S. Lakshmi

Electron-electron interactions on the edge states of graphene: a many-body configuration interaction study

We have studied zigzag and armchair graphene nanoribbons (GNRs), described by the Hubbard Hamiltonian using quantum many-body configuration interaction methods. Due to finite termination, we find that the bipartite nature of the graphene lattice gets destroyed at the edges, making the ground state of the zigzag GNRs a high spin state, whereas the ground state of the armchair GNRs remains a singlet. Our calculations of charge and spin densities suggest that, although the electron density prefers to accumulate on the edges (instead of spin polarization), the up and down spins prefer to mix throughout the GNR lattice. While the many-body charge gap results in insulating behavior for both kinds of GNRs, the conduction upon application of electric field is still possible through the edge channels because of their high electron density. Analysis of optical states suggest differences in quantum efficiency of luminescence for zigzag and armchair GNRs, which can be probed by simple experiments.

Current-voltage characteristics in donor-acceptor systems: Implications of a spatially varying electric field

We have studied the transport properties of a molecular device composed of donor and acceptor moieties between two electrodes on either side. The device is considered to be one-dimensional with different on-site energies and the nonequilibrium properties are calculated using Landauers formalism. The current-voltage characteristics are found to be asymmetric with a sharp negative differential resistance (NDR) at a critical bias on one side and very small current on the other side. The NDR arises primarily due to the bias driven electronic structure change from one kind of insulating phase to another through a highly delocalized conducting phase. Our model can be considered to be the simplest to explain the experimental current-voltage characteristics observed in many molecular devices.

Effect of electron-phonon coupling on the conductance of a one-dimensional molecular wire

The effect of inelastic scattering, particularly that of the electron-phonon interactions, on the current-voltage characteristics of a one-dimensional tight-binding molecular wire has been investigated. The wire has been modeled using the Su-Schreiffer-Heeger Hamiltonian and we compute the current using the Landauers scattering formalism. Our calculations show that the presence of strong electron-lattice coupling in the wire can induce regions of negative differential resistance (NDR) in the I-V curves. The reasons for this can be traced back to the quasidegeneracy in few of the low-energy molecular levels in the presence of electron-phonon coupling and an external applied bias. The molecular levels become highly delocalized at the critical bias at which the NDR is seen, corresponding to the vanishing of the electron-phonon coupling with equal bond lengths.

Effect of electric field on one-dimensional insulators: a density matrix renormalization group study

We perform density matrix renormalization group (DMRG) calculations extensively on one-dimensional Mott and Peierls chains with explicit inclusion of the static bias to study the insulator-metal transition in those systems. We find that the electric field induces a number of insulator-metal transitions for finite-size systems and, at the thermodynamic limit, the insulating system breaks down into a completely conducting state at a critical value of bias that depends strongly on the insulating parameters. Our results indicate that the breakdown, in both the Peierls and Mott insulators, at the thermodynamic limit, does not follow the Landau-Zener mechanism. Calculations on various size systems indicate that an increase in the system size decreases the threshold bias as well as the charge gap at that bias, making the insulator-metal transition sharper in both cases.

Effects of dimerization and spin polarization on the conductance of a molecular wire

We have studied the effects of dimerization on the energy levels of a one-dimensional molecular chain attached between two electrodes. Analytic expressions for the change in energies in the presence of a small perturbing external potential have been obtained for the three limiting cases: (a) uniform, (b) partially dimerized and (c) completely dimerized chains. We find that the presence of dimerization enhances the mixing between low-lying energies in the system resulting in a situation conducive to showing negative differential resistance (NDR) in the current-voltage characteristics. The effect of spin-polarized molecule-electrode couplings on a dimerized chain has also been studied, where both spin-parallel and spin-antiparallel current show NDR behaviour. Strong dimerization however is found to destroy the spin-valve effects that are most essential for spintronic devices.

Electrostatic potential profile and nonlinear current in an interacting one-dimensional molecular wire

We consider an interacting one-dimensional molecular wire attached to two metal electrodes on either side of it. The electrostatic potential profile across the wire-electrode interface has been deduced solving the Schrodinger and Poisson equations self-consistently. Since the Poisson distribution crucially depends on charge densities, we have considered different Hamiltonian parameters to model the nano-scale wire. We find that for very weak electron correlations, the potential gradient is almost zero in the middle of the wire but are large near the chain ends. However, for strong correlations, the potential is essentially a ramp function. The nonlinear current, obtained from the scattering formalism, is found to be less with the ramp potential than for weak correlations. Some of the interesting features in current-voltage characteristics have been explained using one-electron formalism and instabilities in the system.

Negative differential resistance in a one-dimensional molecular wire with odd number of atoms

We have investigated the effects of electron-phonon coupling on the current-voltage characteristics of a one-dimensional molecular wire with odd number of atoms. The wire has been modelled using the Su-Schreiffer-Heeger (SSH) Hamiltonian and the current-voltage characteristics have been obtained using the Landauer’s formalism. In the presence of strong electron-lattice coupling, we find that there are regions of negative differential resistance (NDR) at some critical bias, due to the degeneracy in the energies of the frontier molecular orbitals. The presence of the applied bias and the electron-lattice coupling results in the delocalization of these low-lying molecular states leading to the NDR behaviour.