Single-channel conductance ofH2molecules attached to platinum or palladium electrodes

Abstract

We report a detailed theoretical study of the bonding and conduction properties of an Hydrogen molecule joining either platinum or palladium electrodes. We show that an atomic arrangement where the molecule is placed perpendicular to the electrodes is unstable for all distances between electrodes. In contrast, the configuration where the molecule bridges the electrodes is stable in a wide range of distances. In this last case the bonding state of the molecule does not hybridize with the leads and remains localized within the junction. As a result, this state does not transmit charge so that electronic transport is carried only through the anti-bonding state. This fact leads to conductances of 1 G0 at most, where G0 = 2e 2 /h. We indeed find that G is equal to 0.9 and 0.6 G0 for Pt and Pd contacts respectively. PACS numbers: 73.63.Rt, 71.15.Mb The main obstacle to using single molecules as conducting elements 1 is the difficulty in establishing stable and reproducible contacts to metallic electrodes. Conductance quantization in atomic constrictions was predicted several years ago 2 , but the practical realization in single-molecule contacts only became possible in the mid-90's, with the advent of the scanning tunneling microscope 3,4,5 and mechanically controllable break junctions6 .An important testing ground for both theory and experiment is the simplest of all molecular bridges, namely the H 2 molecule. This molecule has just one bonding and one anti-bonding state in the available energy range. Therefore it should be possible to predict the effect of tuning a range of adjustable parameters, including the position and orientation of the molecule, the distance and voltage between the electrodes and the materials that make up the electrodes. Despite the apparent simplicity of this junction, there is currently no agreement about what are the position and orientation of the molecule for given separation between the electrodes, even for zero applied bias.Smit and coworkers 7 found that the conductance histograms of the molecule, sandwiched between Pt leads, had a sharp peak at about 0.9 G 0 (G 0 = 2e 2 /h is the conductance quantum) and therefore argued that electronic transport was dominated by a single channel with an almost perfect transmission. In addition, from the experimental phonon spectra they concluded that the molecule bonds to the electrodes in a bridge configuration (BC), i.e. with the H 2 bond axis parallel to the transport direction (see Fig.1(a)). This was also confirmed by Cuevas et al. 8 .A different interpretation was given by García and coworkers 9 who showed theoretically that an arrangement where the molecule bonds perpendicularly to the transport direction (perpendicular configuration, PC) has a lower energy, and therefore should also be the preferred atomic configuration * Present address: Department of Physics, Lancaster University, Lancaster, LA1 4YB, U. K.(see Fig. 1(b)). Their simulations gave conductances of order 1 G 0 and 0.2 G 0 for PC and BC, respectively. Very recently, Thygesen ...

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“…The results reported in Ref. 40 are based on Siesta DFT code and show good agreement with our results. The conductance obtained in one of the early theoretical calculations [39] on the hydrogen molecular bridge are considerably lower than our and most other results.…”

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