Parameter dependence of optical conductivity in antiferromagnetic phase of iron pnictides

We investigate the optical conductivity of iron pnictides in antiferromagnetic state by mean-field calculation in a five-band Hubbard model, focusing on its anisotropic behavior by examining several states calculated with different Hund coupling J, such as the states with a low or high magnetization and with or without a strong orbital ordering. In addition, we investigate the J dependence of the Dirac cone structure, which is crucial for the low energy excitation. In our calculations, a weakly ordered state with no orbital ordering shows the anisotropy of optical conductivity in accord with experiments. We conclude that the low energy part of the optical conductivity is relevant to the Dirac electron structure rather than the orbital ordering.     

The metal-insulator transition and the phase transition in metal-ammonia solutions

The ground state of impurity metal (sodium) atoms in liquid ammonia close to the solvated state of the free electrons is considered. It is shown that the critical solubility point lying on the metal side of the metal-insulator transition is determined by the Coulomb interaction between the ions and electrons in the overlapping impurity states, classically accessible spheres of which form an infinite percolation cluster. The percolation conductivity via the impurity states is estimated. The estimate agrees with the experimental data near the critical solubility point. Zh. éksp. Teor. Fiz. 111, 938–948 (March 1997)     

低补偿度n-Hg1-xCdxTe的磁致金属-绝缘体相变和相变后的温度激活输运行为

报道了ｘ＝０．２１４组份、低补偿度（Ｋ《１）ｎ－Ｈｇ_1-xＣｄ_xＴｅ晶体在０．３─３０Ｋ温度范围，０─７Ｔ强磁场下的横向磁阻、电子霍耳迁移率、霍耳系数测量结果，观测到了磁致金属－绝缘体相变和相变后的温度激活输运行为。分析实验数据，提出：低补偿度、组份：ｘ＝０．２附近的ｎ－Ｈｇ_1-xＣｄ_xＴｅ，磁致金属－绝缘体相变（ＭＩＴ）发生的机理是载流子在浅施主杂质态上的磁冻结；发生磁冻结的前提是热冻出（ｔｈｅｒｍａｌ ｆｒｅｅｚｅ 关键词：     

Inelastic tunnel transport of electrons through an anisotropic magnetic structure in an external magnetic field

Quantum transport of electrons through a magnetic impurity located in an external magnetic field and affected by a substrate is considered using the Keldysh diagram technique for the Fermi and Hubbard operators. It is shown that in a strongly nonequilibrium state induced by multiple reflections of electrons from the impurity, the current-voltage (I–V) characteristic of the system contains segments with a negative conductivity. This effect can be controlled by varying the anisotropy parameter of the impurity center as well as the parameters of coupling between the magnetic impurity and metal contacts. The application of the magnetic field is accompanied by an increase in the number of Coulomb steps in the I–V curve of the impurity. The effect of appreciable magnetoresistance appears in this case. We demonstrate the possibility of switching between magnetic impurity states with different total spin projection values in the regime of asymmetric coupling of this impurity with the contacts.     

Energy spectrum of one-particle excitations in liquid dielectrics under high pressures and temperatures

The effect of pressure on the conductivity of molecular liquids (hydrogen, oxygen, and nitrogen) and alkali metals (cesium and rubidium) in the region of the experimentally observed dielectric-metal transition is investigated. It is shown that capture of free electrons by atoms or molecules (resulting in the formation of negative ions) is advantageous from the point of view of energy in liquids under moderately high pressures and temperatures. In spite of the fact that the ionization potential increases with density, the energy of an electron transition to the level of the negative ion decreases and, hence, the forbidden gap also decreases. For high densities, the level of negative ions is broadened and transformed into the conduction band. It is assumed that the exponential dependence of conductivity on density and temperature in the transition region is associated with transfer of quasi-free electrons located at the level of atomic negative ions. The spectrum of negative ions of hydrogen, oxygen, and cesium in the strongly compressed state is determined, and the forbidden gap calculated for these substances is found to be in good agreement with the results obtained for hydrogen and oxygen in single shock-wave experiments.     

Forbidden optical transitions between impurity levels in silicon and gallium phosphide

Experimental estimates are made of absorption cross sections for forbidden optical transitions from the ground state to long-lived excited states of P, As, Sb, In, and Ga impurities in silicon and Te impurities in gallium phosphide. The results can be used to predict the possibility of long-wavelength stimulated emission being excited as a result of the population inversion of long-lived impurity states in these materials.     

Bound states of Dirac fermions in monolayer gapped graphene in the presence of local perturbations

In graphene,conductance electrons behave as massless relativistic particles and obey an analogue of the Dirac equation in two dimensions with a chiral nature.For this reason,the bounding of electrons in graphene in the form of geometries of quantum dots is impossible.In gapless graphene,due to its unique electronic band structure,there is a minimal conductivity at Dirac points,that is,in the limit of zero doping.This creates a problem for using such a highly motivated new material in electronic devices.One of the ways to overcome this problem is the creation of a band gap in the graphene band structure,which is made by inversion symmetry breaking(symmetry of sublattices).We investigate the confined states of the massless Dirac fermions in an impured graphene by the short-range perturbations for "local chemical potential" and "local gap".The calculated energy spectrum exhibits quite different features with and without the perturbations.A characteristic equation for bound states(BSs) has been obtained.It is surprisingly found that the relation between the radial functions of sublattices wave functions,i.e.,f_m~+(r),g_m~+(r),and f_m~-(r),g_m~-(r),can be established by SO(2) group.     

Screening of Local Magnetic Moment by Electrons of Disordered Graphene

Based on the Anderson impurity model and self-consistent approach, we investigate the condition for the screening of a local magnetic moment by electrons in graphene and the influence of the moment on electronic properties of the system. The results of numerical calculations carried out on a finite sheet of graphene show that when the Fermi energy is above the single occupancy energy and below the double occupancy energy of the local impurity, a magnetic state is possible. A phase diagram in a parameter space spanned by the Coulomb energy U and the Fermi energy is obtained to distinguish the parameter regions for the magnetic and nonmagnetic states of the impurity. We find that the combined effect of the impurity and finite size effect results in a large charge density near the edges of the finite graphene sheet. The density of states exhibits a peak at the Dirac point which is caused by the appearance of the edge states localized at the zigzag edges of the sheet.     

Scattering mechanisms and anomalous conductivity of heavily N-doped 3C-SiC in ultraviolet region

Using the first-principles density functional method, we investigate the band structures and conductivity spectra for N-doped 3C-SiC. It is found that conductivity peaks of heavily N-doped 3C-SiC are observed in the ultraviolet (UV), visible and infrared (IR) regions while the peaks can be only seen in the UV region for 3C-SiC. In the UV region, the conductivity peaks of 3C-SiC are obviously higher than those of N-doped 3C-SiC. According to the data of band structures, we calculate the ionized impurity scattering, inter-carrier scattering and neutral impurity scattering. The calculation results show that the scattering by incomplete ionization N to electrons and inter-carrier scattering have large effect on the conductive behavior of heavily N-doped 3C-SiC at room temperature. In the UV region, the conductivity of 3C-SiC depends on long-wavelength optical wave scattering, which has a longer relaxation time than that inter-carrier scattering and neutral scattering. This is the reason of anomalous conductivity of N-doped 3C-SiC in the UV region.     

Anomalous photon-assisted tunneling in graphene

We investigated the transmission of Dirac electrons through a potential barrier in the presence of circularly polarized light. An anomalous photon-assisted enhanced transmission is predicted and explained. It is demonstrated that the perfect transmission for nearly head-on collision in infinite graphene is suppressed in gapped dressed states of electrons, which is further accompanied by a shift of peaks as a function of the incident angle away from head-on collision. In addition, the perfect transmission is partially suppressed by a photon-induced gap in illuminated graphene. After the effect of rough edges of the potential barrier or impurity scattering is included, the perfect transmission with no potential barrier becomes completely suppressed and the energy range for the photon-assisted transmission is reduced at the same time.     

Effect of an impurity on 3-electron quantum dots in magnetic fields

1 Introduction Quantum dots (QDs), often referred to as artificial atoms, are currently under in-tense study because they provide ideal structures used in optical-electronic microdevices, so they are essential in developing microtechniques. They are also essential in the aca-demic aspect, because rich information on microstructures can be extracted both theo-retically and experimentally. Since the early fabrication of the QDs, external magnetic field has been used to control their propertie…     

电子-杂质和电子-声子相互作用对石墨烯磁光导的影响

在外加垂直磁场的石墨烯系统中,基于格林函数方法以自能的形式理论研究了电荷杂质散射和光学声子散射中心对朗道能谱的影响,采用久保(Kubo)公式研究了单层石墨烯的磁光电导谱以及跃迁选择定则。具体计算中电子-杂质库仑相互作用考虑了介电环境的屏蔽效应,对由散射引起的自能以及单粒子格林函数做自洽计算,另外在强磁场下单杂质散射是一个很好的近似模型。理论计算结果表明电荷杂质散射引起朗道能级对称展宽;同时考虑电荷杂质和光学声子两类散射后态密度表现为非对称的展宽。研究结果表明磁光电导谱的峰值和强度强烈依赖于填充因子和态密度。     

Ti和Al共掺杂ZnS的电子结构和光学性质

基于密度泛函理论的第一性原理研究Ti和Al单掺杂和(Ti,Al)共掺杂ZnS的能带结构、电子态密度分布、介电函数、光学吸收系数,分析了掺杂后电子结构与光学性质的变化.计算结果表明:掺杂后禁带中引入了新的杂质能级,费米能级进入导带.掺杂改变了ZnS晶体的导电特性,使它表现出金属特性,导电性能增强;与纯净ZnS相比,Ti单掺杂和(Ti,Al)共掺杂ZnS的吸收边均出现明显的红移,且在1.79eV左右出现了一个新峰;而Al单掺杂ZnS的吸收边则发生明显的蓝移,且不产生新的吸收峰.     

Charge transport and shot noise on the surface of a topological insulator with a magnetic modulation

In this paper, we investigate the transport features and the Fano factor of Dirac electrons on the surface of a three-dimensional topological insulator with a magnetic modulation. We consider a hard wall bounding condition on the edge of the topological insulator, which implies that a surface state of the topological insulator is insulating. We find that a valley of conductivity at the Dirac point is associated with a Fano factor peak, and more interestingly, this topological metal changes from insulating to metallic by controlling the effective exchange field.     

Halleffekt und Leitfähigkeitsmessungen an Zinkoxid-Einkristallen mit Sauerstofflücken als Donatoren

The electrical conductivity and Hall effect of pure zinc oxide single crystals have been measured from 5 to 300 K. The temperature dependence of the conductivity and the charge carrier concentration is similar to silicon and germanium. The low temperature conduction mechanism depending on the impurity concentration is explained by means of hopping conductivity and impurity band conduction. The impurity band is supposed to be built up of overlapping wave functions of the excited donor states. The results have been discussed supposing that the donors are oxygen vacancies occupied by one or by two electrons.     

A new method for impurity states in semiconductors

The causes of inapplicability of the Luttinger method for the theoretical investigations of impurity states in small gap semiconductors are discussed. For this purpose a new method has been developed and applied to GaAs and InSp. It is shown that the impurity states spectrum in these semiconductors must be either of hydrogen-like or Dirac spectrum type.     

Fe-N共掺杂锐钛矿相TiO2电子性质与光学性质的第一性原理研究

近年来,Fe和N掺杂锐钛矿相TiO2半导体在实验中发现许多优异性能,本文采用基于密度泛函理论的平面波超软赝势方法研究了纯锐钛矿相TiO2、Fe和N单掺杂及Fe和N共掺杂TiO2的能带结构、电荷布居、态密度和光学性质.分析发现:Fe掺杂引起杂质能带位于禁带中央,杂质能带最高点与导带相距大约0.6 eV而最低点与价带相距大约0.2 eV;N掺杂引起的杂质能带位于价带顶部附近. Fe和N共掺杂后杂质能带由两部分组成,位于价带顶上方0.62 eV和导带底下方0.22 eV处,其中一层杂质能带主要由N原子的2p轨道和Fe原子的3d轨道杂化形成,而另一条杂质能带主要由Fe原子的3d轨道形成,由于杂质能级的出现,使锐钛矿TiO2的禁带宽度变小.对光学性质分析发现:Fe和N共掺杂会使锐钛矿TiO2光学吸收带边红移,可见光区的光吸收系数明显增大,在低能区出现了新的吸收峰,对应能量为1.82 eV,与实验结果相符.     

硼氢协同掺杂Ib型金刚石大单晶的高温高压合成与电学性能研究

在压力6.0 GPa和温度1600 K条件下, 利用温度梯度法研究了(111)晶面硼氢协同掺杂Ib型金刚石的合成. 傅里叶红外光谱测试表明: 氢以sp³杂化的形式存在于所合成的金刚石中, 其对应的红外特征吸收峰位分别位于2850 cm^-1和2920 cm^-1处. 此外, 霍尔效应测试结果表明: 所合成的硼氢协同掺杂金刚石具有p型半导体材料特性. 相对于硼掺杂金刚石而言, 由于氢的引入导致硼氢协同掺杂金刚石电导率显著提高. 为了揭示硼氢协同掺杂金刚石电导率提高的原因, 对不同体系进行了第一性原理理论计算, 计算结果表明其与实验结果符合. 该研究对金刚石在半导体领域的应用有重要的现实意义.