The electronic transport properties of double-stranded DNA are studied using a tight-binding Hamiltonian. Realistic on-site energies and tunneling constants between adjacent bases are used in the model. The results show that transport properties of DNA molecules can change from insulator to ohmic behavior due to either stronger intra-strand tunneling constants or smaller on-site energy difference between bases. The presence of base mismatches as well as defect vacancies on DNA, the different ways of contacting DNA to electrodes, and the strength of the contact are shown to strongly affect the current flowing through the molecules. Different DNA sequences show different current profiles and conductance. Although more studies are needed, our results suggest the possibility of using transport measurement as a tool in DNA sequencing.
The properties of polaron states in short molecules modeled by electron tunneling among few sites and interacting anharmonically with rotational/twisting vibrational modes have been studied. Angle dependent as well as bare tunnelings are considered. Different models of angle dependent tunneling with built-in asymmetries have been used. The effective tunneling calculated using the ground state orbital in the case of symmetric angle dependent tunneling increases with increasing interaction; however, it typically decreases in the case of asymmetric angle dependent tunneling. For soft phonon modes, symmetric electron phonon coupling gives larger effective tunneling. This behavior may have consequences for molecular transport experiments in flexible molecules.
The effect of local and non-local phonon on transport properties of a molecule model described by two-electronic states has been studied. A Lang-Firsov transformation is used in treating local electron-phonon interaction. Non-local electron-phonon interaction is treated perturbatively up to first nonzero order in the self-energy. Increasing local electron-phonon interaction changes the voltage threshold on the I-V curves, and results in more gradual current increment. The current also increases gradually by increasing non-local electron-phonon interaction, at the same time that the threshold becomes higher. Higher phonon occupation number at higher temperature leads to lower voltage threshold and more gradual current increment after the threshold. However, in the weak interaction regime the effect of temperature is less profound.