Tensor LEED analysis for the electrodeposited Pt(1 1 1)-(3×3)–Ag,I surface structure

A crystallographic analysis is reported using low-energy electron diffraction (LEED) in the tensor LEED approach for the electrodeposited coadsorption (3×3) structure with 4/9 monolayer (ML) of silver and 4/9 ML of iodine on the Pt(1 1 1) surface. The structure approximates a two-layer slice of bulk AgI cut parallel to its (1 1 1) plane and superimposed on the substrate with the Ag atoms in contact with the topmost Pt(1 1 1) layer, and the I atoms forming an overlayer on the Ag atoms. There are two types of Ag atoms in the (3×3) unit mesh; one type bonds to a single Pt atom, while the other type bonds to three Pt atoms. The average Ag–Pt bond distances are close to 2.48 and 2.82 Å respectively for the one and three-coordinate Ag atoms, but both types of Ag atoms bond to three I atoms with an average Ag–I distance of 2.67 Å. No significant corrugation is observed for either the I layer or the Ag layer.     

Observation of the Dipole Blockade Effect in Detecting Rydberg Atoms by the Selective Field Ionization Method

The dipole blockade effect at laser excitation of mesoscopic ensembles of Rydberg atoms lies in the fact that the excitation of one atom to a Rydberg state blocks the excitation of other atoms due to the shift in the collective energy levels of interacting Rydberg atoms. It is used to obtain the entangled qubit states based on single neutral atoms in optical traps. In this paper, we present our experimental results on the observation of the dipole blockade for mesoscopic ensembles of 1–5 atoms when they are detected by the selective field ionization method. We have investigated the spectra of the three-photon laser excitation 5S1/2 → 5P3/2 → 6S1/2 → nP3/2 of cold Rydberg Rb atoms in a magneto-optical trap. We have found that for mesoscopic ensembles this method allows only a partial dipole blockage to be observed. This is most likely related to the presence of parasitic electric fields reducing the interaction energy of Rydberg atoms, the decrease in the probability of detecting high states, and the strong angular dependence of the interaction energy of Rydberg atoms in a single interaction volume.     

Doping effect of hydrogen on Al3 cluster

The doping effect of hydrogen atoms on geometric and electronic properties of small aluminum cluster, Al₃, where deviation from the jellium model may occur, is investigated from a first-principle method. It is found that the most favorable sites for H atoms to be bound to Al atoms depend on amount of H atoms. For each cluster, Al–H bond lengths have the smallest values when the H atoms are on the top sites and have the largest values when the H atoms are on face sites, while those corresponding to bridge-site H atoms are of medium value. The doping of H increases the binding energy of the Al₃ cluster. Al₃H₃ and Al₃H₅ are found to have much lower electron affinities, higher ionization potentials and significantly larger HOMO–LUMO gaps than their neighbors, which are typical characteristics of magic clusters. The high stabilities of the Al₃H₃ and Al₃H₅ suggest that they may have a good potential applications in cluster-assembled materials.     

SrTiO3(001)表面上Au和N原子相互作用的第一性原理研究

采用基于密度泛函理论的第一性原理平面波赝势方法研究了SrTiO₃(001)表面上Au和N原子间相互作用的微观机理.通过比较分析N置换表面层O原子前后SrTiO₃(001)表面吸附Au原子体系的相关能量和电子结构,发现SrTiO₃(001)表面吸附Au原子和N替代表面层O原子的置换过程二者之间存在明显的"协同效应",即N原子置换SrTiO₃(001)表面层O原子的过程增强了相应表面吸附Au原子的稳定性,而SrTiO关键词： 表面结构 相互作用 第一性原理     

A Cu(1 1 1)(√3×√3)R30°-Sb structure by impact collision ion-scattering spectroscopy

We have investigated the structures of Cu(1 1 1)(√3×√3)R30°-Sb using time of flight-impact collision ion-scattering spectroscopy. The experimental data and computer simulations support a structural model for the Cu(1 1 1)(√3×√3)R30°-Sb structure in which Sb atoms displace up to 1/3 of the first layer of Cu atoms and incorporate them into the first Cu layer with the Sb atoms displaced outward 0.40 Å with respect to the first-layer Cu atoms. The outermost first layer of Sb and Cu atoms shift from fcc- to hcp-hollow sites (only the top layer of Sb and Cu atoms occupies hcp hollow sites).     

The electron distribution in superconducting alloys: II. Room temperature analysis of Cr3Si and comparison with V3Si

The charge distribution in Cr₃Si a non-superconducting alloy of the A-15 family was obtained from accurate X-ray diffraction data. The deformation density in the bond between two adjacent Cr atoms in Cr₃Si is 0.17 eA^?3, about three times lower than the density observed in the isomorphous V₃Si structure. Integration of the charge around the Cr and Si atoms leads to a best estimate of 1.3–1.9 electrons for the charge transfer from the Si to three Cr atoms as compared to 1.8 – 2.4 electrons transfered from the Si to the three V atoms in V₃Si.     

Structural, elastic, and electronic properties of orthorhombic NH3BH3: An ab initio study

At the generalized gradient approximation (GGA), the elastic constants of the orthorhombic phase of NH₃BH₃ were calculated with plane-wave pseudo-potential method. Our calculation showed that the orthorhombic phase NH₃BH₃ is a loose and brittle material, as well as hard to be deformed, also we calculated the elastic anisotropies and the Debye temperatures from the elastic constants. And from the band structure and density of state (DOS), we concluded that NH₃BH₃ is a wide-gap semiconductor and the band gap is almost 6.0 eV. The bonds between N atoms and H atoms show a strong covalent characteristic, B atoms and H atoms form ironic bonds, and so as to the B-N bonds. Electrons from the B atoms are absorbed by the H atoms around the B atoms, and the H atoms display electronegativity.     

The effect of doping three Al and N atoms on the chemical shielding tensor parameters of the boron phosphide nanotubes: A DFT study

In this work, an armchair model of the (4,4) boron phosphide nanotubes (BPNTs) with a 1-nm length and consisting of 32 B and 32 P atoms is considered to study the influence of doping three atoms of aluminum in sites of boron (B_3AlPNTs) and three atoms of nitrogen in sites of phosphors (BP_3NNTs) on the electrostatic structure properties. The mouths of nanotubes are capped by hydrogen atoms in order to saturate the dangling bonds of the boundaries and to decrease the calculation time. The structures of BPNTs, B_3AlPNTs and BP_3NNTs are optimized by performing the level of density functional theory (DFT) using 6-31G^? basis set. The optimized structures are used for calculating the chemical shielding (CS) tensors and nuclear magnetic resonance parameters such as isotropic chemical shielding (CS^I) and anisotropic chemical shielding (CS^A). The results reveal that in both models of B_3AlPNTs and BP_3NNTs by doping N atoms the chemical shielding parameters of P and B atoms, which are directly connected to the Al and N atoms decreased and the other sites significantly changed.     

First principles calculations of the relaxed structural and electronic properties of Cu nanobelts

Calculations have been performed for the relaxed structural and electronic properties of Cu nanobelts with the cross-section 3×5, 3×7, 3×9 and 3×11 atomic layers, using the first-principles projector-augmented wave (PAW) potential within the density functional theory (DFT) framework. For all four size Cu nanobelts, most atoms relax inward, and the farther an atom is from the center of the nanobelt, the larger is the amount of inward relaxation and the smaller the total electronic charge. Compared with the (001) plane of Cu bulk crystal, a metallic (delocalized) bonding character obviously appears along the surface atoms as well as along the surface atoms and their first nearest neighbor atoms. The decrease in the coordination number and thus the decrease in restrictions for atoms distant from the center of the nanobelt lead most electrons to range in the higher energy region of the occupancy state.     

First principles study on the magnetocrystalline anisotropy of Fe--Ga magnetostrictive alloys

This paper investigates the electronic structure and magnetocrystalline anisotropy of Fe--Ga magnetostrictive material by means of the full potential-linearized augmented plane-wave method within the generalized gradient approximation. The 3d-orbit splitting of Fe atoms in D0₃, B2-like and L1₂ crystalline structures of Fe--Ga is calculated with consideration of the crystal field as well as the spin--orbit coupling effect. Because of the frozen orbital angular momenta of the 3d-orbit for Fe atoms in Fe--Ga magnetostrictive alloys and the spin--orbit coupling, the distribution of the electron cloud is not isotropic, which leads to the anisotropy of exchange interaction between the different atoms. A method on estimating the magnetocrystalline anisotropy of Fe--Ga alloys by means of calculating orbit-projected density of states for Fe atoms is performed. The anisotropic distribution of the electron cloud of Fe atoms in these three crystalline structures of Fe--Ga is studied based on the above method showing the highest magnetic anisotropy for B2-like structure. This qualitative method comes closer to physical reality with a vivid physical view, which can evaluate the anisotropy of electron cloud for 3d transition atoms directly. The calculated results are in good agreement with both the previous theoretical computation and the tested value on the magnetic anisotropy constant, which confirms that the electron cloud anisotropy of Fe atoms could well characterize the magnetocrystalline anisotropy of Fe--Ga magnetostrictive material.     

Properties of mono-vacancy in L12-type Ni3Al ordered alloy

The relaxed structures and the formation and migration energies of the mono-vacancy in L1₂-type Ni₃Al ordered alloy have been investigated by combining the modified analytical embedded-atom method (MAEAM) with molecular dynamics (MD) simulation. The movements of the atoms in the vicinity of the mono-vacancy are toward the vacancy, except for the second-nearest-neighbor Al atoms and the fifth-nearest-neighbor Ni atoms around an Al vacancy and the third-nearest-neighbor Al atoms and the fourth-nearest-neighbor Ni atoms around a Ni vacancy that move outward from the vacancy. Furthermore, each of the atoms in the vicinity of either an Al or a Ni vacancy moves approximately along a line connected between the atom and the vacancy. From energy minimization, it is easy for a Ni vacancy to form and to migrate in one nearest-neighbor jump.     

XAFS studies of the local environment of Pb impurity atoms in barium,strontium, and calcium titanates

The local environment of Pb impurity atoms in BaTiO₃, SrTiO₃, and CaTiO₃ crystals was studied by XAFS technique. It is shown that, in both polar and nonpolar phases of BaTiO₃ and in SrTiO₃, the Pb atoms are displaced from the A lattice sites by ～0.15 Å; in CaTiO₃ this displacement is absent. Large values of Debye-Waller factors (0.05–0.10 Ų) for the atoms in the first shell of Pb observed in all the three crystals indicate the distortion of the oxygen environment of Pb atoms. The appearance of these features was explained by the fact that the Pb-O chemical bond has a noticeable covalent component and a Pb atom can form strong bonds only with four of the 12 surrounding oxygen atoms. The obtained data were used to determine the main factors responsible for the occurrence of ferroelectric phase transition in SrTiO₃ and CaTiO₃ and for the increase of the Curie temperature of BaTiO₃ when it is doped with Pb.     

Laser cooling of thulium atoms

We demonstrated laser cooling and trapping of thulium atoms at sub-Doppler temperatures in a magneto-optical trap (MOT). Up to 3 × 10⁶ thulium atoms were trapped in the MOT at temperatures down to 25(5) μK which is approximately 10 times lower than the Doppler limit. The lifetime of atoms in the MOT varied between 0.3–1.5 s and was restricted mostly by optical leaks from the upper cooling level. The lower limit for the leaking rate was estimated to be 22(6) s⁻¹. Due to a big magnetic moment of Tm atoms, a part of them were trapped in a magnetic trap from the quadrupole field of the MOT. We observed about 3 × 10⁴ purely magnetically trapped atoms at temperature of 25 μK with a lifetime in the trap of 0.5 s. Also we set up a “dark” MOT consisting of six crossed hollow beams which increased the number of trapped atoms by a factor of 5 leading to 1.5 × 10⁷ atoms at the expense of higher temperature.     

Self-consistent quantum structure of diffuse helium atom clusters

Diffuse clusters of arbitrary numbers of helium atoms are treated in a mean-field theory of the many-body system. The nonlinear Hartree self-consistent-field equations are solved numerically to give their energy eigenvalues and eigenfunctions. Both isotopes ⁴He and ³He are treated according to their bosonic and fermionic statistics and symmetry requirements. Clusters of all sizes of ⁴He atoms are stable, but there is a minimum threshold of ～70 atoms for the formation of ³He clusters. All the excited states in addition to the ground state were found. The wave functions are seen to have interesting differences from electronic wave functions in atoms.     

Stability of (3)He(2)(4)He(n) and (3)He(3)(4)He(n) L=0 clusters

We have studied the stability of mixed (3)He/(4)He clusters in L=0 states by the diffusion Monte Carlo method, employing the Tang-Toennies-Yiu He-He potential. The clusters (3)He(4)He(N) and (3)He(2)(4)He(N) are stable for N>1. The lighter atoms tend to move to the surface of the cluster. The minimum number of 4He atoms able to bind three 3He atoms in a L=0 state is nine. Two of three fermionic helium atoms stay on the surface, while the third one penetrates into the cluster.     

Investigation into the adsorption of atomic nitrogen on an Al2O3 (0001) surface

Computer simulation of sapphire nitridation used to obtain nitride-based heterostructures (GaN) on an Al₂O₃ substrate has been performed. The adhesion of atomic nitrogen to the sapphire (0001) surface is investigated ab initio. The possibility of replacing surface-layer oxygen atoms with nitrogen atoms has been examined. The calculated results indicate that adsorbed nitrogen atoms occupy the most stable positions above surface oxygen atoms at different nitrogen concentrations. The changes in the total system energy after replacement of surface oxygen atoms with nitrogen atoms have been calculated. It turns out that oxygen replacement is energetically unfavorable for a single nitrogen adatom. However, this process becomes energetically favorable if the concentration of nitrogen atoms increases. This outcome, obtained for the first time, enables better understanding of the atomic-scale mechanism of sapphire nitridation.     

Zn1-xMnxS(001)稀磁半导体薄膜的能量稳定性、磁性和电子结构

通过基于广义梯度近似的总能密度泛函理论研究不同Mn掺杂浓度的ZnS(001)薄膜的电学和磁学特性. 计算单个Mn原子和两个Mn原子处于各种掺杂位置及不同的磁耦合状态时的能量稳定性.计算了单个Mn原子掺杂和两个Mn原子掺杂的ZnS(001)薄膜的态密度. 不同掺杂组态的p-d杂化的程度不同. 不同掺杂组态,Mn原子所处的晶场环境不同,所以不同掺杂组态的Mn的3d分波态密度峰的劈裂有很大的不同. 掺杂两个Mn原子时,得到三种稳定组态的基态都是反铁磁态. 分析了以上三种能量稳定的组态中,两个Mn原子在不同磁耦合状态下的3d态密度图. 当两原子为铁磁耦合时,由于d-d电子相互作用,使反键态的态密度峰明显加宽. 随着Mn掺杂浓度的增加,Mn原子有相互靠近,并围绕S原子形成団簇的趋势. 对于这样的组态,Mn原子之间为反铁磁耦合能量更低.     

MoO_3/Si界面区钼掺杂非晶氧化硅层形成的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,通过模拟MoO_3/Si界面反应,研究了MoO_x薄膜沉积中原子、分子的吸附、扩散和成核过程,从原子尺度阐明了缓冲层钼掺杂非晶氧化硅(a-SiO_x(Mo))物质的形成和机理.结果表明,在1500 K温度下, MoO_3/Si界面区由Mo, O, Si三种原子混合,可形成新的稳定的物相.热蒸发沉积初始时, MoO_3中的两个O原子和Si成键更加稳定,同时伴随着电子从Si到O的转移,钝化了硅表面的悬挂键. MoO_3中氧空位的形成能小于SiO_2中氧空位的形成能,使得O原子容易从MoO_3中迁移至Si衬底一侧,从而形成氧化硅层;替位缺陷中, Si替位MoO_3中的Mo的形成能远远大于Mo替位SiO_2中的Si的形成能,使得Mo容易掺杂进入氧化硅中.因此,在晶硅(100)面上沉积MoO_3薄膜时, MoO_3中的O原子先与Si成键,形成氧化硅层,随后部分Mo原子替位氧化硅中的Si原子,最终形成含有钼掺杂的非晶氧化硅层.     

铯47D精细能级超冷里德堡原子自由演化的动力学研究

用连续窄线宽激光器将超冷铯里德堡原子分别激发到47D_3/2, 47D_5/2精细态, 观察了处于里德堡精细态的铯原子向超冷铯等离子体自由演化的过程, 详细对比了不同精细态的铯里德堡原子预电离时间、电离速率以及等离子体的转化效率. 将里德堡原子快速转化为等离子体的过程解释为局域势阱内由预电离产生的电子与里德堡原子的快速碰撞导致的雪崩电离.     

Structural and electronic properties of PtPd and PtNi nanoalloys

The lowest-energy structures of binary (PtPd)_n, (PtNi)_m, (PtNi₃)_s, and (Pt₃Ni)_s nanoclusters, with n=2–28, m=2–20, and s=4–6, modeled by the many-body Gupta potential, were obtained by using a genetic-symbiotic algorithm. These structures were further relaxed within the density functional theory framework in order to obtain the most stable structures for each composition. Segregation is confirmed in all the (PtPd)_n clusters, where the Pt atoms occupy the cluster core and the Pd atoms are situated on the cluster surface. In contrast, for the (PtNi)_m nanoalloys, the Ni atoms are mainly found in the cluster core and the Pt atoms are segregated to the cluster surface. Likewise, for the (PtNi₃)_s nanoalloys, Ni atoms mainly compose the cluster core but there is no clear segregation of the Pt atoms to the surface. Furthermore, for the (Pt₃Ni)_s bimetallic clusters the Pt atoms concentrate in the cluster core and the Ni atoms are segregated to the surface. On the other hand, it has been experimentally found that the Pt_0.75Ni_0.25 supported nanoparticles present a higher catalytic activity for the selective oxidation of CO in the presence of hydrogen than the Pt_0.5Ni_0.5 and Pt_0.25Ni_0.75 nanoparticles. In order to understand this tendency in the catalytic activity, we also performed density functional calculations of the molecular CO adsorption on bimetallic Pt-Ni nanoclusters with the mentioned compositions.