First principle study of the behavior of helium in plutonium dioxide

The incorporation and solution of helium in plutonium dioxide have been investigated based on density functional theory. The GGA and GGA + U approximations were used with the projector-augmented-wave method. Several defects that are likely to accommodate the incorporation of helium in PuO₂, such as oxygen vacancy, plutonium vacancy, divacancy and Schotty defects were considered in this work. With GGA approach, the lowest incorporation energy corresponds to neutral trivacancy, followed by divacancy and plutonium vacancy, while the GGA + U scheme gave us that oxygen vacancy is the most favorable incorporation site for He. Both SP-GGA and SP-GGA + U methods obtained a same conclusion that the most favorable solution site for He is oxygen vacancy, interstitial site and plutonium vacancy for under-, perfect and over-stoichiometry, respectively. Additionally, the concentrations of the point defects and the solution energy of He for the different incorporation sites as a function of the stoichiometry were also obtained based on the point-defect model.     

Ferromagnetism in GaN:Gd: a density functional theory study

First-principle calculations of the electronic structure and magnetic interaction of GaN:Gd have been performed within the generalized gradient approximation (GGA) of the density functional theory with the on-site Coulomb energy U taken into account (also referred to as GGA+U). The ferromagnetic p-d coupling is found to be over 2 orders of magnitude larger than the s-d exchange coupling. The experimental colossal magnetic moments and room-temperature ferromagnetism in GaN:Gd reported recently are explained by the interaction of Gd 4f spins via p-d coupling involving holes introduced by intrinsic defects such as Ga vacancies.     

UO2的电子结构及光学性质的第一性原理研究

采用局域自旋密度近似 (LSDA)和有效库仑相关能 (U) 方法研究了UO₂的晶格参数、能带结构和光学常数. 计算得到的UO₂晶体的晶格常数为5.40 ?,带隙宽度为1.82 eV,正确预测了UO₂的反铁磁性半导体基态性质. 能带结构和介电函数的分析结果表明,铀的6d电子在晶体场中发生劈裂形成两个能级,与实验结果较为符合.     

The effect of 4d states based full Heusler alloy on the electronic and magnetic properties of new half metallic ferromagnetism: DFT+U study

A full heusler alloy Ru₂MoSb is studied using the full potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). Structural, electronic and magnetic properties are investigated for this material. We have used the generalized gradient approximation of Perdew, Burke, and Ernzerh (GGA-PBE), the GGA+U and the modified Becke–Johnson potential of GGA+U (mBJ-GGA+U) to model exchange correlation potential. The Hubbard on-site Coulomb interaction correction U is calculated by constraint local density approximation for both 4d elements Ru and Mo.For both approximations GGA+U and mBJ- GGA+U, density of states and band structure reveal that our compound has a half-metallic character. Magnetic properties show that Ru₂MoSb is ferromagnetic with an integer magnetic moment of 3 µB which is in good agreement with the Slater–Pauling rule. The half-metallicity of Ru₂MoSb is stable under lattice constant changes which makes it a potential contender for spintronic applications.The negative values of the cohesion energy and the formation energy indicate that our Heusler alloy can be synthesized and stabilized experimentally.     

First principle study of the magnetism of Sr2CoMoO6-δ (δ = 0, 1/2) double perovskites

We investigate the electronic structure of bulk Sr₂CoMoO_6-δ double perovskites using the ab initio Full Potential Linearized Augmented Plane Wave method in order to study their magnetic properties within the GGA and GGA+U methods. We discuss the relative stability of ferromagnetic (FM) and antiferromagnetic (AFM) orders (i) without and with taking into account the observed tilting of the oxygen octahedra and (ii) by introducing oxygen vacancies. We show that a very good agreement with experimental results — AFM order for δ= 0 and FM order for δ= 1/2 — is obtained only when the tilting of the oxygen tetrahedra is taking into account and when the GGA+U method is used.     

Phase stability,electronic, magnetic and elastic properties of Ni2CoZ(Z = Ga,Sn): A first principles study with GGA method and GGA+U approach

First principles calculations based on density functional theory are used to investigate the phase stability, electronic, magnetic and elastic properties of ferromagnetic metallic full-Heusler Ni₂CoZ(Z = Ga, Sn) alloys via the FP-LAPW method by the generalized gradient GGA and GGA+U approximations for the exchange and correlation energy, within the Perdew–Burke–Ernzerhof (PBE 96) parameterization. The results of calculating electronic structures and magnetic properties reveal that the both Ni₂CoGa and Ni₂CoSn crystallize in L2₁ phase with regular cubic structure. The two investigated compounds exhibit metallic ferromagnetic behaviors for the GGA+U calculation. The computation of elastic constants with GGA+U approach shows that our compounds are mechanically stable.     

Understanding STM images and EELS spectra of oxides with strongly correlated electrons: a comparison of nickel and uranium oxides

Using a theoretical approach combining the local spin density approximation (LSDA) of density functional theory and the Hubbard U term (LSDA + U), we analyse the connection between the experimentally observed electron energy loss spectra and elevated temperature scanning tunnelling images of surfaces of semiconducting nickel monoxide NiO and uranium dioxide UO2. We show that a combination of electron energy loss spectroscopy, atomic-resolution tunnelling imaging and first-principles ab initio calculations provides a powerful tool for studying electronic and structural properties of surfaces of transition metal and actinide oxides.     

Orbital specificity in the unoccupied states of UO2 from resonant inverse photoelectron spectroscopy

One of the crucial questions of all actinide electronic structure determinations is the issue of 5f versus 6d character and the distribution of these components across the density of states. Here, a breakthough experiment is discussed, which has allowed the direct determination of the U5f and U6d contributions to the unoccupied density of states in uranium dioxide. A novel resonant inverse photoelectron and x-ray emission spectroscopy investigation of UO(2) is presented. It is shown that the U5f and U6d components are isolated and identified unambiguously.     

The spin effect in zinc-blende CdEuS and CdEuSe: GGA and GGA+U studies

We have investigated the structural, electronic and magnetic properties of substitutional europium rare earth impurity in cubic CdS and CdSe by employing the ab-initio method. Calculations were performed by using the full potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the framework of spin-polarized density functional theory (DFT). The electronic exchange-correlation energy is described by generalized gradient approximation GGA and GGA+U (U is the Hubbard correction). The GGA+U method is applied to the rare-earth 4f states. We have calculated the lattice parameters, bulk modulii, the first pressure derivatives of the bulk modulii and the cohesive energies. The calculated densities of states presented in this study identify the metallic behavior of CdEuS and CdEuSe when we use the GGA scheme, whereas when we use the GGA+U, we see that these compounds are half-metallic.     

Orbital-occupancy versus charge ordering and the strength of electron correlations in electron-doped CaMnO3

The structural, electronic, and magnetic properties of mixed-valence compounds are believed to be governed by strong electron correlations. Here we report benchmark density-functional calculations in the spin-polarized generalized-gradient approximation (GGA) for the ground-state properties of doped CaMnO(3). We find excellent agreement with all available data, while inclusion of strong correlations in the GGA+U scheme impairs this agreement. We demonstrate that formal oxidation states reflect only orbital occupancies, not charge transfer, and resolve outstanding controversies about charge ordering.     

Theoretical study of the structural,electronic and magnetic properties of equiatomic quaternary CoTcCrZ (Z = Si,Ge, P) Heusler alloys

In this study, full potential linearized augmented plane wave (FP-LAPW) method has been used to calculate structural, electronic and magnetic characteristics of CoTcCrZ (Z = Si, Ge, P) Equiatomic Quaternary Heusler alloys (EQHAs). Furthermore, Generalized Gradient Approximation (GGA) and Hubbard potential (GGA+U) are adopted in the parametrization of Perdew-Burke-Ernzerhof (PBE). Structural stability calculations confirmed that the Type II is the most stable structure of all considered alloys. Further investigations were carried out for the most stable Type (i.e. Type II). Comparison of electronic structure calculations performed by GGA and GGA+U methods concluded that considered alloys show half-metallic nature for GGA+U method. Magnetic moments for all these alloys are determined which are found in accordance to the Slater-Pauling rule. Half-metallicity has been verified in all these considered Heusler alloys (HAs) from the calculations of spin-polarization at the Fermi level (E_F). Moreover, the Curie temperature (Tc) is estimated by employing mean field approximation (MFA).     

LiV2O4: electronic,magnetic structure and heavy-fermion behaviour from first principles

First principles studies based on density functional theory (DFT) calculations within the generalized gradient approximations (GGA) and GGA + U approach using the full-potential, augmented plane wave + local orbitals (APW + lo) method, as implemented in the WIEN2k code, have been used to investigate the structural, electronic and magnetic properties of spinel-structure LiV₂O₄, in particular regarding the heavy fermion (HF) behaviour. The calculations were performed for ferromagnetic, anti-ferromagnetic, and ferrimagnetic configurations using two kinds of magnetic structures (tetragonal and rhombohedral). The GGA results showed that the Fermi energy lies in the V 3d (t_2g) bands with 1.5 electrons per V atom occupying this band, and the V 3d bands are separated by a ～1.9 eV energy gap from the O 2p bands and further split into t_2g and e_g bands with a ～1.0 eV energy gap, which are in good agreement with the photoelectron spectra. The GGA + U method indicates that the ground state of LiV₂O₄ is the tetragonal anti-ferromagnetic configuration with metallic character, and ferromagnetic order character at slightly higher energy, which is consistent with experimental result. The geometric frustration and hybridization between 3d (V) and 2p (O) could induce spin fluctuation and help to explain the instability of specific heat, susceptibility and HF behaviour.     

Thermomagnetic history effects across the first order magneto-structural transition in the giant magnetocaloric Fe-Rh alloy

We investigate the electronic structure of Sr₂FeMoO₆/SrTiO₃ (SFMO/STO) multilayers using the ab initio Full Potential Linearized Augmented Plane Wave method in order to study their properties within the GGA and GGA+U methods. We examin more especially the role of the interface on the magnetic and transport properties of these multilayers taking into account a possible Fe deficiency at the interface and we show that bulk behaviour is rapidly recovered due to the strong localization of the interfacial perturbation. For perfect interfaces, the whole structure is found half-metallic within the GGA+U method; the situation being ambiguous within the GGA method where SFMO is at the limit of being half-metallic depending on the structural deformation induced by the STO layer. This leads us to the conclusion that such a system could be used as injection electrode and tunnel barrier in magnetic tunnel junctions with a fully spin polarized injected current. For Fe deficient interfaces, we show that the interfacial densities of states are nearly unpolarized showing that this kind of imperfection has potentially a strong impact on the properties of the multilayers.     

掺钇钨酸铅晶体缺陷的理论研究

采用基于密度泛函理论的相对论性离散变分和嵌入团簇方法，计算了掺钇PbWO₄ 晶体中多 种相关缺陷的电荷分布和不同团簇缺陷结合能，由能量最低原理发现［2(Y³⁺Pb)-V″_Pb］缺陷在各相关缺陷形式中最为稳定.并运用过渡 态方法计算了轨 道跃迁的激发能，算出掺Y后晶体中O2p→Y4d的跃迁能量为3.9eV，表明掺Y不会引起晶体中3 50nm和420nm吸收.从掺Y对PbWO₄晶体电子结构的影响来看，其作用机理与掺La 的情况也有较大差异. 关键词： 4晶体')" href="#">PbWO₄晶体 密度泛函 掺Y 态密度分布     

Tuning the magnetic and electronic properties of strontium titanate by carbon doping

The magnetic and electronic properties of strontium titanate with different carbon dopant configurations are explored using first-principles calculations with a generalized gradient approximation (GGA) and the GGA+U approach. Our results show that the structural stability, electronic properties and magnetic properties of C-doped SrTiO₃ strongly depend on the distance between carbon dopants. In both GGA and GGA+U calculations, the doping structure is mostly stable with a nonmagnetic feature when the carbon dopants are nearest neighbors, which can be ascribed to the formation of a C–C dimer pair accompanied by stronger C–C and weaker C–Ti hybridizations as the C–C distance becomes smaller. As the C–C distance increases, C-doped SrTiO₃ changes from an n-type nonmagnetic metal to ferromagnetic/antiferromagnetic half-metal and to an antiferromagnetic/ferromagnetic semiconductor in GGA calculations, while it changes from a nonmagnetic semiconductor to ferromagnetic half-metal and to an antiferromagnetic semiconductor using the GGA+U method. Our work demonstrates the possibility of tailoring the magnetic and electronic properties of C-doped SrTiO₃, which might provide some guidance to extend the applications of strontium titanate as a magnetic or optoelectronic material.     

Suppression of Jahn-Teller distortion by chromium and magnesium doping in spinel LiMn2O4: A first-principles study using GGA and GGA+U

The effect of doping spinel LiMn₂O₄ with chromium and magnesium has been studied using the first-principles spin density functional theory (DFT) within generalized gradient approximation (GGA ) and GGA+U. We find that GGA and GGA+U give different ground states for pristine LiMn₂O₄ and same ground state for doped systems. For LiMn₂O₄, the body-centered tetragonal phase was found to be the ground-state structure using GGA and face-centered orthorhombic using GGA+U, while for LiM_0.5Mn_1.5O₄ (MCr or Mg) it was base-centered monoclinic and for LiMMnO₄ (MCr or Mg) it was body-centered orthorhombic in both GGA and GGA+U. We find that GGA predicts the pristine LiMn₂O₄ to be metallic while GGA+U predicts it to be insulating, which is in accordance with the experimental observations. For doped spinels, GGA predicts the ground state to be half metallic while GGA+U predicts it to be insulating or metallic depending on the doping concentration. GGA+U predicts insulator-metal-insulator transition as a function of doping in case of Cr and in case of Mg the ground state is found to go from insulating to a half metallic state as a function of doping. Analysis of the charge density and the density of states (DOS) suggest a charge transfer from the dopants to the neighboring oxygen atoms and manganese atoms. We have calculated the Jahn-Teller active mode displacement Q₃ for doped compounds using GGA and GGA+U. The bond lengths calculated from GGA+U are found to be in better agreement with experimental bond lengths. Based on the bond lengths of metal and oxygen, we have also estimated the average oxidation states of the dopants.     

Correlation effects and energetics of point defects in uranium dioxide: a first principle investigation

Density functional (DFT) calculations have been used to investigate the stability of point defects in uranium dioxide. Correlation effects are taken into account within the DFT?+?U approach as implemented in the Vienna ab initio simulation package (VASP). More particularly, the formation energies of both intrinsic and extrinsic point defects, i.e. vacancies, interstitials, Frenkel pairs and Schottky trio defects, are calculated. Our results are compared with available experimental data and are also discussed in relation to previous calculations based on conventional functionals, such as the local spin-density approximation and generalized gradient approximations.     

First principles study of half-metallic ferromagnetism in Zn1−xEuxS

Density functional theory calculations by using both generalized gradient approximation (GGA) method and the GGA with considering strong correlation effect (GGA+U) for various Eu concentrations x (=0.00, 0.25, 0.50, and 0.75). It is found that after the Europium incorporation, a new localized band appears between the valence and conduction bands, which corresponds to the majority spin of Eu-4f states, the strong correlation effects is very important for the 4f orbit of the Eu atom in ZnEuS. We find that Zn_1−xEu_xS exhibits a half-metallic characteristic, and the ferromagnetic state is more favorable in energy than the antiferromagnetic state. Structural properties are determined from the total energy calculations, and we discuss the electronic structures, total and partial densities of states and local moments.