Al掺杂浓度对CrSi2电子结构及光学性质的影响

采用基于第一性原理的赝势平面波方法,对不同Al掺杂浓度CrSi2的几何结构、能带结构、态密度和光学性质进行了计算和比较。几何结构和电子结构的计算表明:Al掺杂使得CrSi2的晶格常数a和b增大,c变化不大,晶格体积增大;Cr(Si1-xAlx)2仍然是间接带隙半导体,掺杂使得费米面向价带移动,且随着掺杂量的增大而更深地嵌入价带中,费米能级附近的电子态密度主要由Cr的3d态电子贡献。光学性质计算表明,随着掺杂量的增大,Cr(Si1-xAlx)2的静态介电常数、第一介电峰、折射率n0逐渐增大,平均反射效应减弱,表明Al掺杂有效增强了CrSi2对光的吸收,能够提高其光电转换效率。计算结果为CrSi2光电材料的应用和设计提供了理论指导。     

Rovibrational Intensities of the (00(0)3) <-- (10(0)0) Dyad Absorption Bands of (12)C(16)O(2)

Absolute line intensities of (12)C(16)O(2) are experimentally measured for the first time for the (00(0)3)(I) <-- (10(0)0)(II) band at 5687.17 cm(-1) and the (00(0)3)(I) <-- (10(0)0)(I) band at 5584.39 cm(-1). The spectra were obtained using a Bomem DA8 Fourier transform spectrometer and a 25-m base-path White cell at NASA-Ames Research Center. The rotationless bandstrengths at a temperature of 296 K and the Herman-Wallis parameters are S(0)(vib) = 6.68(30) x 10(-25) cm(-1)/(molecule/cm(2)); A(1) = 1.4(9) x 10(-4), and A(2) = -1.1(5) x 10(-5) for the (00(0)3)(I) <-- (10(0)0)(II) band and S(0)(vib) = 6.07(22) x 10(-25) cm(-1)/(molecule/cm(2)); A(1) = 5.2(1.5) x 10(-4) and A(2) = -4.0(7) x 10(-5) for the (00(0)3)(I) <-- (10(0)0)(I) band.     

Linear and nonlinear optical properties of 3-nitroaniline (m-NA) and 4-nitroaniline (p-NA) crystals: A DFT/TDDFT study

We have studied the electronic structure and optical responses of 3-nitroaniline and 4-nitroaniline crystals within the framework of density functional theory (DFT). In addition, the excitonic effects are investigated by using the recently published bootstrap exchange-correlation kernel within the time dependent density functional theory (TDDFT) framework. Our calculations based on mBJ approximation yield the indirect band gap for both crystals, but the larger one for m-NA. Due to the excitonic effects, the TDDFT calculations gives rise to the enhanced and red-shifted spectra (compared to RPA). Due to the weak intermolecular interactions, band-structure calculations yield bands with low dispersion for both crystals. This study shows that the substituent groups play an important role in the top of valence band and the bottom of conduction band. Due to the linear structure of p-NA molecule, the highest peaks are located in the optical spectra of p-NA crystal, while m-NA has more sharp peaks, especially at lower energies. Both DFT and TDDFT calculations for the energy loss spectra show plasmon peaks around 27 and 28 eV for p-NA and m-NA, respectively. Due to the non-centrosymmetric structure of m-NA crystal, we also have reported its nonlinear spectra and the 2ω/ω intra-band and inter-band contributions to the dominant susceptibilities. Findings indicate the opposite signs for these contributions, especially at higher energies. The comparison between nonlinear spectra and the linear spectra (as a function of both ω and 2ω) reveals the significant resemblance between linear and nonlinear patterns. In addition to the reasonable agreement between our results with experimental data, this study reveals the spectral similarities between crystalline susceptibility and molecular polarizability.     

First-principles study of the co-effect of carbon doping and oxygen vacancies in ZnO photocatalyst

Although tuning band structure of optoelectronic semiconductor-based materials by means of doping single defect is an important approach for potential photocatalysis application,C-doping or oxygen vacancy(Vo)as a single defect in ZnO still has limitations for photocatalytic activity.Meanwhile,the influence of co-existence of various defects in ZnO still lacks sufficient studies.Therefore,we investigate the photocatalytic properties of ZnOx C0.0625(x=0.9375,0.875,0.8125),confirming that the co-effect of various defects has a greater enhancement for photocatalytic activity driven by visible-light than the single defect in ZnO.To clarify the underlying mechanism of co-existence of various defects in ZnO,we perform systematically the electronic properties calculations using density functional theory.It is found that the coeffect of C-doping and Vo in ZnO can achieve a more controllable band gap than doping solely in ZnO.Moreover,the impact of the effective masses of ZnO_xC_0.0625(x=0.9375,0.875,0.8125)is also taken into account.In comparison with heavy Vo concentrations,the light Vo concentration(x=0.875)as the optimal component together with C-doping in ZnO,can significantly improve the visible-light absorption and benefit photocatalytic activity.     

Angle-resolved photoemission study of the cobalt oxide superconductor Na(x)CoO(2) x yH(2)O: observation of the Fermi surface

The cobalt oxide superconductor Na(x)CoO(2) x yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E(F)) in the normal state of Na(x)CoO(2) x yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to Na(x)CoO(2). The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within approximately 30 meV below E(F), closer to E(F) than in Na(x)CoO(2). We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.     

Inversion of two-band superconductivity at the critical electron doping of (Mg, Al)B2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)Al(x)B(2). A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximately 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T(c) < 7 K up to about x approximately 55. Only the pi band had detectable density of states for 0.33 < or =x < or = 0.55, implying an inversion of the two-band hierarchy of MgB(2) in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T(c) are discussed.     

Pseudogap formation in the intermetallic compounds (Fe1-xVx)3Al

Optical conductivity data of the intermetallic compounds (Fe1-xVx)3Al ( 0相似文献   

Electronic structure,phase stability,and vibrational properties of Ir-based intermetallic compound IrX (X=Al,Sc, and Ga)

The phase stability and mechanical properties of B2 type IrX (X=Al, Sc and Ga) compounds are investigated. Self-consistenttotal-energy calculations in the framework of density functional theory using the Generalized Gradient Approximation (GGA) to determine the equations of state and the elastic constants of IrX (X=Al, Sc, and Ga) in the B2 phase have been performed. The calculations predicted the equilibrium lattice constants, which are about 1% greater than experiments for IrAl, 1.81% for IrGa, and 0.71% for IrSc compound. IrAl is shown to be the least compressible, and it is followed by IrGa and the IrSc compound. The phase stability of the studied compounds is checked. The brittleness and ductility properties of IrX (X=Al, Sc, and Ga) are determined by Poisson's ratio σ criterion and Pugh's criterion. IrGa compound is a ductile material; however, IrAl and IrSc show brittleness. The band structure and density of states (DOS), and phonon dispersion curves have been obtained and analyzed. The position of the Fermi level and the contribution of d electrons to the density of states near E_F is studied and discussed in detail. We also used the phonon density of states and quasiharmonic approximation to calculate and predict some thermodynamic properties such as constant-volume specific heat capacity of the B2 phase of IrX (X=Al, Sc and Ga) compounds.     

Zn1-xMgxO电子结构及光学性质的第一性原理GGA+U方法研究

Mg掺杂ZnO形成的固溶体Zn_1-xMg_xO（ZMO）（0 ≤ x ≤ 0.25）是一种带隙较宽、电子学性质可调控的半导体材料,在薄膜太阳电池及光电设备的透明电极等方面具有重要的应用价值。基于密度泛函理论下的第一性原理超软赝势方法,采用GGA+U计算了ZMO的电子结构和光学性质。计算结果表明,随着x值的增加,ZMO的禁带宽度由x=0时的3.32 eV增加到x=0.25时的3.78 eV;光吸收边及反射谱和能量损失谱均发生明显蓝移,峰值存在于紫外光区。计算结果与实验结论相符合。     

Lattice dynamics properties of XAs (X=Al,Ga and In) with zinc-blende structure from first-principle calculations

Band structures, density of states, dielectric and vibrational properties of XAs (X=Al, Ga and In) alloys with zinc-blende structure have been studied using the density functional theory (DFT). The calculated lattice constants, band gap, static dielectric constants and phonon frequencies are all in good agreement with the available experimental data and other theoretical results. The calculated results show that Born effective charges Z_B increase with cation mass. A similar tendency has been observed for phonon frequencies ω_TO and ω_LO. Calculation results prove that static dielectric constants ε(0) increase with atomic weight, i.e. in the sequences AlAs–GaAs–InAs, and show an inverse sequence for band gap.     

First principles study of electronic structure and optical properties of LiMTe2 (M=Al, Ga, In) crystals

The results of first principles calculations of the electronic band structure, density of states and frequency dependent dielectric functions of LiAlTe₂, LiGaTe₂ and LiInTe₂ chalcopyrite crystals are reported. The calculations have been carried out within the density functional theory using norm-conserving pseudopotentials and a plane-wave basis. The peculiarities of the imaginary part ε₂(ω) of the complex permittivity are discussed and interpreted on the basis of the obtained band spectra. Our calculations show that the Ga-containing compound is characterized by the largest optical anisotropy compared to Al- and In-containing compounds and, therefore, is the most promising candidate for nonlinear optical applications among considered crystals.     

Theoretical optoelectronic analysis of intermediate-band photovoltaic material based on ZnY1-xOx （Y = S,Se, Te） semiconductors by first-principles calculations

The structural, energetic, and electronic properties of lattice highly mismatched ZnY1-xOx （Y = S, Se, Te） ternary alloys with dilute O concentrations are calculated from first principles within the density functional theory. We demonstrate the formation of an isolated intermediate electronic band structure through diluted O-substitute in zinc-blende ZnY （Y = S, Se, Te） at octahedral sites in a semiconductor by the calculations of density of states （DOS）, leading to a significant absorption below the band gap of the parent semiconductor and an enhancement of the optical absorption in the whole energy range of the solar spectrum. It is found that the intermediate band states should be described as a result of the coupling between impurity O 2p states with the conduction band states. Moreover, the intermediate bands （IBs） in ZnTeO show high stabilization with the change of O concentration resulting from the largest electronegativity difference between O and Te compared with in the other ZnSO and ZnSeO.     

Magnetic ground state and electronic structure of CeRu(2)Al(10)

We present a combined theoretical and experimental study of the electronic structure for CeRu(2)Al(10) based on ab initio band structure calculations and x-ray photoemission spectroscopy (XPS) data. Our calculations were performed for the base unit cell and for the hypothetical unit cell which enables antiferromagnetic ordering. The stability of the magnetic phase was investigated within fixed spin moment calculations. When additional 4f correlations are not included in the LSDA C U approach, CeRu(2)Al(10) exhibits an unstable magnetic configuration with the difference in total energy per unit cell between the weakly magnetic state and the non-magnetic one of the order ~0.3 meV. We found that Coulomb correlations among 4f electrons, when they are included in the LSDA C U approach, stabilize the magnetic structure. In the weakly correlated system (small U) an antiferromagnetic (AFM) ground state with the lowest total energy is preferred. The situation is, however, the opposite when the 4f correlations are strong. In this case the ferromagnetic (FM) ground state is preferred. By comparing our calculations with the experimental data we conclude that the 4f correlations in CeRu(2)Al(10) are weak. We also carried out a structural relaxation of atomic positions within the Cmcm unit cell and we found that the Al atoms exhibit noticeable displacement from their positions known from x-ray diffraction (XRD) analysis.     

Block spin ground state and three-dimensionality of (K,Tl)(y)Fe(1.6)Se2

The magnetic properties and electronic structure of (K,Tl)(y)Fe(1.6)Se(2) is studied using first-principles calculations. The ground state is checkerboard antiferromagnetically coupled blocks of the minimal Fe(4) squares, with a large block-spin moment ～11.2 μ(B). The magnetic interactions could be modeled with a simple spin model involving both the inter- and intrablock, as well as the nearest-neighbor and next-nearest-neighbor couplings. The calculations also suggest a metallic ground state except for y=0.8 where a band gap ～400-550 meV opens, showing an antiferromagnetic insulator ground state for (K,Tl)(0.8)Fe(1.6)Se(2). The electronic structure of the metallic (K,Tl)(y)Fe(1.6)Se(2) is highly three dimensional with unique Fermi surface structure and topology. These features indicate that the Fe-vacancy ordering is crucial to the physical properties of (K,Tl)(y)Fe(2-x)Se(2).     

A theoretical study of the electronic structure of Pd(111) clean surface

A detailed investigation of the electronic structure of Pd(l 11) clean surface is presented in terms of a LCAO band model. Results including surface bands and local densities of states are given and a comparison with recently performed photoemission experiments is presented. For hω = 21.2 eV we find that the angle resolved distribution curves can be explained in terms of initial state density and k_∥ conserving transitions. For lower photon energies a mixture of bulk transition calculations and surface density of states seems more appropriate. Comparison with previous theoretical work is also presented.     

Optical properties of the semiconducting “metal-like” complex (NMe3H) (I) (TCNQ)

We present the results of a study of the room temperature polarized reflectance between 2.5 meV and 3.8 eV of (NMe₃H)(I)(TCNQ) single crystals. The reflectance found with the electric field parallel to the stacking axis is fit with a Drude-Lorentz model and analyzed using Kramers-Kronig transforms to give the response functions σ₁(ω) and ε₁(ω). The results confirm the model of semiconducting behavior despite a “metal-like” d.c. electrical conductivity at room temperature.     

Electronic structures and mechanical properties of Al(111)/ZrB2(0001) heterojunctions from first-principles calculation

Employing first-principles density functional theory (DFT), the structures and electronic and mechanical properties of Al(111)/ZrB₂(0001) heterojunctions are investigated. It is found that both B-terminated ZrB₂(0001) and Zr-terminated ZrB₂(0001) can form heterojunction interfaces with Al(111) surface. The heterojunction with B-terminated ZrB₂(0001) is demonstrated to be most stable by comparing the surface adhesion energies of six different heterojunction models. In the stable configurations, the Al atom is found projecting to the hexagonal hollow site of neighbouring boron layer for the B-terminated ZrB₂(001), and locating at the top site of the boron atoms for Zr-terminated ZrB₂(001) interface. The mechanisms of interface interaction are investigated by density of states, charge density difference and band structure calculations. It is found that covalent bonds between surface Al atoms and B atoms are formed in the B-terminated heterojunction, whereas the Al atoms and Zr atoms are stabilised by interface metallic bonds for the Zr-terminated case. Mechanical properties of Al/ZrB₂ heterojunctions are also predicted in the current work. The values of moduli of Al/ZrB₂ heterojunctions are determined to be between those of single crystal Al and ZrB₂, which exhibit the transition of mechanical strength between two bulk phases. DFT calculations with the current models provide the mechanical properties for each heterojunction and the corresponding contributions by each type of interface in the composite materials. This work paves the way for industrial applications of Al(111)/ZrB₂(0001) heterojunctions.     

锰掺杂二硅化铬电子结构和光学性质的第一性原理计算

采用基于第一性原理的密度泛函理论(DFT)赝势平面波方法计算了锰掺杂二硅化铬(CrSi2)体系的能带结构、态密度和光学性件质.计算结果表明末掺杂CrSi2属于间接带隙半导体间接带隙宽度△ER=0.35 eV;Mn掺杂后费米能级进入导带,带隙变窄,且间接带隙宽度△Eg=0.24 eV,CrSi2转变为n型半导体.光学参数发生改变,静态介电常数由掺杂前的ε1(O)=32变为掺杂后的ε1(O)=58;进一步分析了掺杂对CrSi2的能带结构、态密度和光学性质的影响,为CrSi2材料掺杂改件的研究提供r理论依据.     

LnZr(BO3)2:Eu3+(Ln=Ba,Sr)的真空紫外光谱特性的研究

采用高温固相法合成了Ba(1-x)SrxZr(BO3)2:Eu3 系列样品,样品Ba(1-x)SrxZr(BO3)2:Eu3 激发谱在130～170nm和230 nm区域有两个很强的吸收带,位于130～170nm的吸收带主要是硼酸盐基质的吸收;位于230nm附近的吸收主要是Eu3 电荷转移态的吸收.当在样品中以Al部分取代Zr时,电荷转移态的吸收明显增强,并且Ba(1-x)SrxZr(BO3)2:Eu3 发射强度也会明显增强;随着x的增大,硼酸盐基质的吸收强度减弱,基质吸收带的主峰值向低能方向移动了大约30 nm.样品Ba(1-x)SrxZr(BO3)2:Eu3 在147nm激发下,发射出主峰值位于616nm的强红光,对应Eu3 电偶极(5D0→7F2)跃迁发射.     

Electronic structures and magnetisms of the Co_2TiSb_(1-x)Sn_x(x= 0, 0.25, 0.5) Heusler alloys: A theoretical study of the shape-memory behavior

The total energy, electronic structures, and magnetisms of the Al Cu2Mn-type Co2TiSb1-xSnx(x = 0, 0.25, 0.5) with the different lattice parameter ratios of c/a are studied by using the first-principles calculations. It is found that the phase transformation from the cubic to the tetragonal structure lowers the total energy, indicating that the martensitic phase is more stable and that a phase transition from austenite to martensite may happen at a lower temperature. Thus, a ferromagnetic shape memory effect can be expected to occur in these alloys. The Al Cu2Mn-type Co2TiSb1-xSnx(x = 0, 0.25, 0.5) alloys are weak ferrimagnets in the austenitic phase and martensitic phase.