Cooling molecular electronic junctions by AC current

Abstract

Electronic current flowing in a molecular electronic junction dissipates significant amounts of energy to vibrational degrees of freedom, straining and rupturing chemical bonds and often quickly destroying the integrity of the molecular device. The infamous mechanical instability of molecular electronic junctions critically limits performance and lifespan and raises questions as to the technological viability of single-molecule electronics. Here, we propose a practical scheme for cooling the molecular vibrational temperature via application of an AC voltage over a large, static operational DC voltage bias. Using nonequilibrium Green’s functions, we computed the viscosity and diffusion coefficient experienced by nuclei surrounded by a nonequilibrium ”sea” of periodically driven, current-carrying electrons. The effective molecular junction temperature is deduced by balancing the viscosity and diffusion coefficients. Our calculations show the opportunity of achieving in excess of 40% cooling of the molecular junction temperature while maintaining the same average current.

First-passage time theory of activated rate chemical processes in electronic molecular junctions

“…In alignment with our previous work [3,4,32,[56][57][58], we assume that the classical motion along the reaction coordinate within the system occurs over long time-scales relative to the characteristic electron tunnelling time. This provides us with the required small parameter to be able to perturbatively solve ( 5) and ( 6) up to the first order in expansion of the exponents with derivatives.…”

“…This enables the consideration of highly non-trivial behaviour on the nuclear dynamics at the cost of a fully quantum description. Nevertheless, the method has proven successful in a range of circumstances [1,3,4,6,11,[26][27][28][29][30][31][32].…”

Emergence of negative viscosities and colored noise under current-driven Ehrenfest molecular dynamics

“…It is instead convenient to adopt a quasi-classical approach in which the nuclei are assumed to behave according to classical equations of motion under the effects of the quantum tunneling electrons. This enables the description of arbitrary molecular potentials for systems along with being able to capture experimentally observed phenomena such as current-induced heating [32][33][34][35][36][37] along with bond rupture and electronically induced chemical reactions [38][39][40][41], at the cost of a fully quantum description.…”

“…A common accompanying assumption is the clear separation of time-scales between the classically described nuclei and the electronic environment, which through the use of a perturbative approximation allows for a Langevin description of the classical particle [32][33][34][35][36][41][42][43][44][45][46][47]. In this regime, the particle dynamics are described by stochastic differential equations in which nuclei evolve under the influence of a frictional force which acts to subdue nuclear vibrations, and a stochastic force which delivers energy to the nuclei; the balance of these two forces yielding the temperature of the molecule.…”

“…In an exact theory, the stochastic force at a given time should have some dependency on the history of the classical coordinate; this corresponds to colored noise [58]. However, it is often computationally infeasible to account for this and it becomes necessary to employ the white-noise approximation; this states that the stochastic force is entirely uncorrelated at different times [32,33,39,41,42,44]. While the white-noise approximation proves to be applicable in regimes showing a clear time-scale separation between electronic and classical coordinates, we show here that it must be called into question in cases where the time-scale separation becomes less defined.…”