Correlation between entanglement and spin density in nitrogen-vacancy center of diamond

Many-body wavefunctions were utilized to calculate von Neumann’s entropy as an entanglement measurement for neutral and negatively charged nitrogen vacancy (NV) centers in diamond. A generalized Hubbard Hamiltonian which considers e-e interaction terms completely was used to calculate many-electron wavefunctions of the ground and excited states. Correlation between entanglement and spin density distributed on neighboring atoms of NV is presented. The behavior of spin density and entanglement under relaxations of neighboring atoms is the same for all investigated ground and excited states. The results suggest that the spin density may be used to quantify the entanglemnt and vice versa.     

Hydrogen incorporation in diamond: the vacancy-hydrogen complex

We report the identification of the vacancy-hydrogen complex in single crystal diamond synthesized by chemical vapor deposition. The S=1 defect is observed by electron paramagnetic resonance in the negative charge state. The hydrogen atom is bonded to one of the carbon atoms neighboring the vacancy. Unlike the analogous defect in silicon, no symmetry lowering reconstruction occurs between the three remaining carbon dangling orbitals. The very small measured hydrogen hyperfine interaction is explained by dipolar coupling between the hydrogen and the unpaired electron probability density delocalized on the three equivalent carbon neighbors.     

氮掺杂的金刚石磁性研究

采用第一性原理计算方法研究金刚石中由空位或者N掺杂引起的磁特性. 发现-1价和-2 价的碳空位能分别产生3 μ_B和2μ_B的磁矩; -2价的碳空位能够引发长程有序的铁磁耦合状态, 而-1价的碳空位更倾向于反铁磁耦合.掺杂N元素能有效地控制空位的荷电状态及相应的磁相互作用, 这一结果为在金刚石中实现非过渡族金属掺杂的铁磁性提供了一条新的路径. 关键词： 第一性原理计算 氮掺杂 金刚石 磁性     

LiF中空位形成能的第一性原理计算

采用平面波展开和基于密度泛函理论框架下的第一性原理赝势法,计算了102GPa下LiF化合物中Li空位和F空位的形成能及空间周围的原子弛豫,讨论了空位形成时电荷密度的重新分布,相应的电子态密度以及能带结构等性质.结果表明:LiF晶体中F空位的形成能比大于Li空位的形成能;F空位对LiF晶体的电子结构等性质的影响要比Li空位的大.     

LiF中空位形成能的第一性原理计算

采用平面波展开和基于密度泛函理论框架下的第一性原理赝势法,计算了102GPa下LiF化合物中Li空位和F空位的形成能及空间周围的原子弛豫,讨论了空位形成时电荷密度的重新分布,相应的电子态密度以及能带结构等性质.结果表明:LiF晶体中F空位的形成能比大于Li空位的形成能;F空位对LiF晶体的电子结构等性质的影响要比Li空位的大.     

空位所引起的晶体弹性常数的改变

认为具有少量空位的晶体是一个均匀的晶体,把完整与不完整晶体的区别,归结为势函数的修改。应用Morse函数推导出弹性常数c₁₁和c₁₂的变化率与空位浓度的关系。因为空位邻近的原子有松弛,考虑了由松弛而改变了的相互作用对弹性常数的影响。首先求得铝中空位的第一和第二最近邻的位移百分率各为1.61和-0.32,这里的正号和负号各表示向空位移进和移出。其次,进行了具体的计算,在铜晶体中,估计1％的空位浓度使弹性常数减小1.3—1.4％。然而,铝晶体中的空位对弹性常数的影响     

Intrinsic magnetism induced by vacancy in GaN

We performed total energy electronic-structure calculations based on DFT that clarify the intrinsic magnetism of undoped GaN. The magnetism is due to Ga, instead of N, vacancies. The origin of magnetism arises from the unpaired 2p electrons of N surrounding Ga vacancy. At a vacancy concentration of 5.6%, the ferromagnetic state is 181 meV lower than the antiferromagnetic state. Our findings are helpful to gain a more novel understanding of structural and spin properties of Ga vacancy in wurtzite GaN and also provide a possible way to generate magnetic GaN by introducing Ga vacancies instead of doping with transition-metal atoms.     

Ion charge state distributions following K-shell ionization in atoms

Ion charge state distributions and the mean charge state following K-shell ionization in atoms are calculated. The Monte Carlo method is applied to calculate the vacancy cascades after K-shell vacancy creation. The radiative and non-radiative transitions are calculated for singly ionized atoms. The transition rates for multi-ionized atoms are obtained using a statistical scaling procedure based on the transition probabilities for singly ionized atoms. The electron shake-off process due to the change of atomic potential, which occurs from core hole production and de-excitation decays, is considered in the calculation. The present results agree well with the available experimental values.     

Effect of lattice relaxation on spin density of nitrogen-vacancy centers in diamond and oscillator strength calculations

Using a generalized Hubbard Hamiltonian, many-electron wavefunctions of negatively charged (NV⁻) and neutral nitrogen-vacancy (NV⁰) centers in diamond were calculated. We report the effect of symmetric relaxation of surrounding atoms on the spin density, calculated from the many electron wavefunctions in the ground and excited states. We evaluated the error, that, arises in estimation of spin density when lattice relaxation effect is neglected in Electron Paramagnetic Resonance experiment and showed that the ground state spin density distribution is accessible in outward relaxations. The computed oscillator strengths give a higher efficiency for the 1.945 eV photoluminescence (PL) line of NV⁻ with respect to 2.156 eV PL line of NV⁰ which agrees well with experiment. This result is explained based on the largest the ground state spin among available values for the NV⁻ with respect to NV⁰. The transition probability between degenerate ground and excited states slightly depends on the S _z value. Finally, we report on the electronic configurations which contribute to the ground and excited states and discuss the population variation of electronic configurations with relaxation.     

Sc掺杂SnO2稀磁半导体的第一性原理研究

采用第一性原理方法,对Sc掺杂SnO2以及含有O空位的Sc掺杂SnO2的电子结构和磁学性质进行了计算。结果表明SnO2晶格中存在两种本征磁性来源,分别为Sc掺杂诱导的未配对O-2p态电子的自旋极化和O空位诱导的未配对Sn-5p态电子的自旋极化。由于两种未配对的弱束缚电子分别由电离施主和受主诱导产生,因此二者之间存在电荷补偿效应,在特定配比下能够使SnO2晶格出现磁性猝灭。     

The use of ENDOR to identify the atomic structure of defects in diamond

Although nearly 100 paramagnetic defects have been catalogued in diamond by spin Hamiltonian parameters measured by electron paramagnetic resonance (EPR), very few of these have been unambiguously associated with an atomic model. It has been necessary to use electron nuclear double resonance (ENDOR) to obtain enough information to make proper assignment of such models. The reason for the limitation of EPR, and the way in which ENDOR overcomes these limitations are discussed. The interpretation of hyperfine structure in terms of unpaired electrons in molecular orbitals, and of quadrupole interactions in terms of all electrons, paired and unpaired, as a source of information about molecular structure in diamond, is evaluated by reference to some well documented examples. The measurements so far made by ENDOR on defects in diamond are reviewed, and the salient contribution for the assignment of a model for each defect is explained. The details revealed by ENDOR considerably increase knowledge about defects, particularly those involving substitutional nitrogen atoms. This in turn helps in understanding the complex electron and atom, migration processes which go on under appropriate conditions of temperature and pressure, or optical excitation. The possibilities are discussed for using ENDOR to increase the number of well characterized centres.     

Electron paramagnetic resonance study of hole trapping at defects in silver halides

Abstract

Hole-trapping at cation vacancies and iodide impurity in silver halides was studied by means of electron paramagnetic resonance (EPR). In AgCl, the hole was found to be preferentially self-trapped on a Ag ion next to a cation vacancy, forming a perturbed self-trapped hole center. The position of the vacancy relative to the center of the STH was determined, by comparison of computer simulations to the experimental EPR spectra, to be in either the nearest-neighbor or next-nearest-neighbor equatorial position. The thermal trap depths of the hole in these centers are estimated to be 0.31 and 0.24 eV, respectively. For lightly doped (several tens of ppm) AgBr:I, an interesting spectrum has been observed, consisting of six equidistant and approximately equi-intense hyperfine EPR lines, consistent with a trapped hole localized at an iodide ion (nuclear spin 5/2), but not with an iodide-perturbed STH.     

The multiple hydrogen location near a α-Fe vacancy

The interaction between 10 hydrogen atoms and a α-Fe structure having a vacancy (V) has been studied using a cluster model and a semi-empirical theoretical method. The energy of the system was calculated by the atom superposition and electron delocalization molecular orbital method. The electronic structure was studied using the concept of density of states and crystal orbital overlap population curves.For the study of a sequential absorption, the hydrogen atoms were positioned in their energy minima configurations, near to the tetrahedral sites neighboring the vacancy, except the last H atom that was located far from the vacancy. The energy difference for H agglomeration was also computed. The vacancy-H_n complexes become less stable than VH species for more than three hydrogen's.The changes in the electronic structure of Fe atoms near the vacancy were also analyzed. The interactions mainly involve Fe 3d and 4s atomic orbitals. The contribution of Fe p orbitals is much less important. The Fe-Fe bond weakened as new Fe-H and H-H pairs were formed. The effect of H atoms is limited to its first Fe neighbors. The detrimental effect of H atoms on the Fe-Fe bonds can be related to the mechanism for embrittlement in α-Fe.     

Quantum-chemical modeling of structural,electronic, and spin characteristics of NV centers in nanostructured diamond: Surface effect

The effect of the surface of diamond on atomic, electronic, and spin properties of diamond nanocrystals containing single nitrogen-vacancy defects ([NV]⁻ centers) is studied. The surface was modeled with clusters C₃₃H₃₀[NV]⁻, C₆₆H₇₂[NV]⁻, which were constructed based on bulk clusters C₃₃H₃₆[NV]⁻ and C₆₉H₈₄[NV]⁻, respectively. In all cases, clusters in the triplet state S = 1 are considered with the cluster charge being −1. The geometric structure of clusters is optimized using the principle of minimization of the total energy of the system; then, the electronic and spin characteristics of clusters are calculated by the density functional theory. The isotropic and anisotropic hyperfine interaction constants of the electron spin of the NV center with the nuclear spin of the nitrogen atom and ¹³C atoms located at different sites in the cluster are calculated. It is found that, in contrast to bulk clusters with [NV]-centers in which the spin density is mainly localized at the three carbon atoms that are the nearest neighbors of the vacancy of the center, upon arrangement of the NV center in the immediate proximity to the surface, the spin density is redistributed such that it is mainly localized at the three carbon atoms that are the nearest neighbors of the nitrogen atom of the center and at C atoms that form the first atomic layer of the (111) surface of the nanocrystal.     

Electronic structure of the isolated vacancy in silicon

The many-electron molecular orbital method predicts a ground state for the various charged vacancy centres in silicon which has a spin multiplicity in agreement with the electron spin resonance results. Symmetric relaxation and Jahn-Teller contributions have been included in first order. The quantitative features of the model are very different to those implied by the one-electron model.     

Study of vacancy on diamond (1 0 0) (2 × 1) surface from first-principles

Structure and energy related properties of neutral and charged vacancies on relaxed diamond (1 0 0) (2 × 1) surface were investigated by means of density functional theory. Calculations indicate that the diffusion of a single vacancy from the top surface layer to the second layer is not energetically favored. Analysis of energies in charged system shows that neutral state is most stable on diamond (1 0 0) (2 × 1) surface. The multiplicity of possible states can exist on diamond (1 0 0) surface in dependence on the surface Fermi level, which supports that surface diffusion of a vacancy is mediated by the change of vacancy charge states. Analysis of density of states shows surface vacancy can be effectively measured by photoelectricity technology.     

六方氮化硼单层中一种(CN)3VB缺陷的第一性原理计算

二维六方氮化硼(hBN)的点缺陷最近被发现可以实现室温下的单光子发射,而成为近年的研究热点.尽管其具有重要的基础和应用研究意义,hBN中发光缺陷的原子结构起源仍然存在争议.本文采用基于密度泛函理论的第一性原理计算,研究hBN单层中一种B空位附近3个N原子被C替代的缺陷(CN)3VB.在hBN的B空位处,3个N原子各自带一个在平面内的悬挂键及相应的未配对电子,而通过C替换可以消除未配对的电子.系统研究了(CN)3VB缺陷的几何结构、电子结构以及光学性质,结果表明,缺陷可以由一个对称的亚稳态经过原子结构弛豫变成1个非对称的、3个C原子连在一起的基态结构.缺陷的形成在hBN中引入了一些由缺陷悬挂σ键及重构的π键贡献的局域缺陷态.这些缺陷态可以导致能量阈值在2.58 eV附近的可见光内部跃迁.本文的工作有助于进一步理解hBN中点缺陷的构成及光学性质,为实验上探讨发光点缺陷的原子结构起源及其性质提供理论依据.     

LiAl中空位形成能的第一原理计算

LiAl是一种非常典型和有重要用途的金属间化合物.采用平面波展开和第一原理赝势法，计算了LiAl化合物中Li空位和Al空位的形成能和空位周围的原子弛豫，讨论了空位形成时电荷密度的重新分布、相应的电子态密度以及能带结构等性质. 关键词： LiAl 空位形成能 第一原理计算     

First-principle study of magnetism in Co-doped SnO2

The effects of Co dopants and oxygen vacancies on the electronic structure and magnetic properties of the Co-doped SnO₂ are studied by the first-principle calculations in full-potential linearized augmented plane wave formalism within generalized gradient approximations. The Co atoms favorably substitute on neighboring sites of the metal sublattice. Without oxygen vacancies, the Co atoms are at low spin state independent of concentration and distribution of Co atoms, and only the magnetic coupling between nearest-neighbor Co atoms is ferromagnetic through direct exchange and super-exchange interaction. Oxygen vacancies tend to locate near the Co atoms. Their presence strongly increases the local magnetic moments of Co atoms, which depend sensitively on the concentration and distribution of Co atoms. Moreover, oxygen vacancies can induce the long-range ferromagnetic coupling between well-separated Co atoms through the spin-split impurity band exchange mechanism. Thus the room temperature ferromagnetism observed experimentally in the Co-doped SnO₂ may originate from the combination of short-range direct exchange and super-exchange interaction and the long-range spin-split impurity band exchange model.     

闪锌矿结构AlSb空位缺陷性质的第一性原理研究

采用基于密度泛函理论(DFT)第一性原理的平面波超软赝势方法，计算含有锑空位和铝空位的电子结构，发现空位引起周围原子弛豫，晶体结构发生畸变。在此基础上研究了空位缺陷对闪锌矿型AlSb体系电子结构的影响，结果表明，铝空位缺陷使锑化铝费米能级降低，带隙变窄；而锑空位缺陷则使锑化铝费米能级升高，带隙变窄，同时，锑化铝的半导体类型由p型变为n型。对光学性质的研究发现，由于空位的引入使其邻近原子电子结构发生变化，使得空位缺陷系统光学性质的变化主要集中在低能量区域。