A theoretical study of the reactivity of Cu2O(111) surfaces: the case of NO dissociation

We compare the electronic properties of Cu(111) and Cu(2)O(111) surfaces in relation to the dissociation of NO using first principles calculations within density functional theory. We note a well-defined three-fold site on both O- and Cu-terminated Cu(2)O surfaces which is verified as the active site for the adsorption and dissociation of NO. The interaction of Cu with O atoms results in the forward shifting of the local density of states and formation of unoccupied states above the Fermi level, compared to the fully occupied d band of pure Cu. These results give valuable insights in the realization of a catalyst without precious metal for the dissociation of NO.     

The electronic structure of surfaces with the matching green function method: II. fcc and bcc transition metal surfaces

Using the matching Green function method, the densities of states at (001) surfaces of Ni (fcc) and Mo (bcc) are calculated. Fixing the wavevector parallel to the surface, the surface density of states at Ni(001) is similar to the bulk, with band edge singularities rounded off and reflected in big surface resonances. Changes are much greater in the case of Mo(001), and the surface density of states has a central peak in the minimum of the bulk density of states. This peak arises from the 4d level in the surface atoms interacting weakly with the substrate and with neighbouring surface atoms. Our results are compared with angular resolved photoemission theory and experiment — in the case of Ni all the experimental peaks correspond to bulk k-conserving transitions, though some surface resonances in the Mo surface density of states show up in photoemission.     

Structural Features of Boron-Doped Si（113） Surfaces Simulated by ab initio Calculations

Based on ab initio calculations, boron-doped Si（113） surfaces have been simulated and atomic structures of the surfaces have been proposed. It has been determined that surface features of empty and filled states that are separately localized at pentamers and adatoms indicates a low surface density of B atoms, while it is attributed to heavy doping of B atoms at the second layer that pentamers and adatoms are both present in an image of scanning tunnelling microscopy. B doping at the second layer should be balanced by adsorbed B or Si atoms beside the adatoms and inserted B interstitials below the adatoms.     

第一性原理研究四元Heusler合金TiZrCoIn (100) 的表面结构、磁性及自旋极化率

本文利用第一性原理计算方法,对具有半金属性的四元Heusler合金TiZrCoIn两个不同终端表面效应进行了理论研究,主要针对表面效应对其结构、磁性、电子结构、自旋极化率及半金属性的影响来展开调研,以便寻求适合于隧道结的表面材料,为后续相关理论研究及实验提供一定参考。研究结果显示,两终端表面不同原子分别发生了不同程度的伸缩,同层原子发生错位,致使表面原子间距改变,进而改变它们之间的杂化作用,同时也影响着原子的磁矩。另外通过分析其态密度,发现TiCo-(100)和ZrIn-(100)两个终端表面由于受到表面效应的影响,其电子结构发生了很大的改变。块体TiZrCoIn原有的宽带隙和半金属性被表面态所破坏,但仍然保留着很高的自旋极化率,尤其是ZrIn-(100)终端表面,其费米面处呈现几乎100%的自旋极化率。     

First-principles study of Ar adsorptions on the （111） surfaces of Pd, Pt, Cu, and Rh

In the present paper we give a detailed report on the results of our first-principles investigations of Ar adsorptions at the four high symmetry sites on M （111） （M =Pd, Pt, Cu, and Rh） surfaces. Our studies indicate that the most stable adsorption sites of Ar on Pd （111） and Pt （111） surfaces are found to be the fcc-hollow sites. However, for Ar adsorptions on Cu （111） and Rh （111） surfaces, the most favorable site is the on-top site. The density of states （DOS） is analyzed for Ar adsorption on M （111） surfaces, and it is concluded that the adsorption behavior is dominated by the interaction between 3s, 3p orbits of Ar atoms and the d orbit of the base metal atoms.     

Chemical fingerprinting at the atomic level with scanning tunneling spectroscopy

Scanning tunneling spectroscopy (STS) based on scanning tunneling microscopy (STM) makes it possible to map the local electronic density of states for clean surfaces and for those with adsorbates. We have developed a protocol that allows us to obtain the spectral fingerprints of halogen atoms on Si(0 0 1), and we use those fingerprints to distinguish between adatom species for surfaces with Cl and Br mixed adsorbates. The key to the process is the energy distribution of the antibonding states that depend on the halogen species.     

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.     

Conical intersections in an ultracold gas

We find that energy surfaces of more than two atoms or molecules interacting via transition dipole-dipole potentials generically possess conical intersections (CIs). Typically only few atoms participate strongly in such an intersection. For the fundamental case, a circular trimer, we show how the CI affects adiabatic excitation transport via electronic decoherence or geometric phase interference. These phenomena may be experimentally accessible if the trimer is realized by light alkali atoms in a ring trap, whose interactions are induced by off-resonant dressing with Rydberg states. Such a setup promises a direct probe of the full many-body density dynamics near a CI.     

Cu 表面性质的第一性原理分析

采用第一性原理赝势平面波方法, 计算并详细分析了面心立方Cu晶体及其 (100), (110) 和 (111) 这3个低指数表面的原子结构、 表面能量及表面电子态密度. 表面能的计算结果表明, Cu (111) 表面的结构稳定性最好, Cu (100) 表面次之, Cu (110)表面的结构稳定性最差. 3个表面的表面原子弛豫量随着层数的增加而逐渐减弱. Cu (110) 表面的最表层原子相对收缩最大, Cu (100)表面次之, Cu (111) 表面的最表层原子相对收缩最小. 表面原子弛豫不仅引起表面几何结构的变化, 而且使表面层原子的电子态密度峰形相对晶体内部发生变化, 这是表面能产生的主要原因, 而Cu (110)表面相对于Cu (100)与Cu (111)表面具有高表面活性的主要原因则源于其表面层原子电子态密度在高能级处的波峰相对晶体内部显著的升高. 关键词： Cu 晶体 表面结构 表面能 态密度     

Two-dimensional electron gas at noble-metal surfaces

The electrons of the surface states on the (111) surfaces of the noble metals Au, Ag, and Cu form a quasi-two-dimensional (2D) free electron gas which is confined to the first few atomic layers at the crystal surface. They are scattered by the potential associated with surface defects, e.g. impurity atoms, adatoms, or step edges, leading to quantum-interference patterns in the local density of states around these defects. We have used the quantum-interference phenomena to quantitatively measure the electron phase-relaxation length and to probe long-range adsorbate interactions. Received: 12 October 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

Theoretical study of TiO2 surfaces

A tight-binding extended Hückel (TBEH) calculation has been performed to investigate the electronic structures of TiO₂ surfaces. By analysing the density of states (DOS) of various surfaces and the orbital interactions between surface atoms, it is found that the continuous drop of the uppermost valence band peak with increasing surface defects is due to the removal of surface bridge O atoms, and that the interaction of the two Ti atoms adjacent to the O vacancy induces a defect gap state.     

Hyperfine-interaction studies of surfaces

Applications of hyperfine-interaction techniques, like NMR, PAC and Mößbauer spectroscopy, to well-characterized surfaces are discussed and the present knowledge of surface hyperfine fields is reviewed. Measurements of nuclear spin relaxation permit to extract the local density of electron states at the Fermi level of adsorbed alkali atoms. From the observed electric-field-gradient properties surface probe sites and diffusion processes can be inferred; the experimentally determined magnetic hyperfine fields give access to the electron-spin behaviour at magnetic surfaces.     

Electronic density of states of group-IV semiconductors: A cluster-Bethe lattice approach for realistic Hamiltonians

We have extended the cluster-Bethe lattice method to study realistic tight-binding Hamiltonians. The numerical solution of the transfer matrix in the Bethe lattice is very stable and requires about 50 steps of our iterative procedure to reach convergency. We apply this method to study the density of states of group-IV semiconductors (C, Si, Ge) using a five-parameter sp³ Hamiltonian, which takes into account all possible interactions between sp³ hybrids in nearest-neighbor atoms. Our results show clearly that the main features of the density of states are due to short-range order. Clusters of about 30 atoms reproduce very well the crystalline density of states. Based on our results we propose a model for the density of states in the gap region of an amorphous semiconductor.     

单晶LaB_6阴极材料典型晶面的电子结构和发射性能研究

单晶LaB_6是一种理想的热发射和场发射阴极材料,其不同晶面表现出不同的发射性能.采用基于密度泛函理论的第一性原理计算分析了LaB_6单晶的(100),(110),(111),(210),(211)和(310)典型晶面的差分电子密度、能带结构和态密度,并对光学区熔法制备的高质量单晶LaB_6的上述典型晶面的热发射性能进行了测试.理论计算结果表明LaB_6各晶面结构的不同和电子结构的差异导致LaB_6发射性能具有各向异性,晶面内La原子的密度越大、费米能级进入导带越深、费米能级附近态密度越大及其在导带区域的分布宽度越宽、导带在费米能级附近分布越多,晶面的逸出功越低,发射性能越好.热发射测试结果表明,当阴极测试温度为1773 K,测试电压为1 k V时,(100),(110),(111),(210),(211)和(310)晶面的最大发射电流密度分别为42.4,36.4,18.4,32.5,30.5和32.2 A/cm~2,其中(100)晶面具有最佳的发射性能.     

Memory function parametrization in the GLE-Ghost atom formalism: A microscopic approach

The memory function used in the GLE approach to solid atom motion is modeled as a generalization of the position autocorrelation function for a multidimensional Brownian oscillator. The exact microscopic memory function and the assumed form are required to agree in two limits: at short times and for the low frequency density of states. This suffices to specify all the parameters, including those which control the memory of correlated motion between primary zone atoms as mediated by the rest of the solid. Determination of these parameters utilizes as the fundamental input the frequency matrix for the solid. An example is presented for various fcc solid surfaces assuming Lennard-Jones (12, 6) forces between the solid atoms. The approach is capable, in principle, of treating such interesting structures as high Miller index surfaces, molecular solids and adsorbed species while retaining only a small number of primary zone atoms.     

Theoretical study of intrinsic surface phonons in hydrogenated amorphous silicon

The extra-mode at 214 cm^-1 which is observed in the infrared spectrum of hydrogenated amorphous silicon is interpreted as being due to the presence of small (? 7 atoms) internal surfaces in the samples. Calculations of the phonon density of states at internal surfaces in bulk Si Bethe lattices show a pronounced peak at the edge of the TA band (≈ 210 cm^-1. It is shown that when hydrogen is present the mode is infrared active through a dynamical charge transfer mechanism.     

Nature of contrast in Ge/Si(111) layers in scanning tunneling microscopy in the presence of Bi and Sb surfactants

It is known that the use of Bi surfactant (unlike Sb) upon the growth of Ge layers on Si(111) increases the contrast between Ge and Si atoms in a scanning tunneling microscope. This makes it possible to distinguish the Ge and Si surfaces. This effect is studied using computer simulation based on the density functional theory. To explain the observed difference between the Ge and Si layers, both structural and electronic effects are considered. The local density of electronic states, as well as the corresponding decay length to vacuum, has been calculated for each of the surfaces. The simulation results have been compared to the previous scanning tunneling microscopy data.     

Molecular dynamics investigation of water adsorption on rutile surfaces

Born-Oppenheimer molecular dynamics (MD) simulations were performed in the framework of the semi-empirical molecular orbital method MSINDO to study water adsorption on rutile surfaces. Monolayer and doublelayer water coverage was considered on the rutile (1 1 0) and (1 0 0) surfaces and the adsorbate structures were determined. Vibrational density of states of hydrogen atoms were calculated by constant temperature MD simulations at 100 K. These were used to interpret the experimental vibrational spectrum by assigning all peaks to the particular types of hydrogen atoms.     

The short range of the electronic promoter effect of potassium

Photoemission of Adsorbed Xenon atoms (PAX) as a local work function probe is used to investigate the range of the electronic promoter effect of potassium submonolayers on a Ru(001) surface. Three Xe states on these bimetallic K/Ru surfaces are clearly distinguishable by their 5p photoemission and are associated with Xe probe atoms at basically unmodified Ru sites, at “mixed” K.Ru sites next to K ions, and on top of potassium, respectively. From the relative intensities of these three states as well as from their 5p electron binding energies as a function of potassium coverage it is concluded that the radius of the “sphere” of modified charge density around one K ion is ～ 6 Å.     

Vibrational states of a cobalt dimer on the (111) and (001) copper surfaces

The results of calculations of the total (lateral and vertical) relaxation of the (001) and (111) copper surfaces in the presence of a small cluster of cobalt adatoms, local vibrational density of states and polarizations of these states are presented. The calculations were performed using the atomic interaction potentials in a tight binding approximation. An analysis of the results obtained showed that the presence of a cobalt dimer gives rise to modification of the vibrational states of the copper surface and generation of new modes localized both on the adatoms of the cluster and the surface atoms of the substrate. The revealed anisotropy of surface relaxation along [001] results in deformation of atomic bonding and splitting of the vibrational modes of the dimer. The lifetimes of the vibrational states of the dimer are found to be nearly equal for both surfaces under study, with a frequency shift being however observed. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp.73–78, December, 2008.