L. Jurczyszyn

Formation of STM images of Ni 3 Al (001) and (111) surfaces

We present the results of scanning tunnel microscope (STM) measurements performed on a Ni 3 Al(111) surface, combined with a theoretical study of the formation of STM images of (001) and (111) surfaces of this alloy. The STM images of the Ni 3 Al(111) surface show a superstructure with the lattice constant corresponding to the size of the surface unit cell. An earlier interpretation of this result assumes that this superstructure represents the distribution of the surface Al atoms, while Ni atoms remain invisible in the STM images. This supposition is confirmed here by presented STM simulations. Moreover, numerical calculations show that a similar effect should also appear in the STM images of the Ni 3 Al(001) surface. A detailed theoretical analysis indicates that the domination of Al atoms in the STM images of both surfaces is mainly caused by the intra-atomic s-p z interference. This kind of intra-atomic interference reduces the s and p z current contributions tunneling through surface Ni atoms and increases considerably the corresponding contributions flowing through Al atoms. As a result, only the surface Al atoms appear in the STM images.

The resonance image states at the (111) surface of fcc metals

Abstract On surfaces of many metals the vacuum level is situated above an energy gap and therefore resonance image states can exist outside the energy gap. The (111) surface of a model fcc crystal, with the vacuum level above the upper edge of the energy gap L 1 was considered here, allowing study of the influence of the crystal band structure on the energy spectrum and broadening of the resonance image states. Calculations, based on the Green function matching, were performed for different values of the matrix element of the crystal potential V 111 . It was found that for decreasing V 111 (which increase the energy distance between the vacuum level and the upper edge of the energy gap) the absolute value of the binding energy of the resonance image state decreased. A comparison of this conclusion with experimental results was presented and discussed.

The resonance image states at Cu(111), Au(111), and Al(111) surfaces

Abstract Using the Green function approach the resonance image states at Cu(111), Au(111), and Al(111) surfaces were studied and the distributions of the density of states were calculated in the crystal surface region. It is shown that the crystal lattice potential influences the resonance image states and is responsible for the difference in their binding energies at (111) surfaces of Cu, Au and Al.

Influence of the atomic and electronic structure of the tip on STM images and STS spectra

Abstract Theoretical studies of the influence of electronic and atomic structure of a tip on STM images and STS spectra are presented. This is discussed for the scanning of clean Al(001) surface performed within aluminium tips of a different geometry. The influence of d states of a tip on STS spectra for the Ni-tip/Al(001)-sample system is also studied.

A new approach to the study of electronic properties of image resonances at clean and Na-covered Al(001) surfaces

Abstract An original one-dimensional semi-infinite approach with a realistic potential, obtained from first-principle slab calculations, is presented and applied to study image resonances at clean Al(001) and Al(001) +Na(2 × 2) surfaces. The obtained results, being in reasonable agreement with experimental data, are compared with those of Silkin and Chulkov [Phys. Solid State 36 (1994) 404]. The presence of the “thin film” state, incorrectly considered by Silkin and Chulkov as the ( n =0) image state, is discussed.

Barrier-resonance states in an external electric field

The Green function approach was used to study the behaviour of barrier-resonance image states in the presence of an external electric field, applied to a free-electron-like metal. Distributions of the density of states were calculated outside the metal for different field intensities F. It was found that the structure of these «barrier resonances» still exists at F≃10 8 V/cm. Therefore, it is possible that they can be involved in the field ionisation process

High energy resonance electronic states inside a W-Pd(111) tunnel junction

Distributions of density-of-states have been calculated between the electrodes of a W-Pd(111) tunnel junction. The effect of the inclusion of the Pd lattice potential is discussed in detail. A comparison is also made between the results obtained using the cut-off interface potential barrier and the results of Jones, Jennings and Jepsen.

Theoretical investigation of image states of hydrogen covered Cu(100)

A model for atomic hydrogen covered Cu(100) is presented and the calculated energy spectrum of localized electronic states in the X gap of Cu(100) is discussed. These states form a series of unoccupied adsorption states (for nα = 2,3,…) lying below the vacuum level V0 and having energies Enα which satisfy the formula Enα = V0−10nα2eV. The lowest state (nα = 1) is expected to lie about 5.5 eV below the Fermi level.

Simulation of the soft-landing and adsorption of C60 molecules on a graphite substrate and computation of their scanning-tunnelling-microscopy-like images

A constant-temperature molecular dynamics (MD) simulation was performed to model the soft-landing and adsorption of C60 molecules on a graphite substrate with the C60s treated as soft molecules and released individually towards the substrate. The intra-molecular and intra-planar covalently bonding interactions were modelled by very accurate many-body potentials, and the non-bonding forces were derived from various pairwise potentials. The simulation extended over 1.6 million time steps covering a significant period of 160 picoseconds. The final alignment of the molecules on the surface agrees closely with that observed in an experiment based on scanning tunnelling microscopy (STM) on the same system, performed at room temperature and under ultrahigh-vacuum (UHV) conditions. Using a tungsten tip in a constant-current mode of imaging, we have also computed the STM-like images of one of the adsorbed molecules using a formulation of the STM tunnelling current based on Keldyshs non-equilibrium Green function formalism. Our aim has been to search for tip-induced states, which were speculated, on the basis of another STM-based experiment, performed in air, to form one of the possible origins of the extra features purported to have been observed in that experiment. We have not obtained any such states.

LCAO calculations of surface electronic structure of zinc-blende-type GaN(100)

Abstract Layer resolved density of states has been calculated for N- and Ga-terminated gallium nitride (100) surfaces, using the semi-empirical LCAO method with self-consistently corrected parameters for the surfaces. Surface states on both N- and Ga-terminated surfaces have been found. Localization properties of these states have also been examined.