Determination of the surface states from the ultrafast electronic states in a thermoelectric material

We reveal the electronic structure in Yb Cd₂Sb₂,a thermoelectric material,by angle-resolved photoemission spectroscopy(ARPES)and time-resolved ARPES(tr ARPES).Specifically,three bulk bands at the vicinity of the Fermi level are evidenced near the Brillouin zone center,consistent with the density functional theory(DFT)calculation.It is interesting that the spin-unpolarized bulk bands respond unexpectedly to right-and left-handed circularly polarized probe.In addition,a hole band of surface states,which is not sensitive to the polarization of the probe beam and is not expected from the DFT calculation,is identified.We find that the non-equilibrium quasiparticle recovery rate is much smaller in the surface states than that of the bulk states.Our results demonstrate that the surface states can be distinguished from the bulk ones from a view of time scale in the nonequilibrium physics.     

缺陷对单层MoS2电子结构的影响

为了研究缺陷对单层MoS2的电子结构, 本文基于密度泛函理论框架下的第一性原理, 采用数值基组的方法计算了MoS2的Mo位缺陷、S位缺陷的能带结构和态密度.结果发现：Mo位缺陷、S位缺陷的MoS2的能带结构中的价带顶与导带底都在Q点, 为直接带隙材料; 其中Mo位缺陷体的禁带区域都出现5条新能级, S位缺陷体的禁带区域出现了3条新能级; 缺陷体能带结构的能量下降与体系中未成键的电子有关.对于态密度而言, Mo位缺陷体的费米能级处出现了峰值, 表明Mo位缺陷会对其光电性质带来影响.同时分析电荷分布发现, Mo缺陷周围存在着负电荷聚集的现象, S缺陷周围存在正电荷聚集的现象.     

缺陷对单层MoS_2电子结构的影响

为了研究缺陷对单层MoS_2的电子结构,本文基于密度泛函理论框架下的第一性原理,采用数值基组的方法计算了MoS_2的Mo位缺陷、S位缺陷的能带结构和态密度.结果发现:Mo位缺陷、S位缺陷的MoS_2的能带结构中的价带顶与导带底都在Q点,为直接带隙材料;其中Mo位缺陷体的禁带区域都出现5条新能级,S位缺陷体的禁带区域出现了3条新能级;缺陷体能带结构的能量下降与体系中未成键的电子有关.对于态密度而言,Mo位缺陷体的费米能级处出现了峰值,表明Mo位缺陷会对其光电性质带来影响.同时分析电荷分布发现,Mo缺陷周围存在着负电荷聚集的现象,S缺陷周围存在正电荷聚集的现象.     

Unusual electronic structure of Dirac material BaMnSb_2 revealed by angle-resolved photoemission spectroscopy

High resolution angle resolved photoemission measurements and band structure calculations are carried out to study the electronic structure of BaMnSb_2. All the observed bands are nearly linear that extend to a wide energy range. The measured Fermi surface mainly consists of one hole pocket around Γ and a strong spot at Y which are formed from the crossing points of the linear bands. The measured electronic structure of BaMnSb_2 is unusual and deviates strongly from the band structure calculations. These results will stimulate further efforts to theoretically understand the electronic structure of BaMnSb_2 and search for novel properties in this Dirac material.     

Comparison of 1300 nm quantum well lasers using different material systems

The band structure and material gain are calculated for 1300-nm band quantum well lasers of GaInNAs, AlGaInAs and GaInAsP material systems. The material compositions for each system are carefully chosen for comparison. The calculated results show that the peak gain is around the same in spite of the difference in band structures for the three systems.     

Ta掺杂对ZnO光电材料性能影响的研究

采用基于密度泛函理论第一性原理的方法, 研究了Ta掺杂ZnO的电子结构和光学性质. 计算结果表明: 掺入Ta原子后, 费米能级进入导带, 随着掺杂浓度的增加, 带隙逐渐变窄, 介电函数虚部、吸收系数、反射率和折射率均发生明显变化, 介电函数虚部和反射率均向高能方向移动, 吸收边发生红移, 从理论上指出了光学性质和电子结构的内在联系.     

MgCNi3的电子结构、光学性质与超导电性

用第一性原理的密度泛函能带计算方法研究了新近发现的超导体MgCNi3的电子能带结构.计算结果表明其电子结构的基本特征是：Ni的3d态和C的2p态的杂化组成了MgCNi3的导带，费米面附近的物理性质主要由来源于Ni的3d电子态决定.在费米能级(EF)以下30eV的范围内，Ni 3d态构成了能带色散微弱的密集电子态，EF恰好落在Ni 3dyz+zx和3d3z2-r2电子态密度.C 2p态分布在EF以下40—70eV的区域内，Mg主要是以二价离子Mg2+的形式存在.Mg原子的掺杂导致了Ni原子的3d态基本上全部占据，引起Ni原子磁矩的消失.费米能级EF处的态密度N(ＥＦ)是550(states/eV·cell)，由此得到的Sommerfeld常数γeal～445mJ/mol·K2.基于第一性原理的光学性质的计算结果表明：在0—12eV的范围内光吸收主要是从占据的Ni 3d态向C　2p和Ni4s的跃迁.根据这些结果得出结论：MgCNi3的超导电性基本上是强耦合的BCS电子-声子作用机理. 关键词： MgCNi3 高温超导体 电子结构 光学性质     

拓扑相MoS2电子结构及光学性质的第一性原理研究

基于密度泛函理论的第一性原理对二维拓扑相1T′-MoS₂和2M-MoS₂的电子结构、有效质量和光学性质进行研究,并将其与二维H-MoS₂进行对比分析.研究表明,电子有效质量大小关系为：2M-MoS₂22,空穴有效质量大小关系为：T′-MoS₂<2M-MoS₂2,但2M-MoS₂的空穴有效质量和T′-MoS₂相差不大,二者均适用于高性能电子器件.由于拓扑相1T′-MoS₂和2M-MoS₂均存在能带反转,导致带间相关性以及导带和价带的波函数重叠增强,进而光电流响应增强,二者的光学性质均优于H-MoS₂. 2M-MoS₂具有较大的吸收系数和光电导率,2M-MoS₂对红外光和紫外光有着优...     

Pd对Ti合金钝化影响的电子理论研究

采用递归法计算了Ti合金的电子态密度、环境敏感镶嵌能、费米能级等电子结构参量.计算发现Pd在晶体体内比在其表面的环境敏感镶嵌能高,说明Pd易于在 Ti合金表面偏聚.Pd在表面时,原子团簇形成能为负值,说明Pd以团簇形式分布于合金表面.态密度计算结果表明,Pd的局域态密度局限在很窄的能量范围内(-20—-15 eV),使Ti合金的总态密度在此区出现尖峰.该尖峰的存在降低了Ti合金的费米能级,于是表面含Pd较多的区域费米能级较低,含Pd少或不含Pd的区域费米能级较高.费米能级不同的两区域接触会形成微电池,在腐 关键词： Ti合金 钝化 电子结构 表面     

Density-functional theory study on the electronic properties of laves phase superconductor CaIr_2

In this work we have used density-functional theory methods such as full-potential local orbital minimum basis(FPLO) and ELK-flapw to study the electronic structure of newly discovered Laves phase superconductor CaIr_2.The calculation of density of states(DOS) indicates that the bands near Fermi level are mostly occupied by the d-electrons of iridium.The simulation of de Haas-van Alphen(dHvA) effect has been performed by using Elk code to check the Fermi surface topology.The results show that there exist four Fermi surfaces in CaIr_2,including two electron-type and two hole-type surfaces.The optical response properties of CaIr_2 have been calculated in the dipole-transition approximations combined with including intra-band Drude-like terms.In the optical spectrum σ(ω) shows that the crossover from intraband to inter-band absorption occur near 1.45 eV.Further analysis on the electron energy loss spectra(EELS) matches the conclusion from that of optical conductivity σ(ω).     

Surface gap and surface electronic states in CuGeO3 single crystal

The surface and bulk electronic excitations of CuGeO₃ are investigated by means of electron energy loss and polarized X-ray absorption spectroscopy. CuGeO₃ shows a surface charge transfer gap of about 3.0±0.3 eV. The unoccupied oxygen derived density of states, as probed by X-ray absorption at the O 1s edge, is in good agreement with recent many-body calculations.     

Soft x-ray spectroscopy for probing electronic and chemical states of battery materials

The formidable challenge of developing high-performance battery system stems from the complication of battery operations, both mechanically and electronically. In the electrodes and at the electrode–electrolyte interfaces, chemical reactions take place with evolving electron states. In addition to the extensive studies of material synthesis, electrochemical, structural, and mechanical properties, soft x-ray spectroscopy provides unique opportunities for revealing the critical electron states in batteries. This review discusses some of the recent soft x-ray spectroscopic results on battery binder,transition-metal based positive electrodes, and the solid-electrolyte-interphase. By virtue of soft x-ray's sensitivity to electron states, the electronic property, the redox during electrochemical operations, and the chemical species of the interphases could be fingerprinted by soft x-ray spectroscopy. Understanding and innovating battery technologies need a multimodal approach, and soft x-ray spectroscopy is one of the incisive tools to probe the chemical and physical evolutions in batteries.     

不同Yb掺杂浓度的ZnO的电子结构与光学特性

本文基于密度泛函理论(DFT)框架下的第一性原理计算方法,研究了不同Yb浓度掺杂ZnO体系的电子结构和光学性质.计算得到的结果证明,Yb掺杂ZnO后会造成电子结构和光学性质的明显改变.增加掺杂浓度使能带带隙逐渐变窄,其费米能级向上移动到导带,表现出n型半导体的特性;在Yb-4f态导带附近的带隙中产生了新的缺陷,同时观察到更好的吸收系数和折射率.因此,Yb掺杂ZnO对其电子性质和光学结构有很大的影响,为进一步深入了解掺杂ZnO性质的影响提供理论基础.     

Sc在Al-Zn-Mg-Cu超高强铝合金中作用机理的电子理论研究

通过晶胞平移获得Al-Zn-Mg-Cu合金中α-Al,Al₃Sc及η相原子集团模型,采用自编软件建立α-Al/液态Al界面、α-Al/Al₃Sc界面原子团模型.用递归法计算合金中各组织的态密度、结合能、费米能级,合金元素Sc与空位相互作用能等电子参数.依据电子参数解释合金晶粒细化、腐蚀的物理本质.研究表明： Al₃Sc从液态金属析出时释放的能量比α-Al从液态金属析出时所释放的能量少,可先于α-Al从液态金属中析出;且α-Al 关键词： 电子结构 腐蚀 超高强Al合金     

Topological Dirac surface states in ternary compounds GeBi2Te4, SnBi2Te4 and Sn0.571Bi2.286Se4

Using high-resolution angle-resolved and time-resolved photoemission spectroscopy, we have studied the low-energy band structures in occupied and unoccupied states of three ternary compounds GeBi₂Te₄, SnBi₂Te₄ and Sn_0.571Bi_2.286Se₄ near the Fermi level. In previously confirmed topological insulator GeBi₂Te₄ compounds, we confirmed the existence of the Dirac surface state and found that the bulk energy gap is much larger than that in the first-principles calculations. In SnBi₂Te₄ compounds, the Dirac surface state was observed, consistent with the first-principles calculations, indicating that it is a topological insulator. The experimental detected bulk gap is a little bit larger than that in calculations. In Sn_0.571Bi_2.286Se₄ compounds, our measurements suggest that this nonstoichiometric compound is a topological insulator although the stoichiometric SnBi₂Se₄ compound was proposed to be topological trivial.     

Study of titanium adsorption on perfect and defected BC3 nanotubes using density functional theory

Density functional theory calculations were used to study the titanium (Ti) adsorption on perfect and defected (4, 0) BC₃ nanotubes, considering Stone–Wales and vacancy defects. The binding energy values for these nanotubes were larger than the corresponding values for carbon nanotubes. The charge transfer from the Ti atom to nanotube was observed for all systems studied. The most exothermic binding process occurred for the Ti adsorption on a native V_B defect, which showed minimum structural deformation with respect to a perfect BC₃ tube. In the case of a nanotube with a reconstructed carbon vacancy, the adsorption of Ti generated a half-metallic anti-ferromagnet. The results obtained in this paper are relevant for spintronics and hydrogen adsorption applications.     

First-principles calculations of structural and electronic properties of Tl_xGa_(1-x)As alloys

The zincblende ternary alloys Tl_xGa_(1-x) As(0 x 1) are studied by numerical analysis based on the plane wave pseudopotential method within the density functional theory and the local density approximation. To model the alloys,16-atom supercells with the 2 × 2 × 2 dimensions are used and the dependency of the lattice parameter, bulk modulus,electronic structure, energy band gap, and optical bowing on the concentration x are analyzed. The results indicate that the ternary Tl_xGa_(1-x) As alloys have an average band gap bowing parameter of 4.48 eV for semiconductor alloys and 2.412 eV for semimetals. It is found that the band gap bowing strongly depends on composition and alloying a small Tl content with GaAs produces important modifications in the band structures of the alloys.     

2,7-二辛基[1]苯并噻吩并[3,2-b]苯并噻吩/Ni(100)的界面能级结构随薄膜厚度的演化

利用紫外光电子能谱、X射线光电子能谱以及原子力显微镜系统研究了2,7-二辛基[1] 苯并噻吩并[3,2-b]苯并噻吩(C8-BTBT)生长在单晶Ni(100)上的能级结构随着薄膜厚度的演化以及薄膜的生长方式. 发现第一层C8-BTBT平躺生长且与Ni基底发生了化学吸附反应. 从第二层起分子直立生长且呈现岛状生长模式. 这种平躺至直立的分子取向转变, 导致薄膜的能级结构在第一层与第二层间发生阶梯式的变化, 真空能级与最高占据能级同步下降. 此后能带结构随着薄膜厚度的增加逐渐向下弯曲, 功函数随着膜厚的增加而减小. 同时还发现由于直立生长的C8-BTBT其层间电导率较差导致实验中的能级未能收敛. 实验结果提示对基于Ni和C8-BTBT的自旋器件需要插入缓冲层并尽可能减少C8-BTBT的层数.