A self-consistent tight binding model for hydrocarbon systems: application to quantumtransport simulation

Abstract

A self-consistent environment-dependent (SC-ED) tight binding (TB) method forhydrocarbons that was developed for quantum transport simulations is presented. Themethod builds on a non-self-consistent environment-dependent TB model for carbon (Tanget al 1996 Phys. Rev. B 53 979) with parameters added to describe hydrocarbon bonds andto account for self-consistent charge transfer. The SC-EDTB model assumes an orthogonalbasis set. Orthogonality is a key element for adapting the SC-EDTB scheme to transportproblems because it substantially increases the efficiency of the Newton–Raphson algorithmused to accelerate self-consistency convergence under non-equilibrium conditions.Compared to most existing TB schemes the SC-EDTB scheme is distinctive in tworespects. First, self-consistency is added through the exact evaluation of Hartree and linearexpansion of exchange integrals. All Hamiltonian elements belonging to the sameatom are affected by charge transfer, not just the diagonal elements. The seconddistinction is the choice of SC-EDTB parameters; they were fitted to Mullikenpopulations and eigenvalue spectra rather than energies or elastic properties. Theformer are directly related to the conductivity and potential profile, which areessential for transport simulation. No two-centre repulsive term parametrizationwas performed. The functionality of the method is exemplified by computingI–V curves, non-equilibrium potential profiles and current density for a resonant tunnellingdevice.