Eric R. Hedin

Tight-binding approach to strain-dependent DNA electronics

Small mechanical strain perturbations are considered in calculations of the poly(G)-poly(C) DNA molecular electronic structure, using a tight-binding framework in conjunction with the theories of Slater-Koster and linear elasticity. Results reveal a strain-induced band gap for DNA which is linearly dependent on the induced strain. Local density of states calculations expose that the contribution of the guanine-cytosine base pairs in the charge transport mechanism is significantly enhanced relative to the backbones when DNA is compressed. Transport investigations also disclose a strain-induced metal-semiconductor transition for the DNA molecule, which suggests possible potential uses for sensing applications.

Enhancement of charge transport in DNA molecules induced by the next nearest-neighbor effects

An advanced two-dimensional tight-binding model including the next nearest-neighbor effects for quantum mechanical electron transport through double-stranded DNA molecules is proposed. Considering the next nearest-neighbor hopping strengths between sites gives a more rational and realistic model for the electron path-way through DNA molecules. We show higher overall transmission and enhanced current for a 30 base-pair poly(G)–poly(C) DNA molecule with the inclusion of diagonal electron hopping between the sites. In addition, an optimum condition of the contact hopping strength and Fermi energy to obtain the maximum current for the system is demonstrated. Finally, we present the current-voltage characteristics showing a transition from a semiconductor-like to a metal-like DNA molecule with the variation of the Fermi energy.

Sensitive spin-polarization effects in an Aharonov-Bohm double quantum dot ring

We study spin-dependent transport and spin polarization through two asymmetric quantum dots (QD’s) embedded in the arms of an Aharonov-Bohm (AB) ring, in which spin splitting produced by external magnetic fields is incorporated into a tight-binding model Hamiltonian. This device shows a sensitive spin-polarization effect by manipulating either in-plane or perpendicular magnetic fields. In particular, an extremely small Zeeman splitting leads to a reversal of the polarization polarity in the differential weighted spin-polarization function. Finally, we demonstrate that the degree of spin polarization is affected by the additional AB effects in the ring. In comparison, the polarization is substantially lower through a single QD in a one-dimensional system.

Electron wave interferometry through an asymmetric Aharonov–Bohm ring

A nanoscale Aharonov–Bohm (AB) ring functioning as an electron wave interferometer is investigated. The total transmission through the ring is calculated as a function of electron energy and threaded magnetic flux for a fixed upper to lower arm length asymmetry ratio. In addition, we model the effects of an asymmetry in the arm transverse widths by inserting an attractive potential well (dot) in one arm. The combined transmission resonance effects as a function of these variables will be presented. It is shown that an attractive potential in one of the arms in the AB ring generates an asymmetric Fano resonance in the transmission. Our theoretical AB-ring model is compared qualitatively to experimental results from measurements of an asymmetric ring fabricated onto a GaAs∕AlGaAs heterostructure, where Fano resonance has been observed.

Spin polarized current from multiply-coupled rings with Zeeman-split quantum dots

We investigate transmission resonances and conductance properties of multiple, serially connected, direct-contact nanoscale rings using the tight-binding model. Quantum dots (QDs) are embedded in the two arms of each ring, and Zeeman-splitting of the QD energy levels is incorporated into the system Hamiltonian. Transmission bands develop as the number of rings in series increases, with a band-gap which is sensitive to the degree of Zeeman splitting and the initial settings of the QD site energy values. The current vs. voltage characteristics of the system can be modulated between Ohmic and semiconducting as a function of the Zeeman splitting. In addition, spin-polarized current results for selected ranges of the Fermi energy.

Serially-connected Aharonov-Bohm rings with embedded quantum dots

Multiple, serially-connected nanoscale rings are analyzed using a tight-binding computational algorithm which allows calculation of the transmission and current characteristics of the system as a function of energy and external magnetic flux. Results show the role of bilateral symmetry in the system response to imposed flux, which can shift the system from metallic to semiconducting.

Optimization of Impedance Plane Reducing Coupling between Antennas

This paper provides a solution for the design optimization of two-dimensional impedance structures for a given elec-tromagnetic field distribution. These structures must provide electromagnetic compatibility between antennas located on a plane. The optimization problem is solved for a given attenuation of the complete field. Since the design optimiza-tion gives a complex law of impedance distribution with a large real part, we employ the method of pointwise synthesis for the optimization of the structure. We also consider the design optimization case where the structure has zero im-pedance on its leading and trailing edges. The method of moments is used to solve the integral equations and the nu-merical solution is presented. The calculated impedance distribution provides the required level of antenna decoupling. The designs are based on the concept of soft and hard surfaces in electromagnetics.

How a Fano Resonance Crosses the Mobility Edge in Quantum Waveguides

Abstractnew scenario for the occurrence of a Fano resonance in the transmission probability of electron waveguides is investigated using a coupled-channel theory. Both a quantum dot and an antidot with either short- or finite-range interaction are embedded in the electron waveguide. Particularly, when the Fano resonance occurs close to the mobility edge (channel threshold), it is shown that Γ~U124/3, where Γ is the resonance width and U12 is the coupling strength between bound state and continuum. This is in contrast to the usual result Γ ~U122, which is valid when the resonance occurs far from the mobility edge. Furthermore, it is shown that increasing the size of both dot and antidot leads to larger resonance width.

Coupled nano-rings: strain and magnetic field effects

Transmission properties of a linear chain of nanorings with 6 quantum dots (QD) per ring are investigated with and without magnetic field effects. The rings are connected serially by a linear segment. A tightbinding Hamiltonian is solved exactly, giving the transmission for any number of rings in series. The Aharonov-Bohm effect shifts and splits the transmission band structure and the Zeeman effect splits the transmission into spin-polarized bands. Multiple system parameters, including coupling integrals and ring number, are varied and shown to have significant effects on the transmission spectrum. The system and results provide generalized conduction properties of a 1-ring wide graphene nanoribbon in the armchair configuration.

The effects of molecular elongation on defective DNA electronics

A defective double stranded poly(dG)-poly(dC) DNA molecule under axial mechanical strain is analyzed using a tight-binding computational model which allows calculation of the transmission and current characteristics of the system as a function of electron energy. Results show the existence of highly sensitive electron transmission behavior with respect to the on-site energy perturbations.