Influence of electron-vibration interactions on electronic current noise of atomic and molecular junctions

We present an ab-initio method to simulate the current noise in the presence of electron-vibration interactions in atomic and molecular junctions at finite temperature. Using a combination of nonequilibrium Keldysh Green's function techniques and density functional theory, we study the elastic and inelastic contributions to electron current and shot noise within a wide range of transmission values in systems exhibiting multiple electronic levels and vibrational modes. Within our model we find the upper threshold, at which the inelastic noise contribution changes sign, at a total transmission between $\tau\approx 0.90$ and $0.95$ for gold contacts. This is higher than predicted by the single-level Holstein model but in agreement with earlier experimental observations. We support our theoretical studies by noise measurements on single-atom gold contacts which confirm previous experiments but make use of a new setup with strongly reduced complexity of electronic circuitry. Furthermore, we identify 1,4-benzenedithiol connected to gold electrodes as a system to observe the lower sign change, which we predict at around $\tau\approx 0.2$. Finally, we discuss the influence of vibrational heating on the current noise.