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

Abstract

Molecules in molecular junctions are subject to current-induced forces that can break chemical bonds, induce reactions, destabilize molecular geometry, and halt the operation of the junction. Theories behind current-driven molecular dynamics simulations rely on a perturbative time-scale separation within the system with subsequent use of nonequilibrium Green's functions (NEGF) to compute conservative, non-conservative, and stochastic forces exerted by electrons on nuclear degrees of freedom. We analyze the effectiveness of this approximation, paying particular attention to the phenomenon of negative viscosities. The perturbative approximation is directly compared to the nonequilibrium Ehrenfest approach. We introduce a novel time-stepping approach to calculate the forces present in the Ehrenfest method via exact integration of the equations of motion for the nonequilibrium Green's functions, which does not necessitate a time-scale separation within the system and provides an exact description for the corresponding classical dynamics. We observe that negative viscosities are not artifacts of a perturbative treatment but also emerge in Ehrenfest dynamics. However, the effects of negative viscosity have the possibility of being overwhelmed by the predominantly positive dissipation due to the higher-order forces unaccounted for by the perturbative approach. Additionally, we assess the validity of the white-noise approximation for the stochastic forces, finding that it is justifiable in the presence of a clear time-scale separation and is more applicable when the current-carrying molecular orbital is moved outside of the voltage window. Finally, we demonstrate the method for molecular junction models consisting of one and two classical degrees of freedom.

References in corpus (21)

• Vibrational and electronic heating in nanoscale junctions
• Resonant electron heating and molecular phonon cooling in single C${}_{60}$ junctions
• Hierarchical Quantum Master Equation Approach to Electronic-Vibrational Coupling in Nonequilibrium Transport through Nanosystems
• Current-induced nonequilibrium vibrations in single-molecule devices
• Current-induced atomic dynamics, instabilities, and Raman signals: Quasi-classical Langevin equation approach
• Cooling mechanisms in molecular conduction junctions
• Pumping of vibrational excitations in a Coulomb blockaded suspended carbon nanotube
• Laser-like vibrational instability in rectifying molecular conductors
• Current noise in molecular junctions: effects of the electron-phonon interaction
• Current-induced forces: a simple derivation
• Semi-classical dynamics of nano-electromechanical systems
• Current-induced forces and hot-spots in biased nano-junctions
• Out-of-equilibrium catalysis of chemical reactions by electronic tunnel currents
• Resonant heating and substrate-mediated cooling of a single C60 molecule in a tunnel junction
• Current Noise in Single-Molecule Junctions Induced by Electronic-Vibrational Coupling
• Electro-Mechanical Transition in Quantum dots
• First-passage time theory of activated rate chemical processes in electronic molecular junctions
• Electronic friction in interacting systems
• Nonconservative dynamics in long atomic wires
• Cooling molecular electronic junctions by AC current
• An ignition key for atomic-scale engines

Cited by in corpus (1)

• Current-induced forces in nanosystems: A hierarchical equations of motion approach