Browsing by Author "Chulkov, E.V."

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Clustering and conductance in breakage of sodium nanowires
(2011-01-10) Zugarramurdi, A.; Borisov, A.G.; Zabala, N.; Chulkov, E.V.; Puska, M.J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We study the conductance during the elongation and breakage of Na nanowires described with the ultimate jellium model. A combined approach is used where the nanowire breakage is simulated self-consistently within the density-functional theory, and the wave packet propagation technique is applied for ballistic electron transport. For certain conditions the breakage of the nanowire is preceded by formation of clusters of magic size in the break junction. This affects the conductance G, in particular the shape of the G=3G0 to G=G0 (=2e2/h) step upon elongation. The observed trends can be explained as due to the transient trapping of ballistic electrons inside the cluster, leading to a resonant character of the electron transport through the break junction. The cluster-derived resonances appear as peak structures in the differential conductance which may serve as an experimental signature of clustering.
Excited states of Na nanoislands on the Cu(111) surface
(2007-04-27) Hakala, Tero; Puska, M.J.; Borisov, A.G.; Silkin, V.M.; Zabala, N.; Chulkov, E.V.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Electronic states of one monolayer high Na nanoislands on the Cu(111) surface are studied as a function of the nanoisland size. Properties of nanoislands such as one-electron states, the electron density, and the associated potential are obtained self-consistently within the density-functional formalism using a one-dimensional pseudopotential for the Cu(111) substrate and the jellium model for Na. A wave packet propagation method is used to study the energies and lifetimes of quasistationary states localized at Na islands. For very large islands, island-localized states merge into the two-dimensional continuum of the Na quantum well state. Thus, we assign the quasistationary states studied as arising from the quantization of the two-dimensional quantum well continuum due to the finite island size. The scattering at the island boundaries results in the energy-conserving resonant electron transfer into the continuum of the substrate states broadening the island-localized states into resonances.
Image potential states of supported metallic nanoislands
(2007-09-26) Borisov, A.G.; Hakala, Tero; Puska, Martti J.; Silkin, V.M.; Zabala, N.; Chulkov, E.V.; Echenique, P.M.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Our theoretical study shows how well-resolved image potential states develop on metal-supported metallic nanoislands. The systems under consideration are one monolayer high Na nanoislands on the Cu(111) surface. However, the phenomenon is general and it requires only that the work function of the adsorbate is lower than that of the substrate, and that the substrate possesses a projected band gap along the surface normal.
Quantum size effects in Pb islands on Cu(111): Electronic-structure calculations
(2004-04-23) Ogando, E.; Zabala, N.; Chulkov, E.V.; Puska, M.J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The appearance of “magic” heights of Pb islands grown on Cu(111) is studied by self-consistent electronic structure calculations. The Cu(111) substrate is modeled with a one-dimensional pseudopotential reproducing the essential features, i.e., the band gap and the work function, of the Cu band structure in the [111] direction. Pb islands are presented as stabilized jellium overlayers. The experimental eigenenergies of the quantum-well states confined in the Pb overlayer are well reproduced. The total energy oscillates as a continuous function of the overlayer thickness reflecting the electronic shell structure. The energies for completed Pb monolayers show a modulated oscillatory pattern reminiscent of the supershell structure of clusters and nanowires. The energy minima correlate remarkably well with the measured most probable heights of Pb islands. The proper modeling of the substrate is crucial to set the quantitative agreement.
Self-consistent study of electron confinement to metallic thin films on solid surfaces
(2005-05-10) Ogando, E.; Zabala, N.; Chulkov, E.V.; Puska, Martti J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We present a method for density-functional modeling of metallic overlayers grown on a support. It offers a useful tool to study nanostructures, combining the power of self-consistent pseudopotential calculations with the simplicity of a one-dimensional approach. The model is illustrated for Pb layers grown on the Cu(111) surface. The analysis provides the strength of the electron confinement barriers in thin slabs with accuracy, supporting the interpretation of the quantum well state spectra measured by scanning tunneling spectroscopy. On the other hand, it offers a benchmark to check the simple analytical models commonly used in the literature to study metallic films on semiconducting or metallic surfaces. As a result, some deficiencies are detected in the applicability of those models, which often lead to an overestimation of the number of wetting layers. Finally, an improved formula is proposed.