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Monatomic metallic wires: structure, electric transport and normal modes
Andres Saul
CRMCN-CNRS Marseille
Thu, Feb. 08th 2007, 11:00
SPEC Bât 466 p.111 (1er ét.), CEA-Saclay
The formation of atomic chains several (up to 7-8) atoms long, has been achieved in recent years using experimental techniques like scanning tunneling microscope and mechanically controllable break junctions. In both cases, a chain formation has been observed in the last stages of pulling an atomic contact for certain metallic elements like Au, Pt and Ir. At low voltages (zero-bias conductance) the experimental results indicate the presence of conductance oscillations with period ~2a (a being the interatomic distance). In the case of Au, these oscillations are superimposed to an almost constant background of the order of the quantum of conductance G0 = 2e2/h, while for Pt the mean value of the conductance exhibits a continuous decrease from ~2.5G0 to ~1.0 G0. For Ir chains the conductance varies between ~2.2 and ~1.8 G0 with a less clear oscillatory behavior. On the other hand, the dependence of the conductance with the applied voltage shows that transport in these wires is non-dissipative up to a threshold of several meV, which corresponds to the energy of the vibrational normal modes of the chain. In this work, we present calculations of the electronic structure and conductance of atomic chains of 5d elements like Au, Pt and Ir. We show that, in addition to the even-odd parity oscillations characteristic of Au, conduction channels associated with the almost full d bands in Pt and Ir give rise to longer periods which could be observed in sufficiently long chains. The results for short chains are in good agreement with recent experimental measurements. We will also show, preliminary results obtained from molecular dynamic simulations using a tight-binding Hamiltonian. For Au, we will present computer simulations showing the formation and compression of atomic wires. We report the atomic processes involved in the formation and crush of the atomic chains, the evolution of the conductance and the normal modes of vibration.


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