R H M Smit et al 2009 New J. Phys. 11 073043 doi:10.1088/1367-2630/11/7/073043
Metallic properties of magnesium point contacts
R H M Smit1, A I Mares1,4, M Häfner2,3, P Pou3, J C Cuevas3 and J M van Ruitenbeek1,5
Show affiliationsWe present an experimental and theoretical study of the conductance and stability of Mg atomic-sized contacts. Using mechanically controllable break junctions (MCBJ), we observed that the room temperature conductance histograms exhibit a series of peaks, which suggests the existence of a shell effect. Its periodicity, however, cannot be simply explained in terms of either an atomic or electronic shell effect. We also found that at room temperature, contacts of the diameter of a single atom are absent. A possible interpretation could be the occurrence of a metal-to-insulator transition as the contact radius is reduced, in analogy with what is known in the context of Mg clusters. However, our first principles calculations show that while an infinite linear chain can be insulating, Mg wires with larger atomic coordinations, as in realistic atomic contacts, are always metallic. Finally, at liquid helium temperature, our measurements show that the conductance histogram is dominated by a pronounced peak at the quantum of conductance. This is in good agreement with our calculations based on a tight-binding model that indicated that the conductance of a Mg one-atom contact is dominated by a single fully open conduction channel.
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GENERAL SCIENTIFIC SUMMARY
Introduction and background. With the ongoing miniaturization of electronics, the interest in molecular conductance has increased greatly. In order to predict the transmission of electrons through complex molecular structures, however, we must first have a profound understanding of the metallic electrodes to which we bind them. In the last decade it was shown that the conduction through metallic contacts at the scale of a single atom is determined by the number of valence orbitals. Frequently used materials such as gold only have one orbital available at the Fermi energy, simplifying the electronic structure greatly. In this study we focus on the more complicated properties of magnesium.Main results. With a full s-shell and an empty p-shell, magnesium contains the right ingredients to be an insulator. Although cluster experiments verify this insulating behavior for small quantities of atoms, our study shows that even the smallest of contacts, consisting of a single atom, show metallic behavior. Moreover, there is only a single electronic state responsible for the conduction. Our model calculations show that this is due to the electronic coupling of the central atom to the electrodes. Just like in bulk magnesium, this leads to a filling of the band gap.
Wider implications. As a consequence, magnesium might be an interesting metal for the study of molecular conduction. Its alternative chemistry might provide additional information on the electronic details.
Figure. Conductance histogram of magnesium atomic-sized contacts measured at helium temperatures with mechanically controllable break junctions. The inset shows a plausible contact geometry giving rise to the peak very close to one quantum of conductance. - PACS
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71.30.+h Metal-insulator transitions and other electronic transitions
71.20.Eh Rare earth metals and alloys
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
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- Dates
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Issue 7 (July 2009)
Received 21 April 2009
Published 23 July 2009
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Metallic properties of magnesium point contacts
R H M Smit et al 2009 New J. Phys. 11 073043
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