Magnetism of Fe clusters embedded in Cu,Ag, and Au fcc matrices: density functional calculations

We present extensive first principles density functional theory (DFT) calculations dedicated to analyze the magnetic properties of small Fe _n clusters (n = 2,3) embedded in Cu fcc, Ag fcc and Au fcc matrices. We consider several dimers and trimers having different interatomic distances. In all cases the Fe atoms are embedded as substitutional impurities in the metallic network. For the case of the Fe dimers we have considered two magnetic configurations: ferromagnetic (antiferromagnetic), when the atomic magnetic moment of the Fe atoms are parallel (antiparallel) each other. For the case of dimers immersed in Cu and Ag matrices, the ground state corresponds to the ferromagnetic Fe dimer whose interatomic distance is a/√2. For Fe dimer immersed in the Au matrix the ground state corresponds to a ferromagnetic coupling when the interatomic distance is a√(3/2). In the case of the Fe trimers we have considered three or four magnetic configurations, depending on the Fe cluster geometry. For the case of Fe trimer immersed in Cu and Ag matrices we have found that the ground state corresponds to the ferromagnetic trimer forming an equilateral triangle with an interatomic distance equal to a/√2. The ground state for the Fe trimer immersed in the Au matrix corresponds to the ferromagnetic Fe trimer forming a right angle triangle.     

Geometries and electronic properties of NbnV(0, ±1) (n = 1−6) clusters studied by density-functional theory

Using density functional theory (DFT) with valence basis set LANL2TZ to study the relative stabilities and electronic properties of the most stable structures of Nb _n V^(0,?±1) (n = 1?6) clusters. The ground state structures of Nb _n V ^(0,?±1) keep the similar geometric structure as the host Nb _n clusters. The doping of vanadium atom enhances the chemical activities of Nb _n clusters. The Nb₃V and Nb₆V are more stable than other clusters. The average binding energy of charged systems (Nb _n V⁺ and Nb _n V^? clusters) are generally larger than neutral Nb _n V clusters natural population analysis shows that there are charge transfers from niobium to vanadium atoms in the small Nb_1?4V, however, for larger clusters (Nb₅V and Nb₆V), the charge transfers are from vanadium to niobium atoms. The vertical and adiabatic ionization potentials (VIP and AIP) are estimated and the vertical one is more close to experimental results.     

The relativistic density functional investigations on geometries, electronic and magnetic properties of Irn (n=1-13) clusters

<正>The Ir_n(n=1-13) clusters are studied using the relativistic density functional method with generalized gradient approximation.A series of low-lying structures with different spin multiplicities have been considered.It is found that all the lowest-energy Ir_n(n=4-13) geometries prefer non-compact structures rather than compact structure growth pattern.And the cube structure is a very stable cell for the lowest-energy Ir_n(n8) clusters.The second-order difference of energy,the vertical ionization potentials,the electron affinities and the atomic average magnetic moments for the lowest-energy Ir_n geometries all show odd-even alternative behaviours.     

Self-field MPD thruster with atomic and molecular propellants

This study is devoted to the design, the operational characteristics and optimization of a self-field magnetoplasmadynamic (MPD) arc in view of its applications as a thruster and as a plasma source for recombination lasers. Its principal characteristics, i.e. its terminal voltageV _E1and the exhaust speedu _eas functions of the square of the discharge currentI divided by the mass flow ratem are determined experimentally for atomic propellants. On the basis of measured diagramsV _E1versus (I ²/m) our discharge configuration is found to be well comparable to those applied by other laboratories. However, in order to further increase the specific impulse, these devices should make use of propellants of low molecular weight. Therefore, we also show that these thrusters can be operated in a quasi-steady mode for atomic as well as for molecular propellants, e.g. H₂, N₂, O₂. Existing theories and interpretations of observations on atomic propellants are reviewed and, if possible, extended to explain the experimental results obtained with molecular propellants. In the framework of these extended theories the high-power limit of a stable operation is well understood.On leave from the Polish Academy of Sciences, Gdansk, Poland     

Non-collinear magnetic moments of seven-atom Cr, Mn and Fe clusters

The non-collinearity of magnetic moments of pentagonal bipyramid Cr₇, Mn₇ and Fe₇ clusters is discussed. The magnetic moments are calculated by the discrete variational non-collinear spin-density functional method. For the Cr₇ cluster, a coplanar magnetic arrangement appears at the large interatomic distance. With decreasing the interatomic distance, the coplanar arrangement changes to the parallel arrangement with a small absolute magnetic moment. For the Mn₇ cluster, the magnetic arrangement changes from coplanar to antiparallel with decreasing the interatomic distance. Also for the Fe₇ cluster, some coplanar magnetic moments appear at the interatomic distance of 2.23 ?. In these coplanar magnetic arrangements, the magnetic moment at the basal site of the pentagon rotates with a step of 144 degrees for the Cr₇ clusters and 72 degrees for the Mn₇ and Fe₇ clusters. Received 30 November 2000     

Geometrical,electronic, and magnetic properties of Au n V (n = 1–8) clusters: A density functional study

The geometrical, electronic, and magnetic properties of small Au _n V (n?=?1–8) clusters have been investigated using density functional theory at the PW91 level. An extensive structural search indicates that the V atom in low-energy Au _n V isomers tends to occupy the most highly coordinated position and the ground-state configuration of Au _n V clusters favors a planar structure. The substitution of a V atom for an Au atom in the Au _{n +1} cluster transforms the structure of the host cluster. Maximum peaks are observed for the ground-state Au _n V clusters at n?=?2 and 4 for the size dependence of the second-order energy differences, implying that the Au₂V and Au₄V clusters possess relatively higher stability. The energy gap of the Au₃V cluster is the largest of all the clusters. This may be ascribed to its highly symmetrical geometry and closed eight-electron shell. For ground-state clusters with the same spin multiplicity, as the clusters size increases, the vertical ionization potential decreases and the electron affinity increases. Magnetism calculations for the most stable Au _n V clusters demonstrate that the V atom enhances the magnetic moment of the host clusters and carries most of the total magnetic moment.     

Ferromagnetism of V-doped ZnO nanowires

The geometry structures,electronic structures,and magnetic properties of Zn46V2O48 nanowires are studied by density functional theory(DFT) calculations.We find that the ferromagnetic(FM) coupling is more stable for six configurations of Zn46V2O48 nanowires,and is mediated by neighboring O as evidenced from the strong hybridization of V 3d and O 2p states,exhibiting strong spin polarization.The spin polarization is found to be 100% in the Zn46V2O48 nanowires,which confirms that it is a half-metallic ferromagnet and very suitable for the injection of the spin carriers,which shows that Zn46V2O48 nanowire is one of the ideal materials to realize spin electronic devices.At the same time,the magnetic coupling mechanisms of Zn46V2O48 nanowires are analyzed with V 3d and O 2p orbitals and their magnetic moments mainly come from the contributions of the unpaired electrons of V 3d orbitals.The above results provide a theoretical basis for the preparation of 3d transition metal-doped ZnO nanowire materials.     

Atomic structure of Pdn (4 ≤ n ≤ 15) nanoclusters

The results of molecular dynamics simulation of the process of atom-by-atom growth of palladium nanoparticles are presented. The structures obtained for the Pd _n clusters agree well with the results of the first-principles calculations. The low-symmetry atomic configurations for the structure of the Pd₄ and Pd₁₃ clusters have been obtained using the semiempirical potentials of the interatomic interaction for the first time.     

Atomic displacements due to interstitial hydrogen in Cu and Pd

The density functional theory (DFT) is used to study the atomic interactions in transition metal-based interstitial alloys. The strain field is calculated in the discrete lattice model using Kanzaki method. The total energy and hence atomic forces between interstitial hydrogen and transition metal hosts are calculated using DFT. The norm-conserving pseudopotentials for H, Cu and Pd are generated self-consistently. The dynamical matrices are evaluated considering interaction up to first nearest neighbors whereas impurity-induced forces are calculated with M₃₂H shell (where M = Cu and Pd). The atomic displacements produced by interstitial hydrogen at the octahedral site in Cu and Pd show displacements of 7.36% and 4.3% of the first nearest neighbors respectively. Both Cu and Pd lattices show lattice expansion due to the presence of hydrogen and the obtained average lattice expansion ΔV/V = 0.177 for Cu and 0.145 for Pd.      

Hydrogen induced change of the atomic magnetic moments in Fe/V-superlattices

We have studied the change of the magnetic saturation of (Fe_n/V_m)₃₀ superlattices (30 periods with n monolayers of Fe and m monolayers of V) upon loading with hydrogen using a highly sensitive Faraday balance and in situ loading with hydrogen. We find that the measured magnetic saturation moment for all samples increases with the hydrogen. The measured magnetic saturation moment for all samples increases with the hydrogen concentration. For the superlattice (Fe₃/V₁₁)₃₀ we find the maximum increase, corresponding to a change of the atomic magnetic moments of +0.35 μ_B/Fe atom. We attribute this remarkable effect to a change of the Fe and V magnetic moments at the interfaces caused by the charge transfer from the hydrogen atoms to the vanadium d bands.     

On the role of correlation energy produced by the spin-spin interaction in the formation of binding energy for two-electron atomic systems

An algorithm is proposed for computing the correlation energy produced by the spin-spin interaction V _SS of electrons in He-like atomic systems. The algorithm takes into account the singular nature of V _SS and the formation of a compact finite motion of electrons in the range of distances Ze ²/mc ² < r ₁₂ < ħ/mc between the electrons under the action of the magnetic fields of spins for a singlet ground state. Good agreement with experimental values of the ionization potential is attained for a wide set of two-electron atomic systems without resorting to variational procedures, but only using hydrogen-like wavefunctions and correctly taking into account the singular nature of the spin-spin interaction of electrons.     

空位的第一性原理及经验势函数的对比研究

利用第一性原理及Stillinger-Weber（SW）,EDIP和Tersoff经验势函数对比研究了硅中单空位(V₁)、双空位(V₂)和六边形空位环(V₆)的结构特性及形成能.讨论了经验势函数描述空位时的优点和缺点.结果发现,第一性原理方法可以精确描述空位的原子结构及能量特性,而短程有效的经验势函数无法描述空位所固有的量子效应,如Jahn-Teller变形等.另外,由于经验势函数自身的缺陷,EDIP和T3无法应用于空位结构特性的计算.虽然 关键词： 空位 第一性原理 经验势函数     

Magnetic ordering of Vn/Mo(0 0 1) systems: Ab-initio calculations

A density-functional theory (DFT) study is performed using a full-potential linearized-augmented-plane-waves (FP-LAPW) method to investigate the magnetic structure of vanadium-molybdenum systems (V_n/Mo(0 0 1), n = 1, 2). The topmost V layers relax inward in both systems with a larger contraction in V₂/Mo(0 0 1) system. A p(1 × 1) in-plane ferromagnetic ordering with appreciable magnetic moments is obtained on V overlayers, which is found to be the ground state in both systems. The layers below the surface exhibit induced magnetism with antiferromagnetic interlayer coupling.     

Chemisorption energetics and surface reactivity: UBI-QEP versus DFT projections

The wealth of information accumulated recently by ab initio/DFT calculations of adsorption energetics and reactivity on metal surfaces makes it possible to systematically compare projections of the DFT and UBI-QEP approaches. We make such comparisons covering both qualitative regularities in and the quantitative accuracy of binding energies and activation barriers. We focus on the following areas: (1) DFT verification of the UBI-QEP assumptions and rigorous projections concerning atomic and molecular binding energies at low coverage; (2) coverage effects revealing in the M-A bond energy competition (metal sharing A-M-A) under atomic co-adsorption; (3) reactivity patterns including both qualitative (periodic dependence of the activation barriers, barrier vs. reaction enthalpy relationships, geometry of the transition state, etc.) and quantitative aspects. We demonstrate the broad agreement between the DFT and UBI-QEP projections and point out the areas where further testing by the ab initio/DFT techniques might be necessary and illuminating. In particular, while discussing the ab initio/DFT absolute values of the atomic binding energies Q _A and their periodic trends, we revised our previous assumptions about monotonic changes of the Q _A values with the changing atom A valence and along the VIII and IB group metals, where the periodic trends appear to be complicated. Accordingly, we make corrections to the original UBI-QEP parameters, summarizing the current recommended values of Q _A.     

Local atomic structures of amorphous Pd80Si20 alloys and their configuration heredity in the rapid solidification

The local atomic structures of amorphous Pd₈₀Si₂₀ alloys and their configuration heredity in the rapid solidification are investigated by a molecular dynamics simulation with the help of cluster-type index method based on Honeycutt–Anderson bond-type index and an inversely tracking technique of atomic trajectories. Their short-range orders are found to be various Kasper clusters as well as their distorted configurations, and among which (10 2/1441 8/1551) bi-capped square Archimedean anti-prism (BSAP) clusters are dominated, e.g. Si-centred Pd₁₀Si₁ clusters. These Kasper clusters mainly exist in the form of isolated basic clusters. Few medium-range orders can be detected, especially for Si-centred Kasper clusters. Similarly to icosahedrons of Cu–Zr amorphous alloys, their sustainable configuration heredity also occurs firstly in the super-cooled liquid region, and BSAP clusters have higher onset temperature T_onset and bigger descendible fraction F than other Kasper clusters in the rapid solidification of Pd₈₀Si₂₀ alloys.     

密度泛函理论对CoBen(n=1—12)团簇结构和性质的研究

采用密度泛函理论中的广义梯度近似(GGA)对CoBe_n(n=1—12)团簇的几何构型进行优化，并对能量、频率和磁性进行了计算，同时考虑了电子的自旋多重度.得到了CoBe_n(n=1—12)团簇最低能量结构的自旋多重度是2和4.在CoBe_n(n=1—12)团簇中，Co原子的磁矩出现了奇偶振荡，当n=6时，Co原子的4s，3d和Be原子的2s，2p较强杂化、Co-Be键长的减小以及对称性的降低导致Co原子的磁矩最小.通过对CoBe_n(n=1—12)团簇电子性质的分析，得出了掺杂可以增强团簇稳定性和有利于增加合金化学活性的结论.n=5，10是团簇的幻数. 关键词： n团簇')" href="#">CoBe_n团簇 自旋多重度 磁矩 电子性质     

氧化钒薄膜的微结构及阻变特性研究

采用射频反应溅射法于室温下在Cu/Ti/SiO₂/Si基底上制备了氧化钒薄膜. X-射线衍射、X射线光电子能谱分析仪及原子力显微镜结果表明, 室温下制备的氧化钒薄膜除微弱的V₂O₅ (101)和V₂O₃ (110)峰外, 没有明显的结晶取向, 是VO₂, V₂O₅, V₂O₃及VO的混合相薄膜, 且薄膜表面颗粒大小均匀, 表面均方根粗糙度约为1 nm. 采用半导体参数分析仪对薄膜的电开关特性进行测试. 结果表明薄膜具有较低的开关电压(V_Set<1 V, V_Reset<-0.5 V), 并且具有稳定的可逆开关特性. 薄膜从低阻态转变为高阻态的电流(I_Reset)随限流的增大而增大.通过高低阻态时I-V对数曲线的拟合(高阻态斜率>1, 低阻态斜率=1), 认为Cu离子在薄膜中扩散形成的导电细丝是该体系发生电阻转变的主要机制. 关键词： 氧化钒薄膜 电阻开关 电阻式非挥发存储器 导电细丝     

Modulation structure in the STM images of the non-metallic misfit-layer compound (PbS)1.12VS2

This paper presents the scanning tunneling microscopy (STM) results of the misfit-layer compound (PbS)_1.12VS₂, which is constructed of alternately stacking of PbS (Q) and VS₂ (H) layers. Temperature dependent resistivity measurements show a semiconducting behavior with small activation energies. Unlike the metallic 1Q/1H type of compounds we have succeeded to take both the STM images of a Q layer and a H layer, because electron tunneling from the underlying H layer is suppressed when intermediate positive bias voltage (V_b) is applied to a tip. At V_b = 0.15 V the image shows pseudo-tetragonal arrays of bright spots, although it is obscure with decreasing bias voltage and disappears at less than 10 mV. A modulation structure is found on the H layer of a stepped surface on which surface atoms are undulated in a period being twice the V-V interatomic distance in the [1 0]_H or the [1 1]_H direction.     

Structures,energetics and magnetic properties of (NiSn)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> clusters with <Emphasis Type="Italic">n</Emphasis> = 1–6

We report the results of calculations which were performed to investigate equilibrium structures, electronic and magnetic properties of stoichiometric (NiSn) _n clusters with n = 1–6 within the framework of density functional theory. The calculated results show that the structural arrangement of (NiSn) _n clusters is dominated by the Ni-Sn and Ni-Ni interactions. We find that these binary clusters show significant variation in the geometries as compared to that of the host nickel clusters. The preference for tetrahedron unit of Ni₃Sn is seen in the lowest-energy configuration of these clusters. The multi-centre bonding between Ni atoms play an important role in stabilizing the stoichiometric Ni-Sn clusters. Doping of Sn atoms enhances the binding energy and reduces the ionization potential of nickel clusters. These binary clusters prefer the lowest spin state. For (NiSn)₆ the magnetic moment is 0 μB. The complete quenching of the cluster magnetic moment appears to be due to the antiferromagnetic alignment of atomic spins as revealed by the spin density plots.