1.225 eV,吸附距离分别为2.475?、2.854?,具有明显的化学吸附特征.同时Hirshfeld转移电荷分别为0.213e和0.325e,具有明显的受体特征,提高了气敏性.La和Ce掺杂后能明显改变功函数的大小,也说明La和Ce能提高h-MoS₂对NO的气敏性.施加电场后能有效提高Pr掺杂体系的吸附强度,增大Hirshfeld转移电荷,对功函数的改变最大,所以电场对Pr掺杂体系的影响非常明显.通过分析掺杂体系的伏安特性曲线,La、Ce两掺杂体系的电流分别由0增大到5.3μA和4.8μA,而Pr、Nd...     

Pr3+或Sm3+掺杂YAG:Ce的发光特性及其荧光寿命

研究了Pr3 ,Sm3 掺杂对YAG:Ce发射光谱及其荧光寿命的影响.观察到当掺杂Pr3 时,在609 nm处出现Pr3 的发射峰,而掺杂Sm3 时,在616 nm处呈现Sm3 的发射峰.掺杂Pr3 或Sm3 增加红光区的发射峰将有利于提高YAG:Ce荧光粉的显色性.实验中测定了(Y0.95Sm0.01Ce0.04)3Al5O12、(Y0.95Pr0.01Ce0.04)3Al5O12、(Y0.96Ce0.04)3Al5O12的荧光寿命(τ),观察到在YAG:Ce中掺入Pr3 或Sm3 使Ce3 的荧光寿命减小.实验结果表明,少量掺杂Pr3 或Sm3 并未引起基质的结构的变化.     

锂离子电池正极材料Li_2MnO_3稀土掺杂的第一性原理研究

掺杂是锂离子电池电极材料优化改性的一种有效的方法.稀土元素因其具有高的电子电荷、大的离子半径以及强的自极化能力,成为掺杂改性的重要选择.本文利用基于密度泛函理论的第一性原理方法研究了稀土元素(La, Ce, Pr, Sm)掺杂的锂离子电池正极材料Li_2MnO_3的性质.通过稀土元素的掺杂, Li_2MnO_3材料的晶格常数和晶胞体积都有不同程度的增大.由于稀土原子的价态不同,导致掺杂后的Li_2MnO_3的电子结构性质不同. La掺杂的Li_2MnO_3表现出金属性,而Ce, Pr, Sm掺杂的结构为半导体性质,但带隙与未掺杂情况下相比有所减小. Li_2MnO_3中的Li离子迁移在La和Ce掺杂后展示出复杂的能垒变化.在远离稀土离子处,Li离子迁移势垒比未掺杂时减小,但在靠近稀土离子处则表现为势垒变化的多样性.当Li离子在离稀土离子最近的位置处进行迁移,势垒有明显的增加,这一结果与稀土离子周围的局域结构变化大密切相关.     

Pr^3＋或Sm^3＋掺杂YAG∶Ce的发光特性及其荧光寿命

研究了Pr3 ,Sm3 掺杂对YAG∶Ce发射光谱及其荧光寿命的影响。观察到当掺杂Pr3 时,在609nm处出现Pr3 的发射峰,而掺杂Sm3 时,在616nm处呈现Sm3 的发射峰。掺杂Pr3 或Sm3 增加红光区的发射峰将有利于提高YAG∶Ce荧光粉的显色性。实验中测定了(Y0.95Sm0.01Ce0.04)3Al5O12、(Y0.95Pr0.01Ce0.04)3Al5O12、(Y0.96Ce0.04)3Al5O12的荧光寿命(τ),观察到在YAG∶Ce中掺入Pr3 或Sm3 使Ce3 的荧光寿命减小。实验结果表明,少量掺杂Pr3 或Sm3 并未引起基质的结构的变化。     

Chiral perturbation theory for fDS/FD and BBS/BB

The decay constants for D and D_s mesons, denoted f_D and f_DS respectively, are equal in the SU(3)_V limit, as are the hadronic amplitudes for and mixing. The leading SU(3)_V violating contribution to (F_DS/F_D) and to the ration of hadronic matrix elements relevant for and mixing amplitudes are calculated in chiral perturbatiion theory. We discuss the formalism needed to include both meson and anti-meson fields in the heavy quark effective theory.     

Magnetism in nanometer-thick TiOx/Co/TiOx/TiN/Si multilayered structures

Most studies on Co-doped TiO₂ system were focused on thin films grown by MBE-based methods. In this work we report the ferromagnetism of nanometer-thick-layered TiO₂/Co/TiO₂/TiN film grown on Si substrate by conventional magnetron sputtering. For the growth of TiO₂ on silicon, a non-oxide thermally stable material, TiN, was introduced to prevent Ti penetration into the Si substrate. Structural, magnetic, and transport measurements respectively by Raman, SQUID and Hall effect show that our samples are n-type semiconductors and exchange bias effect due to exchange coupling between Co and interfacial CoO. For the rapid vacuum annealed specimen, we found an enhanced loss and a Perminvar-type constricted hysteresis loop, which attributed to pinning of domain walls due to an induced anisotropy by the pair ordering in the metallic alloy of Co-Ti-Si.     

Electrical property of HfOxNy-HfO2-HfOxNy sandwich-stack films

The effects of HfO_xN_y on the electrical property of HfO_xN_y-HfO₂-HfO_xN_y sandwich-stack (signed as SS) films were investigated. Excellent electrical performances were achieved in SS films, with a high dielectric constant of 16 and a low leakage current of ∼2 × 10⁻⁸ A/cm² at 1 MV/cm. Schottky (SK) emission and Frenkel-Poole (PF) emission are found to be the dominant mechanisms for the current conduction behavior. After a long time stress, the flat-band voltage shift in the SS film is much smaller than that in a pure HfO_xN_y film indicating fewer charge traps existed in the SS film. Based on the experiments, the new SS structure is more favorable for the improvement of electrical performances than a pure HfO_xN_y or HfO₂ structure.     

Electronic and magnetic structures of V-doped zinc blende Zn1-xVxNyO1-y and Zn1-xVxPyO1-y

Electronic and magnetic structures of zinc blende ZnO doped with V impurities are studied by first-principles calculations based on the Korringa-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA).Calculations for the substitution of O by N or P are performed and the magnetic moment is found to be sensitive to the N or P content.Furthermore,the system exhibits a half-metallic band structure accompanied by the broadening of vanadium bands.The mechanism responsible for ferromagnetism is also discussed and the stability of the ferromagnetic state compared with that of the paramagnetic state is systematically investigated by calculating the total energy difference between them by using supercell method.     

Different temperature dependence of carrier transport properties between AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures

The temperature dependence of carrier transport properties of AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures has been investigated.It is shown that the Hall mobility in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures is higher than that in Al0.25Ga0.75N/GaN heterostructures at temperatures above 500 K,even the mobility in the former is much lower than that in the latter at 300 K.More importantly,the electron sheet density in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures decreases slightly,whereas the electron sheet density in Al0.25Ga0.75N/GaN heterostructures gradually increases with increasing temperature above 500 K.It is believed that an electron depletion layer is formed due to the negative polarization charges at the InyGa1-yN/GaN heterointerface induced by the compressive strain in the InyGa1-yN channel,which e-ectively suppresses the parallel conductivity originating from the thermal excitation in the underlying GaN layer at high temperatures.     

Spinor techniques for calculating pp → W±/Z0 + jets

We present techniques which enable one to calculate quickly the amplitudes for many scattering processes in the high-energy limit. As an illustration of the method, these are applied to the diagrams for $\text{p}\text{p}\text{→}\text{V}\text{+ 0, 1}\text{or}\text{2}\text{jets}$, where V = W^± or Z⁰. The form of the results lends itself to immediate numerical evaluation.     

Near-infrared bands of S2: 3Πgi-3Σu+ system

Vibrational and rotational analyses of the near-infrared bands of S₂ lying in the region 7440–8085 Å are reported. They form a new band system involving a ${}^3\text{Π}{}_{\mathrm{gi}}\text{-}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ transition and arise from the same initial ³Π_gi state of the ³Π_gi-³Δ_ui band system reported earlier. The analyses of the bands of this system due to the isotopic molecules ³²S³⁴S and ³⁴S₂ are also reported.     

Spin-glass-like behavior in ZnxCryAlzSe4

The magnetic and electrical properties of the Al-doped polycrystalline spinels Zn_xCr_yAl_zSe₄ (0.13≤z≤0.55) with the antiferromagnetic (AFM) order and semiconducting behavior were investigated. A complex antiferromagnetic structure below a Néel temperature T_N≈23 K for the samples with z up to 0.4 contrasting with the strong ferromagnetic (FM) interactions evidenced by a large positive Curie-Weiss temperature θ_CW decreasing from 62.2 K for z=0.13 to 37.5 K for z=0.55 was observed. Detailed investigations revealed a divergence between the zero-field-cooling (ZFC) and field-cooling (FC) susceptibilities at temperature less than T_N suggesting bond frustration due to competing ferromagnetic and antiferromagnetic exchange interactions in the compositional range 0.13≤z≤0.4. Meanwhile, for z=0.55 a spin-glass-like behavior of cluster type with randomly oriented magnetic moments is observed as the ZFC-FC splitting goes up to the freezing temperature T_f=11.5 K and the critical fields connected both with a transformation of the antiferromagnetic spin spiral via conical magnetic structure into ferromagnetic phase disappear.     

The decay processes Λ8Li → π− + 8Be1 (∼ 17 MeV)

The branching ratio is calculated for _Λ⁸Li decay to the (2⁺) ⁸Be¹ states near 17 MeV, using intermediate coupling wave functions for _Λ⁸Li and for the relevant ⁸Be¹ states. It is pointed out that this ratio is sensitive primarily to a mixing angle ? in the _Λ⁸Li wave function. Within one standard deviation, the data allow two ranges (+0.05 to +0.25 rad and +1.10 to +1.25 rad) for the value of ?. The further requirement that there also be acceptable agreement between the angular distribution expected for the subsequent ${}^8\text{Be}{}^1\text{(? 17}\text{MeV → 2}{}^4\text{He}$ decay and the data, shifts these allowed ranges for ?, to (+0.13 to 0.40) rad and (+0.9 to +1.2) rad. It is predicted that the dominant transition should be to ⁸Be¹ (16.6 MeV), as is observed to be the case, rather than to ⁸Be¹ (16.9 MeV). The interpretation of these values for ? is discussed in some detail and their implications for intermediate coupling shell-model calculations of Λ-hypernuclear wave functions are considered.     

The 5T2g → 5Eg absorption in MgO: Fe2+

At helium temperatures two sharp lines at 9350 and 9510 cm^?1 have been observed for the first tune on the low-energy side of the broad double-peaked absorption corresponding to the ${}^5\text{T}{}_{\mathrm{2g}}\text{→}{}^5\text{E}{}_{\mathrm{g}}$ transition in Fe²⁺ at the octahedral site in MgO. The lower energy line has a half width of 4 cm^?1; Zeeman measurements show that it is of magnetic dipole origin. The Zeeman spectra are consistent with those expected for a pure electronic transition from the (⁵T_2g)T_2g ground state to the ⁵E_g excited state. The second line, with a halfwidth of ~ 35 cm^?1, a vibrational sideband.