潜在超导母体材料LaOMnSe的第一性原理研究

通过第一性原理计算,我们研究了LaOMnSe的结构、电子性质和磁性质．我们发现LaOMnSe在11点附近存在三个类空穴费米面,在M点附近有两个类电子费米面．如果将类空穴费米面平移（π,π,O）,则类空穴费米面就会和类电子费米面在很大程度上重叠,这种费米面嵌套将会导致磁不稳定性和自旋密度波（SI）、V）,和LaOFeAs...     

二能带交迭高温超导理论的Van－Hove奇异性与超导转变温度和反常同位素效应

本文利用二交迭能带理论，计算费米面附近电子态密度的Ｖａｎ－Ｈｏｖｅ奇异性对超导转变温度和同位素指数因子的影响，计算结果表明：费米面附近电子态密度的Ｖａｎ－Ｈｏｖｅ奇异性能同时提高超导转变温度和降低同位素指数因子，这是普通的ＢＣＳ理论不能解释的［３］．     

GaN及其Ga空位的电子结构

用团簇埋入自洽计算法对宽禁带半导体GaN的电子结构进行了自旋极化的、全电子、全势场的从头计算,得到了与实验值符合的GaN晶体禁带宽度以及价带中N2p带、N2s带和Ga3d带之间的相对位置.在此基础上Ga空位计算(无晶格畸变)显示,Ga空位周围的费米面显著高于正常GaN晶格的费米面.因此Ga空位周围N原子的处于费米面上的2p电子很易被激发成正常晶格处的传导电子 关键词： GaN 电子结构 团簇埋入自洽计算     

二维应变作用下超导薄膜LiFeAs的磁性和电子性质

基于密度泛函理论的第一性原理计算,研究了二维应变作用下LiFeAs超导薄膜的磁性结构、电子能带和态密度变化,分析了应变对其超导电性的作用.结果显示,对体系施加1%—6%的二维平面张、压应变均不改变其基态条形反铁磁性结构,费米面附近的电子态密度主要来自于Fe-3d轨道电子以及少量的As-4p电子.研究发现,与无应变情形相比,当施加压应变时,体系中Fe离子的反平行的电子自旋局域磁矩减小,薄膜反铁磁性受到抑制,费米面上电子态密度增加,超导电性来自于以反铁磁超交换耦合作用为媒介的空穴型费米面和电子型费米面间嵌套的Cooper电子对.而在张应变作用时,局域反铁磁性增强,费米面上电子态密度减小,金属性减弱,特别是张应变时费米面上空穴型能带消失, Cooper电子对出现概率显著降低,将抑制超导相变.     

石墨烯能带中的重叠矩阵效应:Tight-Binding方法在模拟中的研究

采用紧束缚方法计算了石墨烯的价带(π)和导带(π*),考虑了非正交基矢下重叠矩阵效应,重叠积分参量s越小,导带越靠近费米面,而价带越远离费米面.在重叠积分参量s≤0.1时,基本保持了原子在实际空间中重叠所引起的能带的改变,太大(s=0.4)则会导致物理上失效.计算了石墨烯的能态密度,在费米面ε=0处(对应Dirac点)的能态密度为零,并且在Dirac点附近呈线性变化.     

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     

手性ABM态p波超导体上临界磁场的角依赖关系

利用格林函数方法,通过Klemm-Clem变换,计算具有正交晶格结构椭球形费米面的手性ABM态p波超导体上临界磁场的角依赖关系.超导序参量选取具有手性ABM对称性的等自旋配对单分量形式.当椭球形费米面满足一定条件时,上临界磁场随角度呈现非单调变化,表明除配对电子有效质量的各向异性外,手性ABM态p波超导体也具有超导序参量的各向异性.计算结果可用于判断重费米子超导体Sr2RuO4的配对电子空间结构.     

近反铁磁费米液体理论的推广

把主同温超导体的唯象近反铁磁费米液体理论作了推广，得到了自旋磁化率对整个频率范围成立。因此，原来理论中的低频限制被去掉，计算了推广的ＮＡＦＬ理论中的电子自能，发现类似于边缘费米液体理论，准粒子的谱权重随着趋向于费米面以对数的形式趋于零。     

新型铁基超导体角分辨光电子能谱研究

最近发现的新型122结构的铁基超导体掀起了铁基高温超导研究的又一轮热潮.文章利用角分辨光电子能谱实验手段,研究了这类新型铁基材料的电子结构、费米面拓扑和超导能隙.实验结果表明,其在布里渊区中心的能带结构及费米面与其他铁基超导体存在明显差异,并导致嵌套在粒子-空穴通道的费米面消失.另外,在布里渊区边缘的电子型费米面发现了较强的并且有各向同性的超导能隙.这些结果对可能的超导配对机制提出了严格的限制.     

铁基超导体的角分辨光电子能谱研究进展

最近发现的超导转变温度高达55K铁基高温超导体结束了铜氧化物在超导转变温度高于40K的领域一统天下的局面.与铜氧化物高温超导体一样,超导配对对称性对于理解这一新的体系有着重要的作用.文章利用角分辨光电子能谱实验手段,全面地研究了铁基材料的能带结构和费米面以及它们随载流子掺杂浓度变化的演化过程,发现了铁基超导体中依赖费米面的无节点的超导能隙,指出了费米面间的相互作用对超导配对起着关键作用.     

Relative energies of surface and defect states: ab initio calculations for the MgO (001) surface

We present the results of calculations of the energy levels of defects at the (001) surface of MgO relative to the top of the valence band and values of defect ionisation potentials and electron affinities. The calculations were made using an embedded cluster method in which a cluster of several tens of ions treated quantum mechanically is embedded in a finite array of polarisable and point ions modelling the crystalline potential and the classical polarisation of the host lattice. The calculated ionisation potential of the ideal surface, which fixes the position of the top of the valence band with respect to the vacuum level, is about 6.7 eV. This value is used as a reference for positioning the energy levels of three charge states of a surface anion vacancy, which are also calculated as ionisation energies with respect to the vacuum level. The surface and vacancy electron affinities are calculated using the same method. As a prototype low-coordinated surface site, we have considered a cube corner. Our calculations predict the splitting of the corner states from the top of the surface valence band by about 1.0 eV. Both unrelaxed and relaxed holes are strongly localised at the corner oxygen ion. The ionisation energies and electron affinities of the corner anion vacancy are calculated. The electrons in the F and F⁺ centres at the corner are shown to be significantly delocalised over surrounding Mg ions.     

Surface properties of Si from photoelectric emission at room temperature and 80 °K

The position of the Fermi level with respect to the energy bands at a semiconductor surface as well as changes in the work function can be determined from the energy distributions of photoelectrically emitted electrons. Prior studies involved the photoelectric yield spectrum and required assumptions concerning the photoelectric threshold; the present method is free of such assumptions. Measurements at room temperature indicate that the Fermi level lies 0.23 eV and 0.41 eV above the top of the valence band for degenerate p and n-type materials, respectively. These results confirm those of Allen and Gobeli¹). Cooling to 80 °K increases the work function of p-type material by 0.025 eV while that of n-type Si remains unchanged; the results show that the electron and hole gases in the surface states are degenerate. The density of surface states lies between 7 × 10¹³ and 10¹⁵ eV⁻¹ cm⁻². On the cesiated surface, the Fermi level lies 0.16 eV below the conduction band at room temperature and coincides with its bottom at 80 °K.     

Indication of charge-density-wave formation in Bi(111)

Photoemission spectroscopy of Bi(111) reveals a small hexagonal two-dimensional Fermi surface (FS) associated with an electron band centered in the surface Brillouin zone. Along the hexagon the Fermi momentum k(F) ranges from 0.053 to 0.061 A(-1). Temperature dependent valence band spectra show an anisotropic energy gap Delta near the Fermi level. We find a transition temperature of about 75 K. At 11 K, the gap is Delta=4 meV at the corner and Delta=7.5 meV at the side of the hexagon. Arguments based on susceptibility chi(--> q) calculations of a hexagonal FS are used to discuss an incommensurate charge-density-wave (CDW) formation associated with a q(CDW)=0.106 A(-1).     

A new approach for a multistage depressed collector for gyrotrons

A feasibility study for a two-stage depressed gyrotron collector has been performed. A new approach for an adiabatic magnetic decompression of the hollow electron beam has been used. It permits control of the radius of the constant magnetic flux surface, which determines the radial extension of the electron beam. Independent of the value of the magnetic field around the beam. For this purpose, either solenoidal coils or a ferromagnetic insert can be placed inside the hollow electron beam. Thus, the radial dimensions of a multistage depressed collector of a high-power high-frequency gyrotron can be kept within limits given by technological constraints. The energy sorting of the electron beam is improved by using electrodes inside the hollow electron beam for controlling the potential distribution. The additional control electrodes make it possible to eliminate almost all of the effect of secondary electrons on the operation of the collector. In order to demonstrate the proposed approach, a compact two-stage depressed collector has been designed for a 1.5-MW coaxial cavity gyrotron operating at 165 GHz in the transverse electric (TE)_31,17 mode, which is under development at FZK, Karlsruhe, Germany. Including the effect due to secondary electrons, a collector efficiency of 73% has been calculated with an average and peak heat dissipation density of about 240 W/cm² and 500 W/cm², respectively. This results in an increase of the output gyrotron efficiency from 36.5% to 62.6% when internal radio frequency (RF)-losses inside the gyrotron tube of 15% are taken into account     

Ion-induced secondary electron and negative ion emission from Mo/O

Absolute yields of secondary electrons and negative ions resulting from collisions of Na⁺ with Mo(100) and a polycrystalline molybdenum surface have been measured as a function of the oxygen coverage of the surface for impact energies below 500 eV. The sputtered negative ions have been identified with mass spectroscopy, and O⁻ is found to be the dominant sputtered negative ion for the surfaces at all oxygen coverages and impact energies. Both the electron and O⁻ yields have an impact energy threshold at about 50 eV and exhibit a strong dependence on oxygen coverage. The kinetic energy distributions of the secondary electrons and sputtered O⁻ were determined as functions of the oxygen coverage and impact energy. The distributions for O⁻ are characterized by a narrow low-energy peak (at 1–2 eV) followed by a low-level high-energy tail. The secondary electrons have a narrow (FWHM 1–2 eV) kinetic energy distribution, centered approximately at 1–2 eV. The shapes of the distributions and their most probable energies are essentially invariant with impact energy, oxygen coverage and the nature of the Mo surface. The emission is explained and analyzed in terms of a simple model which involves a collision-induced electronic excitation of the MoO⁻ surface state. The decay of this excited state leads to the production of both secondary electrons and O⁻ with energy distributions and yields comparable to those observed.     

Realization of an interacting two-valley AlAs bilayer system

By using different widths for two AlAs quantum wells comprising a bilayer system, we force the X-point conduction-band electrons in the two layers to occupy valleys with different Fermi contours, electron effective masses, and g factors. Since the occupied valleys are at different X points of the Brillouin zone, the interlayer tunneling is negligibly small despite the close electron layer spacing. We demonstrate the realization of this system via magnetotransport measurements and the observation of a phase-coherent, bilayer nu=1 quantum Hall state flanked by a reentrant insulating phase.     

Deplacements du niveau de Fermi de composes d'insertion sodium-pyrocarbone-bore

The Fermi level of pyrolytic carbons is displaced through boron doping and intercalation and de-intercalation of sodium. Diamagnetic anisotropy and Hall effect measurements show that, whatever the initial doping level, the intercalation of sodium proceeds until the Fermi level is raised about 3 eV above the Brillouin zone corner. The average ionization of sodium atoms is estimated at 0.2−0.4 electronic charge. As theoretically predicted the diamagnetic anisotropy is maximum when the Fermi level is at the Brillouin zone corner.     

Origin of the blueshift in the infrared absorbance of intersubband transitions in N/GaN multiple quantum wells

A self-consistent calculation of the subband energy levels of n-doped quantum wells is studied. A comparison is made between theoretical results and experimental data. In order to account for the deviations between them, the ground-state electron–electron exchange interactions, the ground-state direct Coulomb interactions, the depolarization effect, and the exciton-like effect are considered in the simulations. The agreement between theory and experiment is greatly improved when all these aspects are taken into account. The ground-to-excited-state energy difference increases by 8 meV from its self-consistent value if one considers the depolarization effect and the exciton-like effect only. It appears that the electron–electron exchange interactions account for most of the observed residual blueshift for the infrared intersubband absorbance in Al_xGa_1-xN/GaN multiple quantum wells. It seems that electrons on the surface of the k-space Fermi gas make the main contribution to the electron–electron exchange interactions, while for electrons further inside the Fermi gas it is difficult to exchange their positions.     

InAlN/GaN异质结二维电子气波函数的变分法研究

使用变分法推导了InAlN/GaN异质结二维电子气波函数和基态能级的解析表达式,并讨论了InAlN/GaN异质结结构参数对二维电子气电学特性的影响.在假设二维电子气来源于表面态的前提下,使用了一个包含两个变分参数的尝试波函数推导电子总能量期望值,并通过寻找能量期望极小值确定变分参数.计算结果显示,二维电子气面密度随InAlN厚度的增大而增大,且理论结果与实验结果一致.二维电子气面密度增大抬高了基态能级与费米能级,并保持二者之差增大以容纳更多电子.InAlN/GaN界面处的极化强度失配随着In组分增大而减弱,二维电子气面密度随之减小,并导致基态能级与费米能级减小.所建立的模型能够解释InAlN/GaN异质结二维电子气的部分电学行为,并为电子输运与光学跃迁的研究提供了解析表达式.     

空间电子辐照聚合物的充电特性和微观机理

空间电子辐照聚合物的充电特性和微观机理是研究和防护航天器聚合物充放电特性的基础.采用蒙特卡罗方法模拟空间电子的散射过程,快二次电子模型模拟二次电子的产生,有限差分法求解电荷连续性方程、电流密度方程和泊松方程的电荷输运过程,俘获过程基于Poole-Frenkel效应来实现.基于电子散射/输运同步模型基础,结合法国国家航空航天科研局(ONERA)的地球同步轨道电子能谱分布理论公式和欧空局(SIRENE)机构的地面实验方法,建立了基于地球同步轨道电子能谱分布的空间多能电子的散射模型.通过空间电子辐照聚合物充电过程的数值模拟,获得了空间电荷密度、电位、电场和空间电位分布.阐明了空间电子辐照聚合物的充电特性和样品微观参数与表面电位的关联性.表面电位特性与实验结果相吻合,单能电子的电位强度高于多能电子的电位.充电达到稳态时,电子迁移率较小时(小于10~(–11)cm~2·V~(–1)·s~(–1)),空间电位绝对值随电子迁移率的降低明显加强;复合率较大时(大于10~(–14)cm~3·s~(–1)),空间电位绝对值随复合率的增大而增大.研究结果对于揭示空间电子辐照聚合物的充电特性和微观机理、提高航天器充放电故障机理研究水平具有重要科学意义和价值.