Current-Induced Vibrational Instabilities in Molecular Wires with Broken Conjugation

The stability of molecular scale circuits in presence of an external bias is particularly sensitive to the subtle balance between heat generation and dissipation associated to the flow of electrical current [1]. In this talk I will present first-principles calculations based on density functional theory (DFT) and the nonequilibrium Green's function formalism (NEGF) of the current-induced heating dynamics in a terphenyl-based molecular junction [2].
I will consider a molecule in which steric repulsion localizes wave functions on either end of the molecule and show how this left-right decoupling creates a condition for the effective heating of molecular vibrations in junctions with LUMO-dominated transport. Depending on the slope of the spectral function projected on the positive electrode and the structure of the electron-vibration coupling matrix, the emission rate can exceed inter-electrode absorption which could lead to negative vibronic damping.
Our findings generalize the conditions for vibrational stability and have important implications for both theoretical as well as experimental investigations on molecular wires with broken conjugation.

[1] J.-T. Lü, P. Hedegård, and M. Brandbyge, Phys. Rev. Lett. 107, 046801 (2011)
[2] G. Foti and H. Vázquez, (in preparation).