Current-induced mechanical torque in chiral molecular rotors

Headnote

Full Research Paper

Keywords:

molecular junctions; molecular motors; molecular switches

Abstract

There has been great endeavor to engineer molecular rotors operated by an electrical current. A frequently met operation principle is the transfer of angular momentum taken from the incident flux. In this paper, we present an alternative driving agent that works also in situations where angular momentum of the incoming flux is conserved. This situation arises typically with molecular rotors that exhibit an easy axis of rotation. For quantitative analysis we investigate here a classical model where molecule and wires are represented by a rigid curved path. We demonstrate that in the presence of chirality, the rotor generically undergoes a directed motion, provided that the incident current exceeds a threshold value. Above this threshold, the corresponding rotation frequency (per incoming particle current) for helical geometries turns out to be 2ρm/M₁, where m/M₁ is the ratio of the mass of an incident charge carrier and the mass of the helix per winding number.

(ProQuest: ... denotes formulae omitted.)

Introduction

Experiments employing scanning tunneling microscopy (STM) have achieved the directed rotation of molecules controlled by an electrical current. Correspondingly, realizations of molecular switches and rotors have been reported. [1-9], with potential relevance for future molecular technologies.

The theory describing the working principle of such molecular motors often employs angular Langevin equations [9.10]. This method has been established by Hänggi [11] and Astumian [12] and their collaborators in the context of Brownian motors. It describes the dynamics of a classical angular variable 9 that is subject to a "ratchef'-type potential in the presence of a (phenomenologically treated) driving torque. Ab initio expressions for the current-induced torques have been obtained within the non-equilibrium Green's function formalism [13.14]. The current excites a variety of molecular vibrational modes, rendering the atomistic analysis of the torque very complex (see [6] for an ab initio calculation of the vibrations).

To bring about a controlled unidirectional rotation in the STM setup requires a degree of symmetry breaking. There are two typical situations, that is. either the molecule by itself exhibits a handedness (chirality ) or chirality is imposed by the geometry of the molecular junction [6,10]. The purpose of this article is to provide a qualitative description of the...