Spin-polarized scanning tunneling spectroscopy of nanoscale cobalt islands on Cu(111)

Spin-averaged and spin-polarized scanning tunneling spectroscopy at low temperature was performed on nanometer-scale triangular Co islands grown epitaxially on Cu(111) in the submonolayer coverage regime. Two structurally different island types can clearly be distinguished by their spin-averaged electronic structure. Spin-polarized measurements allow a separation of spectral contributions arising from different island stacking or from opposite magnetization states, respectively. In an applied magnetic field, both island types are found to be magnetized perpendicular to the surface, with large values of saturation field, remanence, and coercivity.     

Interplay of electronic,magnetic and structural properties of surface-supported clusters

We present first theoretical evidence revealing the influence of structural changes on the spin-polarized surface states of large Co nanoislands grown on Cu(111). The minority density of electronic states possesses a pronounced peak whose energetic position depends sensitively on the Co layers stacking order. Our results suggest a way to deduce the stacking order of large Co nanoislands using scanning tunnelling microscopy/spectroscopy.     

A spin-selective approach for surface states at Co nanoislands

During recent years the surface electronic states of cobalt nanoislands grown on Cu(111) and Au(111) have been extensively studied and still yield fascinating results. Among magnetic surfaces, cobalt islands are particularly appealing because of their spin-polarized electronic states near the Fermi energy, involving localized d states of minority character, as well as free-like s–p states of majority character. We show here that these states are a sensitive probe to minute changes of structural details such as strain and stacking, and therefore constitute an ideal playground to study the interplay between structural and spin-related properties. Due to their size, cobalt islands on Cu(111) offer the additional opportunity to host single-magnetic adsorbates suitable for spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM and SP-STS). We establish here that, in an energy interval just below the Fermi level, the spin-polarization of a transition-metal atom is governed by surface-induced states opposite in sign compared to the island, while the spin-polarization of Co-Phthalocyanine molecules is governed by molecular states. This opens up interesting perspectives for controlling and engineering spin-polarized phenomena at the nanoscale.     

Doping effects on the stacking fault energies of the γ’ phase in Ni-based superalloys

The doping effects on the stacking fault energies(SFEs),including the superlattice intrinsic stacking fault and superlattice extrinsic stacking fault,were studied by first principles calculation of the/phase in the Ni-based superalloys.The formation energy results show that the main alloying elements in Ni-based superalloys,such as Re,Cr,Mo,Ta,and W,prefer to occupy the Al-site in Ni3 AI,Co shows a weak tendency to occupy the Ni-site,and Ru shows a weak tendency to occupy the Al-site.The SFE results show that Co and Ru could decrease the SFEs when added to fault planes,while other main elements increase SFEs.The double-packed superlattice intrinsic stacking fault energies are lower than superlattice extrinsic stacking fault energies when elements(except Co) occupy an Al-site.Furthermore,the SFEs show a symmetrical distribution with the location of the elements in the ternary model.A detailed electronic structure analysis of the Ru effects shows that SFEs correlated with not only the symmetry reduction of the charge accumulation but also the changes in structural energy.     

Size-dependent surface states of strained cobalt nanoislands on Cu(111)

Low-temperature scanning tunneling spectroscopy over Co nanoislands on Cu(111) showed that the surface states of the islands vary with their size. Occupied states exhibit a sizable downward energy shift as the island size decreases. The position of the occupied states also significantly changes across the islands. Atomic-scale simulations and ab initio calculations demonstrate that the driving force for the observed shift is related to size-dependent mesoscopic relaxations in the nanoislands.     

Effect of 3d doping on the electronic structure of BaFe2As2

The electronic structure of BaFe(2)As(2) doped with Co, Ni and Cu has been studied by a variety of experimental and theoretical methods, but a clear picture of the dopant 3d states has not yet emerged. Herein we provide experimental evidence of the distribution of Co, Ni and Cu 3d states in the valence band. We conclude that the Co and Ni 3d states provide additional free carriers to the Fermi level, while the Cu 3d states are found at the bottom of the valence band in a localized 3d(10) shell. These findings help shed light on why superconductivity can occur in BaFe(2)As(2) doped with Co and Ni but not Cu.     

Crystallography and morphology of the early stages of the growth of CoCu(111) by LEED and STM

We report on a quantitative investigation of the structure and morphology of ultrathin films of Co deposited on Cu(111) by an in situ combination of real space (STM) and diffraction (LEED) techniques. The film grows initially as bilayer islands with strict fcc stacking. part of them are covered by Cu. The lateral distribution of Co- and Cu-terminated domains as well as of uncovered patches is both quantitatively determined by intensity analysis and visualized by STM images. Their simultaneous presence correlates nicely to known magnetic properties of ultrathin films, in particular with the observed magnetic anisotropy.     

Electronic structure and ordering of multilayers of Co and Ag on Cu(111) investigated by photoelectron spectroscopy

The growth and the electronic structure of multilayers of Co and Ag on Cu(111) at room temperature have been studied with photoelectron spectroscopy and low-energy electron diffraction (LEED). The coverage range spans from Co and Ag layers between one monolayer (ML) to stacking of several monolayers. Surface states and ordered structures have been identified at room temperature. A Ag-related surface state with a binding energy of 0.30 eV is identified in normal emission in the ultraviolet photoelectron spectra when silver constitutes the top layer. Core-level binding energy shifts of Ag 3d_5/2 reflect the changing surroundings of Ag. Hexagonal diffraction patterns are observed for sandwiches of consecutive layers of Co and Ag up to 5 layers. Since no interlayer diffusion is observed in the layer-by-layer formation of the films, multilayers of consecutive silver and cobalt on Cu(111) offer preparation of sandwiched magnetic–non-magnetic structures.     

Local spectroscopy of two-dimensional states in Na films on Cu(111)

Alkali overlayers on noble metal surfaces exhibit two-dimensional electronic states. We investigated the electronic structure of Na on Cu(111) at T=5 K with scanning tunneling spectroscopy. In agreement with photoelectron spectroscopy we found two electronic bands close to the Fermi energy which correspond to quantum well states (QWS). In second monolayer islands, confinement of the QWS was observed.     

Structure and electronic properties of ultrathin Co films on W(1 1 0)

The structure and electronic properties of ultrathin Co films on W(1 1 0) grown by molecular beam epitaxy in UHV were investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy and spectroscopy (STM and STS). For coverages above 0.7 ML the pseudomorphic (ps) monolayer is transformed gradually into close-packed (cp-) monolayer areas, showing up as separated islands that increase in size with coverage until the cp-monolayer is complete. Two different structures of the cp-monolayer were observed by atomically resolved STM, both leading to a 8 × 1 superstructure in the LEED pattern. Higher coverages continue to grow in the Stransky–Krastanov growth mode forming simultaneously double layer islands and triple layer islands in fcc(1 1 1) and hcp(0 0 0 1) stacking. STS reveals tunneling spectra that differ considerably depending on the thickness and on the structure. Two different classes of triple layer islands can be distinguished by a resonant peak at +0.3 eV appearing in only one of the two classes. We attributed this behavior to a different stacking according to a fcc or hcp structure.     

Quantum size effects in metal thin films grown on quasicrystalline substrates

We have investigated by scanning tunneling microscopy the growth of Bi and Ag thin films on the fivefold surface of Al63Cu24Fe13 and Al72Pd19.5Mn8.5 quasicrystal, respectively. For both systems, we observe the formation of islands with magic height, corresponding to the stacking of a specific number of atomic layers. We interpret this unusual growth morphology in terms of quantum size effects, arising from the confinement of the electron within the film. The magic island heights are thus a direct manifestation of the electronic structure of the quasicrystalline substrates.     

Surface states of cobalt nanoislands on Cu111

The electronic structure of thin Co nanoislands on Cu(111) has been investigated below and above the Fermi level (E(F)) by scanning tunneling spectroscopy at low temperature. Two surface related electronic states are found: a strong localized peak 0.31 eV below E(F) and a mainly unoccupied dispersive state, giving rise to quantum interference patterns of standing electron waves on the Co surface. Ab initio calculations reveal that the electronic states are spin polarized, originating from d3(z(2)-r(2))-minority and sp-majority bands, respectively.     

Atomistic processes and the strain distribution in the early stages of thin film growth

Structural relaxations in small Co islands on the Cu(001) surface are investigated performing atomistic calculations. We demonstrate that the strain relief at the metal interface in the early stages of heteroepitaxy is more complicated than suggested by simple considerations based on the small mismatch between the Co and Cu bulk metals. We found that the strain distribution in the surface region near the islands varies strongly on an atomic scale. The effect of strain on the shape of the Co islands is revealed. Diffusion on the top of strained islands and edge diffusion are considered. Received: 10 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001     

新型超导材料MgCNi3的电子结构与超导电性研究

用MS-Xα方法研究了非氧化物超导材料MgCNi₃的电子结构. 研究结果显示, 态密度分布曲线的主峰靠近Fermi面, 主要来自于Ni的d电子的贡献. 用T(T=Co，Mn，Cu)替代MgCNi₃中的部分Ni形成化合物MgCNi₂T，替代使Ni的价电子数减小, 价态发生变化, Fermi面处态密度N(E_F)减小. 计算结果表明：无论是电子掺杂（Cu）还是空穴掺杂（Co，Mn），MgCNi₃的超导电 关键词： 电子结构 态密度 超导电性     

Charge confinement in epitaxial metallic islands: a study at semiempirical level

Hartree-Fock calculations at semiempirical level, using the extended Debey-Hückel approximation, and tests calculations using LDA with pseudopotentials are used to describe the dispersion relationship, the density of states and the conduction charge of homoepitaxial islands formed by Cu and Ag. The islands have a three-dimensional shape with a dimension of the height and basis in the range of the bulk Fermi wavelength. The calculations indicate that at these small sizes the island steps, at variance with the ones on vicinal surfaces, act as weak and permeable barriers. In fact, the electronic charge retains bulk-like features, though faint traces of confinement can be identified.     

Modified gap states in Fe/MgO/SrTiO3 interfaces studied with scanning tunneling microscopy

The geometric and electronic structures of Fe islands on MgO film layers were studied with scanning tunneling microscopy and spectroscopy. The MgO layers were grown on a Nb-doped single crystal SrTiO₃ (100) surface. Deposited Fe atoms aggregate into islands, the height and diameter of which are about 2.5 and 9.4 nm respectively. Fe islands modify the electronic structure of MgO surface; a ring type depression in the scanning tunneling microscope topography appears by lowered local electron density of states around Fe islands. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, which is attributed to the reason for the depletion of the electronic density of states.     

Doping of monatomic Cu chains with single Co atoms

Close-packed Co-Cu chains of various length and composition were assembled from single Co and Cu atoms on Cu(111) by atom manipulation in a low-temperature scanning tunneling microscope. Local spectroscopy reveals significant electronic Co-Cu coupling leading to confined quantum states delocalized along the heteroatomic chain. Composite Co-Cu chains provide a model case in which the quantum state of an atomic-scale host structure can be tuned by the controlled incorporation of foreign atoms.     

Nuclear magnetic resonance study of epitaxial Co layers on single-crystal substrates

Nuclear magnetic resonance (NMR) is a powerful tool to study the local structure of ultrathin magnetic films and multilayers. This is demonstrated with an NMR study of ultrathin molecular beam epitaxy-grown Co layers on single-crystal Cu(111), Cu(110) and Pd(111) substrates. Co on Cu(111) results in a mixture of fcc and hep phases and the Cu(111)/Co interface is near to perfect. Co on Cu(110) grows mainly fcc in long stripe-shaped islands with rough Co/Cu interfaces. Finally, Co on Pd(111) gives a (111)-oriented fcc structure. The top surface is flat, but the Pd(111)/Co surface is intermixed.     

Synthesis of bimetallic systems using replacement reactions

Series of bimetallic systems were prepared by replacement reactions and characterized by XRD and XPS. The results suggest that the ad-metals are monolayer dispersed on the surface of sub-metal in Pd(Pt, Cu)/Co(Ni) systems, while in Pd(Pt, Au)/Cu systems surface solid solution is formed. In Ag(Au)/Co(Ni) and Ag/Cu systems no interaction between the metals is observed just as in the simple mixture of the respective crystallites. The outermost electronic configurations, the atomic radius of the metals, and the low-preparation temperature seem to be important factors for the different states of these bimetallic catalysts.     

Fast dimer diffusion near the island: A computational study

Performing atomic scale simulations, we study the diffusion of small Co clusters on Cu(111) surface in the presence of Co islands. It is revealed that mesoscopic relaxations at the island-substrate interface play an important role during the diffusion events of the clusters. The diffusion barrier of small Co clusters near the islands increases with increasing cluster size. Especially, we find that dimer diffusion near the island with B step is as fast as monomer’s, which might be responsible for the formation of dendritic shaped islands at low temperature.