Origin of Vibrational Instabilities in Molecular Wires with Separated Electronic States

Abstract

Current-induced heating in molecular junctions stems from the interaction between tunneling electrons and localized molecular vibrations. If the electronic excitation of a given vibrational mode exceeds heat dissipation, a situation known as vibrational instability is established, which can seriously compromise the integrity of the junction. Using out of equilibrium first-principles calculations, we demonstrate that vibrational instabilities can take place in the general case of molecular wires with separated unoccupied electronic states. From the ab initio results, we derive a model to characterize unstable vibrational modes and construct a diagram that maps mode stability. These results generalize previous theoretical work and predict vibrational instabilities in a new regime.

Kinetic Schemes in Open Interacting Systems

“…(10). In agreement with previous considerations, 28,29,31 the zero-order calculation predicts instability for resonance condition, showing runaway heating of the vibration when the electron hopping matrix element t is small (see low right corner of the dissipation rate map shown in Fig. 3a).…”

Single-molecule quantum-transport phenomena in break junctions

“…To verify the level position, high bias voltages would need to be applied to reach the resonant transport condition. However, it is difficult to reach this condition; bond breaking 46 , Coulomb interactions or instabilities associated with vibrational heating 47 or charging of the molecule have been proposed as explanations for this difficulty.…”

Unraveling current-induced dissociation mechanisms in single-molecule junctions

“…As a side note, the results obtained for larger moleculelead coupling Γ can be used to evaluate the range of validity of low-order kinetic schemes. 38,43 We compared the results obtained using different hierarchical truncation tiers for the parameters shown in Fig. 13 and found that when Γ > Ω 0 /2, the first-tier results (equivalent to a second order perturbative treatment of molecule-lead coupling) always overestimate the dissociation rate.…”

“…This has been achieved within a classical treatment of the nuclei based on the Ehrenfest approach [39][40][41][42] or using perturbative theories. 11,43 Similarly, the study of mechanical instabilities of molecular junctions under the influence of nonconservative current-induced forces has so far been based on classical treatments of the nuclei and/or used the harmonic approximation for the description of the nuclear potentials. [44][45][46][47][48] It is also noted that the theoretical framework to study the related process of dissociative electron attachment in the gas phase is well established, [49][50][51][52] but this problem is conceptually simpler because only a single electron that is scattered from the molecule has to be considered.…”