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.     

Magnetization reversal of individual Co nanoislands

We investigate the magnetization reversal of individual Co islands on Cu(111) in the size range of N=700 to 18,000 atoms by spin-polarized scanning tunneling microscopy at 8 K. The switching field H(sw) changes with island size in a nonmonotonic manner: it increases with island size and reaches a maximum value of 2.4 T at N=5500 atoms, and it decreases for larger islands. We extract the energy barrier for magnetization reversal as a function of island size. The maximum H(sw) corresponds to an energy barrier of 1 eV. Our results elucidate a crossover of the magnetization reversal from an exchange-spring behavior to domain wall formation with increasing size at around 7500 atoms.     

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.     

Transfer matrix approach to surface states

The tranfer matrix approach is utilized to study the effect of an impurity states within the framework of the one-dimensional Kronig-Penney model.     

On surface states at superconductor oxide interfaces

Localized electron states in oxides on metal surfaces hybridize with conduction electrons leaking into the oxide and form so a metallic layer and a new type of surface state. For a superconductor such surface states weaken the superconductivity. Because these surface states extend into the oxide, they cause an enhanced tunnel conductivity, which is finite below the gap voltage due to resonance tunneling via the surface states below the gap.     

Non-orthogonal tight-binding approach to surface states of covalent semiconductors

A non-orthogonal tight-binding approach to the surface electronic structure of covalent semiconductors is formulated. Using a non-orthogonal basis of bonding, antibonding and dangling-bond orbital surface states, in particular empty surface resonances in the conduction band are computed. Calculations are performed for Si (111) ideal and relaxed surfaces, leading to results in good agreement with self-consistent calculations. The advantage of using a non-orthogonal basis which effectively results in improved orbital localization is demonstrated.     

Synthesis and characterization of palladium nanoislands on the silicon surface

Samples representing a system of Pd nanoparticles on a Si surface is obtained. The metal is deposited onto the substrate by thermal evaporation in vacuum. The system of nanoparticles is formed using a thin screen located at an angle to the substrate and by vacuum annealing of ultrathin continuous palladium films. The chemical composition of the deposited substance is studied by Auger electron spectroscopy. The size and topology of the islands on the surface is established by scanning electron microscopy.     

Theoretical study of the effect of spin-selective adsorption of water molecules on MgO surface

The effect of spin-selective adsorption of water molecules on the surface of MgO crystal is theoretically studied. The study is performed using two different approaches, i.e., quantum-chemical simulation and an analytical calculation in a quasiclassical approximation. The adsorption energy is calculated using the B3LYP density functional and the 6–311G* basis set. The calculated value of the adsorption energy 0.70 eV agrees well with an experimental value of 0.65 eV. It is established that the energy difference of adsorbed ortho- and para-water molecules is negligible and, thus, the difference of the adsorption energies is completely determined by the energy difference of free molecules in ortho- and para-states. It follows from the analytical calculation that this result is essentially general and is related not only to an MgO surface, but to any other surface on which the energy barrier for rotation of the adsorbed molecule is much larger than the corresponding rotational constant. Based on this, the conclusion is reached that the effect of spin-selective adsorption on these surfaces should not be observed under normal conditions.     

Electron states at Zn(0001) surface

The electronic structure of the Zn(0001) surface is analyzed through a realistic slab calculation with an accurate tight-binding sp-d model hamiltonian. The results are compared with a recent photoemission investigation of this surface. The comparison with noble and near-noble metal surfaces, with reference to the surface states in the s-p gap at the centre of the surface Brillouin zone, is also discussed.     

Mixed-valence states at a crystal surface: SmS

We present a model calculation, based on the Falicov-Kimball model for metal-insulator transitions, which shows that for a mixed-valence solid with a surface, the average valence of the atoms at the surface may be substantially different from that in the bulk. The effect, which we have calculated only for T = 0 and neglecting hybridization, is due to the different local density of itinerant states at the surface and bulk atoms. Surface states contribute to the valence difference but are not solely responsible for it.     

A unified approach for least-squares surface fitting

This paper presents a novel approach for least-squares fitting of complex surface to measured 3D coordinate points by adjusting its location and/or shape. For a point expressed in the machine reference frame and a deformable smooth surface represented in its own model frame, a signed point-to-surface distance function is defined,and its increment with respect to the differential motion and differential deformation of the surface is derived. On this basis, localization, surface reconstruction and geometric variation characterization are formulated as a unified nonlinear least-squares problem defined on the product space SE(3)×m. By using Levenberg-Marquardt method, a sequential approximation surface fitting algorithm is developed. It has the advantages of implementational simplicity, computational efficiency and robustness. Applications confirm the validity of the proposed approach.     

A unified approach for least-squares surface fitting

This paper presents a novel approach for least-squares fitting of complex surface to measured 3D coordinate points by adjusting its location and/or shape. For a point expressed in the machine reference frame and a deformable smooth surface represented in its own model frame, a signed point-to-surface distance function is defined, and its increment with respect to the differential motion and differential deformation of the surface is derived. On this basis, localization, surface reconstruction and geometric variation characterization are formulated as a unified nonlinear least-squares problem defined on the product space SE(3) × R ^m . By using Levenberg-Marquardt method, a sequential approximation surface fitting algorithm is developed. It has the advantages of implementational simplicity, computational efficiency and robustness. Applications confirm the validity of the proposed approach.     

A new approach to the calculation of tight-binding surface density of states in thin films

We present a new approach to the calculation of planar electronic density of states of thin films within the tight-binding scheme. The diagonal elements of the electronic Green's function are obtained by an iteration procedure derived from the transfer-matrix approach for semi-infinite crystals. An application is made to the study of the (100) planes of a model transition metal film with a simple cubic structure.     

Effective surface potential method for calculating surface states

A novel formalism (the effective surface potential method) is developed for calculating surface states. Like the Green function method of Kalkstein and Soven and the transfer matrix method of Falicov and Yndurain, the technique is exact for simple tight binding Hamiltonians. As well as offering an alternative viewpoint, the present method provides a simple analytic expression describing the surface states. At each point k_s in the surface Brillouin zone the semi-infinite solid is viewed as an effective linear chain where each element of the chain is a planar layer. The solution to the linear chain problem can be expressed in terms of an effective potential h(k_s,E) at each energy E. A number of examples are presented in detail; “sp^d” Hamiltonians for a linear chain (d = 1), the honeycomb lattice (d = 2), the 111 surface of silicon (d = 3), and a dissected Bethe lattice. Various exact results are given, e.g. the extremities of surface state bands and the surface density of states of p-like (delta function) bands. The results of Kalkstein and Soven for the 100 surface of a simple cubic solid with a perturbation on the surface layer are rederived.     

A conceptual model for surface states and catalysis ond-band perovskites

This paper proposes a conceptual model to describe symmetry factors believed to be most important in catalyzing a reaction on a transition metal oxide. An hypothetical situation is constructed from a model reaction, the dimerization of ethylene to form cyclobutane, and a model perovskite, SrTiO₃. A detailed review is given of the symmetry imposed energy barrier for the gas phase reaction. Then it is shown how thed _xy, d_xz, d_yz atomic orbitals have the symmetry required to reduce this energy barrier when a transition metal atom is included in the reaction. It is also shown how the evergy barrier reduction depends upon the electronic occupation of thed orbitals and upon the relative energies of thed orbitals involved in the reaction. Using SrTiO₃ as an example, it is shown that the perovskites possess highly localized, atomic-liked surface states of the required symmetry. Two semiquantitative examples are developed for the catalyzed dimerization of ethylene on the (100) surface of SrTiO₃. A preliminary discussion is given of the role played by surface potentials in promoting the reaction.     

Model Eliashberg functions for surface states

We present a simplified procedure for the analysis of the phonon-induced lifetimes of surface states. The model includes information about the electron and phonon structure and is thus more reliable than procedures based on phonon Debye models. We apply the model to calculate the lifetime broadening of Cu(1 1 1) and Al(0 0 1) surface states. The obtained Eliashberg functions and lifetimes are in reasonable agreement with previous detailed studies.     

A new particle-hole approach to collective states

The relevance of the particle-hole space is demonstrated by showing that in some commonly used formalisms the first excited state lies entirely within the particle-hole space generated from the correlated ground state. This property is proved for several cases of angularmomentum projection — projected Hartree-Fock method (PHF) — and for the generator coordinate method in the Gaussian overlap approximation, while in other cases it has been verified only numerically. A new method is presented for the approximate calculation of energies and transition amplitudes of particle-hole excited states. Only hermitian one-body operators are used to generate the excited states. The two-body density matrix of a correlated state approximating the ground state is required as input data. The formula is tested on the 2⁺ and 3^? states of ⁸Be and ¹²C by using the PHF ground state. Where comparison is possible the method gives better agreement with PHF and experiment than the extended random-phase approximation.     

A density-matrix approach to thermal equilibrium states

Different solutions for thermal equilibrium states arising from both density matrix properties and quasi-spin conservation were examined in a constrained approach, with reference to a nuclear model.     

Scattering of surface states at step edges in nanostripe arrays

Surface states of noble metal surfaces split into Ag-like and Cu-like subbands in stepped Ag/Cu nanostripe arrays. The latter self-assemble by depositing Ag on vicinal Cu(111). Ag-like states scatter at nude step edges in Ag stripes, leading to umklapp bands, quantum size effects, and peak broadening. By contrast, Ag stripe boundaries become transparent to Cu-like states, which display band dispersion as in flat Cu(111). We find a linear relationship between the quantum size shift and peak broadening that applies in a variety of stepped systems, revealing the complex nature of step barrier potentials.