1. Introduction

Current transfer is defined as a charge transfer transition characterized by the relocation of both charge and its momentum. In a recent paper, we have proposed a tight binding charge transfer model for recent observations that indicate that photo-electron transfer induced by circularly polarized light through helical molecular bridges depends on the relative handedness of the bridge helicity and on the optical circular polarization. Another recent example of current transfer in photoemission is provided by Ref. 4, in which the signature of a biased linear momentum distribution created on Cu (100) surface is observed in the angular distribution of the photoemitted current.

Our rationalization of the experimental results of Refs. 2, 3 was based on the assumption (supported by theoretical analysis5-8 that excitation by circularly polarized light can create a circular electronic current in the absorbing molecule and that chiral control of transmission of these currents results from a coupling scheme associated with atoms proximity. Figure 1 illustrates this idea.

It should be clear from Fig.1 that the current transfer phenomenon originates from the coupling scheme, which results here from proximity of circular molecular structures. Simpler structures that show the same physical behavior are displayed in Figure 2, where each structure corresponds to a tight binding Hamiltonian with nearest neighbor coupling indicated by the bond connecting different sites. In model 2a we consider a wire D (the “driver”) carrying a current DJ and investigate the possibility of current transfer to wire A through a coupling region defined by coupling matrix elements Vij between DAN pairs of neighboring atoms i and j on different wires.

2. Model Hamiltonian and the steady-state problem

For definiteness we focus on the model of Fig. 2b, which depicts the driving wire D, the bridging wire B and the accepting wire A as linear tight-binding chains. The corresponding Hamiltonian is ˆˆ ˆ ˆ ˆ ˆ DA BB AB D HH H H V V , where () () ,1 1; , , jKK K KK KKK KK jK jK HEjjVjjKDAB     (2) and   ' ' (,') ,' ' , ,' ;,', , KK KK KK KK K K jj jKj K VVjjKKDBorBA   (3) where ˆ D H, ˆ B H, and ˆ A H are the Hamiltonians for the D, B and A moieties, respectively and ()ˆ DB V, ()ˆ BA V are the D-B and A-B interactions.

In Ref. 1 we have considered the time evolution that follows the excitation of ring current in the driving wire. If the latter is a ring of DN equivalent sites, this is represented by the Bloch wavefunction  (1) 1 0 D D D N ij ka D j a te j L      . (5a) with 2 D M ka N   ; 0, 1,..., 1 D M N   ; D L Na (5b) where α is the inter-site distance. Here the driving wire is restricted to remain in this state, and we require the steady state assumed by the rest of the system under this restriction. Obviously, the only relevant sites on the driver are those that are directly'