Yong S. Joe

Classical analogy of Fano resonances

We present an analogy of Fano resonances in quantum interference to classical resonances in the harmonic oscillator system. It has a manifestation as a coupled behaviour of two effective oscillators associated with propagating and evanescent waves. We illustrate this point by considering a classical system of two coupled oscillators and interfering electron waves in a quasi-one-dimensional narrow constriction with a quantum dot. Our approach provides a novel insight into Fano resonance physics and provides a helpful view in teaching Fano resonances.

Characteristics of transmission resonance in a quantum-dot superlattice

We investigate phase-coherent electron transport through height-varying potential barriers in a quantum-dot superlattice. Due to the aspect ratio variations of two alternating potential heights in the quantum channel, well-arranged resonant peaks in the first miniband of each plateau are divided into the paired peaks of two groups, which produce an extra gap inside each miniband. In addition, for a five barrier case, the second and third resonant peaks in the miniband are no longer distinguishable at a critical aspect ratio, and the amplitude of this degenerate peak becomes smaller than one and eventually approaches zero. The mean lifetimes of the resonant peaks whose amplitudes remain unity are studied. We also examine the resonant tunneling with under-unity transmission in the one-dimensional superlattice system with alternating potential barriers. Finally, it is found that the “quasi-resonance” appears in a quantum-dot superlattice with 13 barriers consisting of 2 alternating potential heights.

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.

Anti-resonance in a one-dimensional chain of driven coupled oscillators

We investigate a driven system of N one-dimensional coupled oscillators with identical masses. The first mass is connected to a sinusoidal driving force of frequency ω. In the steady state, when all the masses perform simple harmonic motion, we analytically obtain the dependence of their amplitudes on ω and show that there are resonance and anti-resonance frequencies. At an anti-resonance frequency, the amplitude of one of the masses becomes exactly zero. The mass directly connected to the driving force has the largest number of anti-resonance frequencies, N – 1. The phase of each masss motion is either 0 or π with respect to the driving force. The case where damping forces are present is also considered, and the amplitude dependence on driving frequency is analytically obtained. In the presence of damping, there is no anti-resonance.

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.

Collapse of resonance in quasi-one-dimensional quantum channels

We study the resonance structure of the conductance (transmissivity) of a quasi-one-dimensional channel that contains an attractive impurity of finite dimensions and derive an exact expression for the scattering matrix. We show that an impurity of finite dimensions may cause a set of Fano resonances to appear in the transmissivity. We also find that due to the coherent interaction the Fano resonances can collapse and discrete levels may appear in the continuum. Finally, we establish the wave function of the discrete levels and study the channel transmissivity in the critical regime.

Radiation characteristics of waveguide antennas located on the same impedance plane

The radiation characteristics of waveguide antennas located on the same impedance plane are theoretically and experimentally investigated. In order to study the influence of surface impedance, we calculate the key parameters of antennas, such as the coefficient of standing waves, radiation patterns and decoupling level between antennas using the moment method. Since a decoupling level for a fixed geometry reaches a saturation point when the reactivity of normalized impedance becomes capacitive impedance, we incorporate a corrugated structure on a conducting surface with dielectric materials. The radiation patterns in this system are presented for both an ideal conducting flange and capacitive impedance. Finally, we perform an experiment on the frequency dependence of the decoupling level by preparing a sample decoupling device with constant depths of corrugation and compare numerical and experimental results.

Resonance characteristics through double quantum dots embedded in series in an Aharonov-Bohm ring

Stimulated by recent intriguing experiments with a quantum dot in an Aharonov–Bohm (AB) ring, we investigate novel resonant phenomena by studying the total transmission probability of nanoscale AB rings with embedded double quantum dots in one arm and a magnetic flux passing through its centre. In this system, we show the overlapping and merging of both Breit-Wiger (BW) and Fano resonances as the interaction parameter between the dots changes. In the strong overlapping regime of Fano resonances, the transmission zeros leave the real-energy axis and move away in opposite directions in the complex-energy plane. The simultaneous swings (from Fano to BW and then back to Fano resonance) of a pair of Fano resonances in the overlapping regime are observed by modulating the magnetic flux threading the AB ring. The periodic tuning of the Fano resonance for a fixed interaction parameter is also discussed as the magnetic flux increases.