表面Cu原子间相互作用对Cu(001)表面跳跃扩散行为的影响

采用嵌入原子方法的原子间相互作用势，利用准静态分子动力学模拟研究了Cu原子在Cu(001)表面吸附所导致的基体晶格畸变以及对其附近的另一个吸附原子自扩散行为的影响.研究结果表明，吸附原子的存在可以导致多达10层的Cu基体晶格产生畸变.两个吸附原子所产生的晶格畸变应力场之间的相互作用，可以导致吸附原子运动活性的增加.通过比较同一路径上往返跳跃扩散势垒的差异发现，在原子间相互作用势的有效距离之外，两个吸附原子的扩散行为可以认为是存在晶格畸变应力场相互作用的两个独立吸附原子的扩散；在原子间相互作用势的有效距离之 关键词： 表面吸附原子 晶格畸变 表面二聚体 扩散     

Diffusion mechanism of Cu adatoms on a Cu(001) surface

Ab initio calculations on surface diffusion of Cu adatoms on Cu(001) are presented. The hopping mechanism with a calculated energy barrier of 0.69 eV is found to be favorable over the exchange mechanism with 0.97 eV. We find from the geometry relaxations that adatoms are significantly attracted to the surface and push away nearest-neighbor atoms in the surface. Lateral relaxations of atoms in the surface are larger than vertical ones.     

Scattering of electrons on the Cu(001) surface by Co adatoms

Excited electrons at surfaces can be scattered by adsorbate atoms or defects, which changes the energy or momentum. Such scattering processes can be studied by energy, time and angle-resolved two-photon photoelectron spectroscopy. In this article the influence of statistically distributed Co adatoms on a Cu(001) surface on the dynamics of electrons in image-potential states is investigated. Different scattering mechanisms, such as interband, intraband, and bulk scattering are identified and analyzed quantitatively. Cobalt adatoms cause mainly quasielastic scattering of electrons in image-potential states. Inelastic processes are due to interactions with electrons in the substrate and are not significantly increased by Co adatoms. The results are compared to previous experimental and theoretical work on Cu adatoms. PACS 73.20.At; 68.49.Jk; 79.60.Ht     

Strongly enhanced orbital moments and anisotropies of adatoms on the Ag(001) surface

We present ab initio calculations for orbital moments and anisotropy energies of 3d and 5d adatoms on the Ag(001) surface, based on density functional theory, including Brooks' orbital polarization (OP) term, and applying a fully relativistic Korringa-Kohn-Rostoker-Green's function method. In general, we find unusually large orbital moments and anisotropy energies, e.g., in the 3d series, 2.57 mu(B) and +74 meV for Co, and, in the 5d series, 1.78 mu(B) and +42 meV for Os. These magnetic properties are determined mainly by the OP and even exist without spin-orbit coupling.     

Scattering by Cu adatoms between image-potential bands on Cu(001)

With the increasing resolution and sensitivity of photoelectron spectroscopy, the influence of defects is becoming more and more obvious. Scattering processes induced by adsorbate atoms can be studied by time- and angle-resolved two-photon photoemission. We have examined the dynamics of electrons in image-potential states on the Cu(001) surface for statistically distributed Cu adatoms and have identified different scattering mechanisms. Scattering of electrons from the second (n=2) to the bottom of the first (n=1) image-potential band is observed, which we attribute to inelastic interband scattering with electrons in the bulk. At energies above the bottom of the n=2 band, resonant interband scattering from the n=2 to the n=1 image-potential band is found. The rate for these processes can be determined by modeling the time-resolved measurements via optical Bloch equations of a four-level system. Comparison of the transition and decay rates reveals that the decay rate of the n=2 electrons is almost exclusively changed by additional resonant interband-scattering processes upon adsorption. PACS 73.20.At; 79.60.Ht; 68.49.Jk     

Dynamical motion of adatoms and their neighbours at the Ni(111) surface

We have calculated mean square displacements and one atom spectral densities for the adatoms H, O, S, Ni, and their neighbours on a Ni(111) surface. The adatoms are isolated on the substrate. They lie at a site of three-fold symmetry. The upward shift in frequency of the adatom's normal mode of vibration perpendicular to the surface (a shift from the value on an infinitely massive substrate) has been calculated. A simple analytical formula is obtained by means of which the shift can be estimated for any adatom —surface combination.     

Sulfur-modified surface of InP(001): Evidence for sulfur incorporation and surface oxidation

Received: 21 March 1997/Accepted: 12 August 1997     

Elastic interactions between surface adatoms and between surface clusters

One major term is omitted in most conventional treatments of interactions between surface adatoms or groups of adatoms. This is the elastic interaction, in which adatoms interact through mutual distortion of the substrate. The indirect elastic interaction explains a range of observed surface phenomena in a consistent quantitative way. These phenomena include static and dynamic effects ranging from ordered structures to correlated motions of adatom clusters. It is likely also that substrate distortion is important in clean surface reconstruction. The elastic interaction is typically comparable with or larger than the indirect electronic interactions usually presumed. It follows that detailed calculations which ignore the substrate distortions produced will be of limited value only.     

Evidence of Sn adatoms quantum tunneling at the alpha-Sn/Si(111) surface

We present a low-temperature scanning tunneling microscopy study of the alpha-Sn/Si(111) surface that demonstrates the fluctuating behavior of the Sn adatoms. The dynamical fluctuation model, successfully applied in describing the alpha-Sn/Ge(111) surface, is proposed for the related alpha-Sn/Si(111) surface too, although with a much lower transition temperature. In addition, a new phenomenon appears responsible for the unexpected evidence that the average oscillation frequency remains constant at temperatures lower than 15 K, in contradiction to the Arrhenius law. We explain this phenomenon as quantum tunneling of Sn adatoms.     

Characterization of the interfacial reaction between sputter-deposited Ni film and Si substrate

In the present work, we report the interfacial reaction characteristics between sputter-deposited Ni film and a single crystalline Si substrate. The effect of substrate bias during the deposition process on the interfacial reaction is also discussed. It was found that sputter deposition with a substrate bias can promote the interfacial reaction between Ni film and Si substrate. Under our experimental conditions, Ni₂Si (orthorhombic) with good crystallinity formed in the film when it was deposited at 200 °C and with a -80 V substrate bias. Such films had relatively low resistivity. Upon thermal annealing at 500 °C, NiSi formed through further reaction, however there is almost no change in resistivity. PACS 67.57.Np; 68.55.-a; 68.35.Fx     

From antiferromagnetic to ferromagnetic coupling for V adatoms on Co(001) substrates

We discuss the polarization of V atoms on Co(001) substrates within density functional calculations. For sub-monolayer coverage the coupling between V and Co is clearly of antiferromagnetic type whereas it changes to ferromagnetic coupling in the case of a full V monolayer on Co(001). The results obtained in the case of a sub-monolayer coverage are in agreement with recent X-ray magnetic circular dichroism by Huttel et al. [Phys. Rev. B 68, 174405 (2003)]. The transition from antiferromagnetic coupling (in the case of sub-monolayer coverage) to ferromagnetic coupling (for a full monolayer coverage) is discussed in terms of local coordination numbers and V-Co hybridization. Comparison with Cr and Mn coverages on Co(001) complicates the problem: i) submonolayer Cr coverage stabilizes the antiferromagnetic coupling between Cr and Co atoms (like for V on Co(001) whereas a Cr monolayer on Co presents in-plane antiferromagnetic coupling; ii) submonolayer Mn coverage stabilizes now the ferromagnetic coupling between Mn and Co whereas a Mn monolayer on Co(001) presents an in-plane antiferromagnetic coupling. Competition between Co induced magnetism and surface induced magnetism at V sites is discussed.     

Analysis of interactions between Co adatoms on the vicinal Cu(111) surface

The energies of magnetic interactions between Co adatoms at the vicinal Cu(111) surface are calculated in the framework of the density functional theory using the Korringa-Kohn-Rostoker type Green’s functions. It is demonstrated that the interactions between Co adatoms appreciably depend on the distance from a surface step. Our calculations show that the magnitude of the repulsive barrier related to the surface step is larger for Co adatoms located at the upper surface terrace than for those located at the lower surface terrace.     

Evidence of substrate metallization by Li adsorption on the Si(001) surface

We report an evidence of substrate metallization induced by Li adsorption on the Si(001) surface, based on the combined results of electron energy-loss (EEL) and angle-resolved photoemission (ARP) measurements. The metallic surface at a low dose of Li manifests itself as a loss peak due to an intraband surface plasmon in EEL spectra and a metallic peak in ARP spectra. These peaks are coherently understood in terms of substrate metallization, where electrons from Li adatoms partially occupy the empty substrate surface bands. Furthermore, the unique negative dispersion of the plasmon reveals that local field effects may cause such an anomalous dispersion.