Structural and electronic properties of endohedral phosphorus fullerene P@C60: an off-centre displacement of P inside the cage

Single P-doped endohedral P@C₆₀ is investigated via semiempirical and first-principles calculations. Unlike the encased N atom, which is situated on the centre of the C₆₀ cage and not covalently bound to the carbon atoms of the fullerene cage, static geometric optimization shows that the encased P atom occupies an off-centre position and is bound to the carbon atoms of the fullerene cage. The electronic ground state of the doped system is the spin quarter state, with spin density distribution significantly compressed by the cage.     

Spin resonance transport properties of a single Au atom in S–Au–S junction and Au–Au–Au junction

The spin transport properties of S–Au–S junction and Au–Au–Au junction between Au nanowires are investigated with density functional theory and the non-equilibrium Green's function. We mainly focus on the spin resonance transport properties of the center Au atom. The breaking of chemical bonds between anchor atoms and center Au atom significantly influences their spin transmission characteristics. We find the 0.8 eV orbital energy shift between anchor S atoms and the center Au atom can well protect the spin state stored in the S–Au–S junction and efficiently extract its spin state to the current by spin resonance mechanism, while the spin interaction of itinerant electrons and the valence electron of the center Au atom in the Au–Au–Au junction can extract the current spin information into the center Au atom. Fermi energy drift and bias-dependent spin filtering properties of the Au–Au–Au junction may transform information between distance, bias,and electron spin. Those unique properties make them potential candidates for a logical nanocircuit.     

Electron paramagnetic resonance investigation of endohedral fullerenes N@C(60) and N@C(70) in a liquid crystal

Endohedral fullerenes N@C(60) and N@C(70) were dissolved in the liquid crystal 4-methoxybenzylidene-4'-n-butylaniline (MBBA) and investigated by electron paramagnetic resonance. In both cases well resolved EPR spectra give proof for molecular orientation in the nematic mesophase. Spectral features are dominated by a nonvanishing zero-field interaction, indicating a deviation from spherical spin density distribution at the encased nitrogen atom. In N@C(70), a maximum order parameter O(33) = 0.18(3), correlated with the long axis of the cage, and a zero-field-splitting parameter D = -2.6(4) MHz were determined. A persistent zero-field splitting is also observed in C(60) via the quartet spin of the encapsulated nitrogen, although no assignment of the director with respect to the molecular frame is possible. The observed line splitting is indicative of pseudo orientation of the rapidly rotating cage in this case.     

Stepwise computational synthesis of fullerene C<Subscript>60</Subscript> derivatives. Fluorinated fullerenes C<Subscript>60</Subscript>F<Subscript>2<Emphasis Type="Italic">k</Emphasis></Subscript>

Reactions of fullerene C₆₀ with atomic fluorine are studied by the unrestricted broken spin symmetry Hartree-Fock (UBS HF) approach implemented in semiempirical codes based on the AMI technique. The calculations are focused on a successive addition of a fluorine atom to the fullerene cage following the indication of the highest chemical susceptibility of the cage atom, which is calculated at each step. The proposed computational synthesis is based on the effectively unpaired-electron concept of the chemical susceptibility of fullerene atoms. The obtained results are analyzed from the standpoints of energy, symmetry, and the composition abundance. A good fitting of the data to experimental findings proves a creative role of the suggested synthetic methodology.     

Spin couplings in small magnetic systems

Various spin couplings for magnetic atoms in dilute alloys are described by treating the symmetry of configuration space wave function and its connection with spin properties. All possible degenerate atomic levels are considered for describing the direct couplings between two nearlying atoms and the coupling between the atom and the conduction sea. Although, the qualitative form of the expressions is known from general arguments, such a detailed treatment is necessary to account for the actual appearance of spin couplings.     

Squeezing of nitrogen atomic orbitals in a chemical trap

Fullerenes can act as inert cages for highly reactive nitrogen atoms even at room temperature. Confinement in a cage of less than spherical symmetry as realized in C70 leads to a characteristic deformation of the atomic charge and spin distributions which can be sensed by magnetic resonance techniques. A quantitative analysis of the amount of orbital squeezing is possible by comparison with data of free nitrogen ions.     

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.     

Nuclear magnetic resonance chemical shift in an arbitrary electronic spin state

We present a general and systematic electronic structure theory of the nuclear magnetic resonance shielding tensor and the associated chemical shift for paramagnetic atoms, molecules, and nonmetallic solids. The approach is for the first time rigorous for an arbitrary spin state as well as arbitrary spatial symmetry and is formulated without reference to spin susceptibility. The leading-order magnetic-field dependence of shielding is derived. The theory is demonstrated by first principles calculations of organometallic molecules.     

Transformation of As-Grown Phosphorus-Related Centers in HPHT Treated Synthetic Diamonds

This communication presents new data on phosphorus-containing centers in synthetic diamonds grown in the P–C system by high-pressure high-temperature (HTHP) method and annealed in the temperature range of 2,073–2,573 K. The electron paramagnetic resonance (EPR) study has shown that as-grown at 1,873 K diamonds contain single substitutional nitrogen (P1) and single substitutional phosphorus (MA1) centers. The main part of the spin density in the MA1 center locates on the carbon atom C₁ separated from phosphorus by one carbon atom. HPHT annealing (7 GPa, 2,073–2,273 K) results in aggregating substitutional nitrogen and phosphorus atoms. On the first step of annealing (2,073 K) of as-grown diamonds nitrogen–phosphorus NIRIM8 (NP1) centers are created. It is supposed that nitrogen and phosphorus atoms in this center are separated by two carbons. Further temperature increasing shifts the nitrogen atom toward phosphorus and creates two new nitrogen–phosphorus centers NP2 and NP3 with the supposed structures C₁–N–C–P and N–P–C₁, respectively. The main part of the spin density in MA1, NIRIM8 (NP1), NP2 and NP3 is located on the carbon atom C₁. Annealing these samples in the temperature range of 2,073–2,273 K has shown vanishing of NIRIM8 and increasing of NP2 and NP3 centers. HPHT annealing of diamonds at 2,573 K significantly changes the electron paramagnetic resonance (EPR) spectra: all previous nitrogen–phosphorus centers disappear and two new phosphorus centers NP4 and NP5 are created. Features of these centers are g ≈ 2.001 and high spin density located on the phosphorus atoms. The NP5 center is sensitive to X-ray irradiation and low-temperature annealing. The EPR spectra of both these centers are due to the hyperfine structure of one phosphorus atom. The structures of all phosphorus-containing centers are discussed.     

SiC单层内Co空间选位对自旋组态的调控

基于第一性原理的密度泛函理论,对SiC单层不同位置掺杂Co进行了能带结构、电子态密度、净自旋密度和自旋纹理等计算,结果表明不同位置的掺杂引起不同特征的自旋积累及单层的电子结构特性。由于Co的不同选位掺杂而产生一些新奇现象,如扭曲的Co-C键在掺杂SiC内激发了自旋流而诱导了自旋重新分布,不同选位的Co原子通过调整内磁场改变了小极化子内巡游电子的定域属性,增加了Dirac点附近磁振子的色散强度等。这些研究结果为得到一个人工调控量子自旋电路和选频自旋波器件内自旋谷电子提供了理想平台。     

The solution conformation of triarylmethyl radicals

Hyperfine coupling tensors to 1H, 2H, and natural abundance 13C were measured using X-band pulsed electron nuclear double resonance (ENDOR) spectroscopy for two triarylmethyl (trityl) radicals used in electron paramagnetic resonance imaging and oximetry: methyl tris(8-carboxy-2,2,6,6-tetramethyl-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl) and methyl tris(8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl). Quantum chemical calculations using density functional theory predict a structure that reproduces the experimentally determined hyperfine tensors. The radicals are propeller-shaped with the three aryl rings nearly mutually orthogonal. The central carbon atom carrying most of the unpaired electron spin density is surrounded by the sulfur atoms in the radical and is completely shielded from solvent. This structure explains features of the electron spin relaxation of these radicals and suggests ways in which the radicals can be chemically modified to improve their characteristics for imaging and oximetry.     

单层GaSe表面Fe原子吸附体系电子自旋性质调控

Ⅲ族金属单硫化物因其优越的光电和自旋电子特性而备受关注,实现对其自旋性质的有效调控是发展器件应用的关键.本文采用密度泛函理论系统地研究了GaSe表面Fe原子吸附体系的几何构型及自旋电子特性.Fe/GaSe体系中Fe吸附原子与最近邻Ga,Se原子存在较强的轨道耦合效应,使体系呈现100%自旋极化的半金属性.其自旋极化贡献主要来源于Fe-3d电子的转移及Fe-3d,Se-4p和Ga-4p轨道杂化效应.对于Fe双原子吸附体系,两Fe原子之间的自旋局域导致原本从Fe转移至GaSe的自旋极化电荷量减少,从而费米能级附近的单自旋通道转变为双自旋通道,费米能级处的自旋极化率转变为0.研究结果揭示了Fe_n/GaSe吸附体系自旋极化特性的形成和转变机制,可为未来二维自旋纳米器件的设计与构建提供参考.     

Magnetism of palladium clusters in nanoporous carbon

To determine the electronic properties of metal-carbon clusters, palladium clusters are introduced into nanoporous carbon samples. The structural and magnetic properties of the nanocomposite thus obtained are investigated. A ferromagnetic resonance (FMR) component is observed in the electron spin resonance (ESR) spectra measured in a wide temperature range. The electron density of states (DOS) of clusters consisting of a palladium atom surrounded by carbon atoms is calculated, and it is shown that, due to the interaction between electronic terms of carbon and metal, such a cluster should exhibit magnetism. The investigation of the structures of the porous skeleton of nanoporous carbon and of palladium clusters has allowed us to reveal how carbon and palladium atoms are brought into close contact so that to give rise to the observed ferromagnetism.     

硅中Jahn-Teller畸变的磷-空位复合缺陷的电子结构

本文采用扩展的缺陷势,利用一个紧束缚的Koster-Slater格林函数方法,确定了磷-空位缺陷的波函数为深能级E的函数,以深能级的实验值为输入参数,得到的波函数定量地描述了EPR和ENDOR实验资料,特别是,理论给出空位的四个近邻原子上的超精细相互作用常数α和b,同实验符合得很好。 关键词：     

Spin-polarized electron scattering at single oxygen adsorbates on a magnetic surface

Scanning tunneling spectroscopy (STS) on the system of isolated oxygen atoms adsorbed on the double layer of Fe on W(110) shows highly anisotropic spatial oscillations in the local density of states in the vicinity of the adsorbates. We explain this in terms of a single-particle model as electron waves being scattered by the potential induced by the presence of the oxygen atoms. Analysis of the wavelength of the standing electron waves and comparison with ab initio spin-resolved electronic structure calculations reveal that minority-spin bands of d-like symmetry are involved in the scattering process. By applying spin-polarized STS, we observe this standing wave pattern on one particular type of magnetic domain of Fe on W(110) only, thereby proving that the standing electron waves are highly spin polarized.     

Evidence of a critical Mn-Mn distance for the onset of ferromagnetism in NiAs type compounds

Recently, the technique of spin wave resonance has been used to measure the exchange coupling constants as a function of composition in several solid solutions of MnSb. From an examination of the results we show that in those alloys in which the linear chains of magnetic atoms are not disturbed, the exchange coupling constants become positive leading to ferromagnetic coupling when the distance between Mn atoms becomes equal to 2.83 Å or more. This experimental value of the Mn-Mn distance for the onset of ferromagnetism is equal to the critical distance derived by Forrer after examining a large number of materials which owe their ferromagnetism predominantly to the direct exchange interaction.     

Specific features of the resonance interaction of a coherent wave with an ensemble of impurity atoms of a photonic crystal

Two-quantum relaxation transitions in an impurity atom that are activated by its resonance interaction with a monochromatic wave lead to hole burning in the distribution of atoms populating the lower energy level over detunings from the resonance. In this case, the levels of an impurity atom lying within the gap or the pseudogap in the density of photon states of a photonic crystal act as a trap. The impurity atoms change to these levels in the process of interaction of a monochromatic wave with the atomic transition that is not influenced by specific spectral features of a photonic crystal.     

氢原子吸附对金表面金属酞菁分子的吸附位置、自旋和手征性的调控

实现单个功能有机分子构型、电子结构和自旋态的可逆调控, 是未来分子电子学和分子自旋电子学应用的关键. 近年来, 我们利用极低温强磁场超高真空扫描隧道显微镜系统, 结合第一性原理计算, 系统研究了氢原子吸附对金表面吸附的金属酞菁分子的自旋、手性和吸附位置的调控. 通过将金表面吸附的酞菁锰分子暴露于氢气或氢原子环境, 使得分子中心的磁性离子吸附单个氢原子, 从而实现了体系近藤效应由“开”到“关”的转变. 基于密度泛函理论的第一性原理计算表明, 氢原子吸附使得锰离子3d轨道内的电荷重排导致了分子的自旋由3/2降为1; 同时分子与金基底的间距增大, 使得近藤效应消失. 通过施加局域电压脉冲或者给样品加热, 可以实现单个或所有分子脱氢, 从而恢复体系的自旋态和近藤效应. 氢原子吸附还导致分子的优先吸附位置从金表面的面心立方堆垛区域变成了六角密排堆垛区域. 三个氢原子吸附于同一酞菁锰分子上, 可导致分子对称性的降低及分子镜面对称轴与金基底镜面对称轴的偏离, 从而导致手征性的出现. 这种分子吸附结构的手征性, 导致分子轨道也呈现出手征性. 这项工作为金属酞菁未来在分子电子学、自旋电子学、气体传感器等方面的应用提供了新思路.     

类锂体系激发态的精确判据

利用Gálvez Porras关于原子体系电子在核处密度和径向期待值的精确不等式，分别建立了波函数的两套精确判据，它们是独立于能量判据之外的检验波函数精确度的判据. 以类锂体系激发态为研究对象，首先对1s2 3s态的电子在核处密度和径向期待值的上界进行了检查，然后对锂原子1s2 3s态在不同计算方法下由电子在核处密度上界定义的R6值和的上界定义的R4值分别进行了测试，同时还对锂原子双激发态的电子在核处密度和径向期待值的上界进行了检验. 研究表明这两套不等式都可作为原子体系各种能态波函数的精确判据. 关键词： 电子在核处密度 径向期待值 全实加关联 精确判据     

Quantum Teleportation and Resonance Information Transfer from One Atom to Another at Arbitrary Interatomic Distances

The feasibility of resonance transfer of quantum information from one double-level atom to another that is at an arbitrary distance from the former one has been proved. Symmetric and antisymmetric combinations of the wave functions of individual atoms are considered. When taking into account the interatomic dipole–dipole interaction, a certain energy corresponds to each wave function. A solution has been found to a system of equations for the amplitudes of the probability that a resonance photon will be absorbed by one of the system atoms, and it has been shown that the interaction of the system with actual photons has the result that the wave function of the final state of the system can be represented as a linear combination of the functions < 00|, < 0n|, and < n0| corresponding to the ground and excited states of individual atoms. The amplitude of the probability of each of these states depends on the interatomic distance and on the parameters of the action of actual photons on atoms. Three types of solution to the system of equations have been investigated for the resonance and nonresonance absorption of photons and different interatomic distances. It has been shown that when atoms are at an infinite distance from one another, so that there is no dipole–dipole interaction of atoms, quantum information can be transferred from one atom to another with a characteristic time considerably shorter than the time it takes for a photon to cover the interatomic distance. This effect is referred to as the effect of quantum teleportation in a system of resonance atoms.