Abnormal Modulation of Dielectric Band Transmittance of Polystyrene Opal

The abnormal transmittance in the dielectric band edge of a polystyrene opal is observed and analysed. The transmittance is periodically modulated and the period of modulation varies with the wavelength, which destroys the perfect structure of the photonic band gap. The transmittance modulation originates from the propagation of the low order whispering-gallery mode excited in polystyrene spheres. These results indicate that the whisperinggallery mode has a great influence on practical applications of polystyrene opal.     

Magnetic and Electronic Properties of Double Perovskite Ba_2SmNbO_6 without Octahedral Tilting by First Principle Calculations

The structural, magnetic and electronic properties of the double perovskite Ba_2 SmNbO_6(for the simple cubic structure where no octahedral tilting exists anymore) are studied using the density functional theory within the generalized gradient approximation as well as taking into account the on-site Coulomb repulsive interaction.The total energy, the spin magnetic moment, the band structure and the density of states are calculated. The optimization of the lattice constants is 8.5173 A, which is in good agreement with the experimental value 8.5180 A.The calculations reveal that Ba_2 SmNbO_6 has a stable ferromagnetic ground state and the spin magnetic moment per molecule is 5.00μB/f.u. which comes mostly from the Sm~(3+) ion only. By analysis of the band structure,the compound exhibits the direct band gap material and half-metallic ferromagnetic nature with 100% spin-up polarization, which implies potential applications of this new lanthanide compound in magneto-electronic and spintronic devices.     

Analysis of two-dimensional photonic band gap structure with a rhombus lattice

The relative band gap for a rhombus lattice photonic crystal is studied by plane wave expansion method and high frequency structure simulator(HFSS)simulation.General wave vectors in the first Briliouin zone are derived.The relative band gap as a function of air-filling factor and background material is investigated,respectively,and the nature of photonic band gap for different lattice angles is analyzed by the distribution of electric energy.These results would provide theoretical instruction for designing optical integrated devices using photonic crystal with a rhombus lattice.     

Electronic and magnetic properties of BiFeO_3 with intrinsic defects:First-principles prediction

The electronic structure, magnetism, and dielectric functions of BiFeO3 with intrinsic vacancies, including Bi-, Fe-, and O-vacancies （denoted as VFe, VBi, and Vo, respectively） are investigated using the first-principles density functional theory plus U calculations. It is revealed that the structural distortions associated with those vacancies impose significant influences on the total density of state and magnetic behaviors. The existence of VBi favors the excitation of the O2p state into the band gap at 0.4 eV, while the O2p and Fe3d orbitals are co-excited into the band gap around 0.45 eV in VFe- Consequently, a giant net magnetic moment of 1.96 P-B is generated in VFe, and a relatively small moment of 0.13 P-B is induced in VBi, whereas Vo seems magnetically inactive. The giant magnetic moment generated in VFe originates from the suppression of the spatially modulated antiferromagnetic spin structure. Furthermore, VFe and VBi have strong influences on dielectric function, and induce some strong peaks to occur in the lower energy level. In contrast, VO has a small effect.     

A novel two-dimensional SiO sheet with high-stability,strain tunable electronic structure,and excellent mechanical properties

Using the structure search of particle swarm optimization(PSO) algorithm combined with density functional theory(DFT), we conduct a systematic two-dimensional(2D) material research on the SiO and discover a P2 monolayer structure.The phonon spectrum shows that the 2D P2 is dynamic-stable under ambient pressure. Molecular dynamics simulations show that 2D P2 can still exist stably at a high temperature of 1000 K, indicating that 2D P2 has application potential in high-temperature environments. The intrinsic 2D P2 structure has a quasi-direct band gap of 3.2 e V. The 2D P2 structure can be transformed into a direct band gap semiconductor by appropriate strain, and the band gap can be adjusted to the ideal band gap of 1.2 e V–1.6 e V for photovoltaic materials. These unique properties of the 2D P2 structure make it expected to have potential applications in nanomechanics and nanoelectronics.     

Effect of defects on the electronic structure of a PbI2/MoS2 van der Waals heterostructure: A first-principles study

PbI2/MoS2,as a typical van der Waals(vdW)heterostructure,has attracted intensive attention owing to its remarkable electronic and optoelectronic properties.In this work,the effect of defects on the electronic structures of a PbI2/MoS2 heterointerface has been systematically investigated.The manner in which the defects modulate the band structure of PbI2/MoS2,including the band gap,band edge,band alignment,and defect energy-level density within the band gap is discussed herein.It is shown that sulfur defects tune the band gaps,iodine defects shift the positions of the band edge and Fermi level,and lead defects realize the conversions between the straddling-gap band alignment and valence-band-aligned gap,thus enhancing the light-absorption ability of the material.     

Surface Structure and Electronic Property of InP（001）-（2×1）S Surface： A First-Principles Study

The surface structure and electronic property of InP（001）-（2 ×1）S surface under S-rich condition are investigated based on first-principles simulations. The analyses of phase transition show that the 3B model is the most stable structure and the S-S dimer is difficult to form. The geometry of the 3B structure agrees well with the experiments. It is also found that the 3B structure has a good passivation with a band gap of about 1.24eV. The results indicate that the 3B structure is the best candidate for the sulfur-rich InP（001）（2 × 1）A phase.     

Pressure induced magnetic and semiconductor–metal phase transitions in Cr_2MoO_6

We investigate magnetic ordering and electronic structures of Cr_2MoO_6under hydrostatic pressure. To overcome the band gap problem, the modified Becke and Johnson exchange potential is used to investigate the electronic structures of Cr_2MoO_6. The insulating nature at the experimental crystal structure is produced, with a band gap of 1.04 eV, and the magnetic moment of the Cr atom is 2.50 μB, compared to an experimental value of about 2.47 μB. The calculated results show that an antiferromagnetic inter-bilayer coupling–ferromagnetic intra-bilayer coupling to a ferromagnetic inter-bilayer coupling–antiferromagnetic intra-bilayer coupling phase transition is produced with the pressure increasing. The magnetic phase transition is simultaneously accompanied by a semiconductor–metal phase transition. The magnetic phase transition can be explained by the Mo–O hybridization strength, and ferromagnetic coupling between two Cr atoms can be understood by empty Mo-d bands perturbing the nearest O-p orbital.     

Generation of Fabry–Pérot oscillations and Dirac state in two-dimensional topological insulators by gate voltage

We investigate electron transport through Hg Te ribbons embedded by strip-shape gate voltage through using a nonequilibrium Green function technique. The numerical calculations show that as the gate voltage is increased, an edgerelated state in the valence band structure of the system shifts upwards, then hangs inside the band gap and merges into the conduction band finally. It is interesting that as the gate voltage is increased continuously, another edge-related state in the valence band also shifts upwards in the small-k region and contacts the previous one to form a Dirac cone in the band structure. Meanwhile in this process, the conductance spectrum displays as multiple resonance peaks characterized by some strong antiresonance valleys in the band gap, then behaves as Fabry–P′erot oscillations and finally develops into a nearly perfect quantum plateau with a value of 2e~2/h. These results give a physical picture to understand the formation process of the Dirac state driven by the gate voltage and provide a route to achieving particular quantum oscillations of the electronic transport in nanodevices.     

Lattice strain and p-d repulsion affecting electronic structure of wurtzite Zn1-xCdxO alloys

In the paper, density of states, band structure and electron density difference of Zn1-xCdxO are calculated by first principles, here x varies from 0 to 0.75 at intervals of 0.125, and the band gap obtained from band structure changes from 0.968 eV to 0.043 eV. The lattice strain and p-d repulsion theory are used to investigate variation of the band gap, the results obtained show that the variation is mainly due to the lattice tensile strain. The p-d repulsion in Zn1-xCdxO cannot be neglected. In addition, electron density difference can be used to verify the results.     

Quantum Impurity Models with Coupled Cluster Method

We investigate the ground-state properties of the Anderson single impurity model （finite Coulomb impurity repulsion） with the Coupled Cluster Method. We consider different CCM reference states and approximation schemes and make comparison with exact Green＇s function results for the non-interacting model and with Brillouin-Wigner perturbation theory for the full interacting model. Our results show that coupled cluster techniques are well suited to quantum impurity problems.     

Ti：Al2O3晶体中Ti^3＋离子的电子顺磁共振研究

通过对Ｔｉ：Ａｌ２Ｏ３晶体的不同取向的电子顺磁共振（ＥＰＲ）研究，认为在９３Ｋ温度下观测到的ｇ＝２．００的强烈各向异性的共振线是来自于Ｔｉ：Ａｌ２Ｏ３晶体中的Ｔｉ３＋离子２Ｔ２ｇ能态的中间能级１Ｅ１／２的顺磁共振吸收。而室温下观察到的ｇ≈２．００的吸收线是由Ｔｉ３＋离子２Ｔ２ｇ能态上能级２Ａ１的共振吸收产生的。由晶体场理论进行的计算与上述结果基本符合。     

First-principles study of structural,electronic and optical properties of ZnF_2

The structural, electronic, and optical properties of rutile-, CaC12-, and PdF2-ZnF2 are calculated by the plane-wave pseudopotential method within the density functional theory. The calculated equilibrium lattice constants are in reasonable agreement with the available experimental and other calculated results. The band structures show that the rutile-, CaCl2-, and PdF2-ZnF2 are all direct band insulator. The band gaps are 3.63, 3.62, and 3.36 eV, respectively. The contribution of the different bands was analyzed by the density of states. The Mulliken population analysis is performed. A mixture of covalent and weak ionic chemical bonding exists in ZnF2. Furthermore, in order to understand the optical properties of ZnF2, the dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, and optical reflectivity are also performed in the energy range from 0 to 30 eV. It is found that the main absorption parts locate in the UV region for ZnF2. This is the first quantitative theoretical prediction of the electronic and optical properties of ZnF2 compound, and it still awaits experimental confirmation.     

First-Principles Study of Structural,Elastic and Electronic Properties of OsSi

First-principles study of structural, elastic, and electronic properties of the B20 structure OsSi has been reported using the plane-wave pseudopotential density functional theory method. The calculated equilibrium lattice and elastic constants are in good agreement with the experimented data and other theoretical results. The dependence of the elastic constants, the aggregate elastic modulus, the deviation from the Cauchy relation, the elastic wave velocities in different directions and the elastic anisotropy on pressure have been obtained and discussed. This could be the first quantitative theoretical prediction of the elastic properties under high pressure of OsSi compound. Moreover, the electronic structure calculations show that OsSi is a degenerate semiconductor with the gap value of 0.68 eV, which is higher than the experimental value of 0.26 eV. The analysis of the PDOS reveals that hybridization between Os d and Sip states indicates a certain covalency of the Os-Si bonds.     

First-principles study of the structural,elastic, and optical properties for Sr0.5Ca0.5TiO3

A detailed theoretical study of the structural, elastic, and optical properties for Sr0.5Ca0.5TiO3 is carried out by first- principles calculations. The band structure exhibits a direct bandgap of 2.08 eV at the F point in the Brillouin zone. The bulk modulus, shear modulus, Young＇s modulus, and Poisson＇s ratio are derived based on the calculated elastic constants. The bulk modulus B = 153 GPa and shear modulus G = 81GPa are in good agreement with available experimental data. Poisson＇s ratio v = 0.275 suggests that Sr0.sCa0.sTiO3 should be classified as being a ductile material. Using the electronic band structure and density of states, we analyze the interband contribution to the optical properties. The real and imaginary parts of the dielectric function, as well as the optical properties such as the optical absorption coefficient, refractive index, extinction coefficient, and energy-loss spectrum are calculated. The static dielectric constant ε1 （0） and the refractive index n（0） are also investigated.     

Electronic and Optical Properties of Rock-Salt AIN under High Pressure via First-Principles Analysis

Electronic and optical properties of rock-salt AIN under high pressure are investigated by first -principles method based on the plane-wave basis set. Analysis of band structures suggests that the rock-salt AIN has an indirect gap of 4.53 eV, which is in good agreement with other results. By investigating the effects of pressure on the energy gap, the different movement of conduction band at X point below and above 22.5 GPa is predicted. The optical properties including dielectric function, absorption, reflectivity, and refractive index are also calculated and analyzed. It is found that the rock-salt AIN is transparent from the partially ultra-violet to the visible light area and hardly does the transparence affected by the pressure. Furthermore, the curve of optical spectrum will shift to high energy area （blue shift） with increasing pressure.     

Effect of Cr,Mo, and Nb additions on intergranular cohesion of ferritic stainless steel: First-principles determination

Effects of Cr, Mo, and Nb on the ferritic stainless steel ]2（210） grain boundary and intragranularity are investigated using the first-principles principle. Different positions of solute atoms are considered. Structural stability is lowered by Cr doping and enhanced by Mo and Nb doping. A ranking on the effect of solute atoms enhancing the cohesive strength of the grain boundary, from the strongest to the weakest is Cr, Mo, and Nb. Cr clearly prefers to locate in the intragranular region of Fe rather than in the grain boundary, while Mo and Nb tend to segregate to the grain boundary. Solute Mo and Nb atoms possess a strong driving force for segregation to the grain boundary from the intragranular region, which increases the grain boundary embrittlement. For Mo- and Nb-doped systems, a remarkable quantity of electrons accumulate in the region close to Mo （Nb）. Therefore, the bond strength may increase. With Cr, Mo, and Nb additions, an anti-parallel island is formed around the center of the grain boundary.     

First-principles study of the electronic structure and optical properties of cubic Perovskite NaMgF_3

The structural, electronic, and optical properties of cubic perovskite NaMgF3 are calculated by plane-wave pseudopo- tential density functional theory. The calculated lattice constant a0, bulk modulus B0, and the derivative of bulk modulus B~ are 3.872/~, 78.2 GPa, and 3.97, respectively. The results are in good agreement with the available experimental and theo- retical values. The electronic structure shows that cubic NaMgF3 is an indirect insulator with a wide forbidden band gap of Eg = 5.90 eV. The contribution of the different bands is analyzed by total and partial density of states curves. Population analysis of NaMgF3 indicates that there is strong ionic bonding in the MgF2 unit, and a mixture of ionic and weak covalent bonding in the NaF unit. Calculations of dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, optical reflectivity, and conductivity are also performed in the energy range 0 to 70 eV.     

Electronic structures and optical properties of IIIA-doped wurtzite Mg0.25Zn0.75O

The energy band structures, density of states, and optical properties of IliA-doped wurtzite Mg0.25Zn0.75O （IIIA= A1, Ga, In） are investigated by a first-principles method based on the density functional theory. The calculated results show that the optical bandgaps of Mg0.25Zn0.75O：IIIA are larger than those of Mg0.25Zn0.75O because of the Burstein-Moss effect and the bandgap renormalization effect. The electron effective mass values of Mg0.25Zn0.75O：IIIA are heavier than those of Mgo.25Zno.750, which is in agreement with the previous experimental result. The formation energies of MgZnO：Al and MgZnO：Ga are smaller than that of MgZnO：In, while their optical bandgaps are larger, so MgZnO：Al and MgZnO：Ga are suitable to be fabricated and used as transparent conductive oxide films in the ultra-violet （UV） and deep UV optoelectronic devices.     

Characteristics of the magnetic analysis system for a compact MPR-type spectrometer

The magnetic proton recoil（MPR）spectrometer is a novel diagnostic instrument with high perfor-mance for measurements of neutron spectra in inertial confinement fusion（ICF）experiments and high power fusion devices.A compact MPR-type spectrometer dedicated to the research of pulsed deuterium-tritium（DT）neutron spectroscopy of special experimental conditions is currently under design.Analyses of the main parameters and performance of the magnetic analysis system through 3-D particle transport calculations and MonteCarlo simulations and calibration of the system performance as a test using CR-39 solid track detector and α particle from 239pu and 226Ra radioactive sources are presented in this paper.The results indicate that the magnetic analysis system will achieve a detection efficiency level of 10-5-10-4 at an energy resolution of 1.5%-2.1%,and fulfills the design goals of the spectrometer.