Tunable Fano effect in parallel-coupled double quantum dot system

With the help of the Green function technique and the equation of motion approach, the electronic transport through a parallel-coupled double quantum dot (DQD) is theoretically studied. Owing to the interdot coupling, the bonding and antibonding states of the artificial quantum-dot molecule may constitute an appropriate basis set. Based on this picture, the Fano interference in the conductance spectra of the DQD system is readily explained. The possibility of manipulating the Fano line shape in the tunneling spectra of the DQD system is explored by tuning the dot-lead coupling, the interdot coupling, the magnetic flux threading the ring connecting dots and leads, and the flux difference between two subrings. It has been found that by making use of various tunings, the direction of the asymmetric tail of Fano line shape may be flipped by external fields and the continuous conductance spectra may be magnetically manipulated with the line shape retained. More importantly, by adjusting the magnetic flux, the function of two molecular states can be exchanged, giving rise to a swap effect, which might play a role as a qubit in the quantum computation.

Figure 1

(a) Schematic diagram for a tunneling-coupled parallel DQD system. (b) Schematic diagram of the parallel-coupled DQD system in the molecular orbital representation.

Figure 2

Three structures investigated, in which the solid and dashed lines stand for the stronger and weaker tunnel couplings, respectively.

Figure 3

(Color online) The density of states for two molecular states in structures 1 and 2 as shown in Fig. 2, where $t_c=\gamma$ which is taken as an energy scale.

Figure 4

(Color online) Conductance spectra in structures 1(a) and 2(b). The parameters for calculations are $\theta =0$, $\varphi =0$, ${\epsilon }_0=0$, and $t_c=\gamma$. Notice that the conductance spectra in two structures have quite different interference patterns though the identical DOS as in Fig. 3.

Figure 5

(Color online) (a) Calculated conductance in structure 2 for three different values of the interdot coupling $t_c$, where the Fermi energy is roughly at the antibonding level. (b), (c), and (d) Simulation of the normalized Fano line shape with different asymmetric factors $q$ as compared to the results obtained by Green functions in (a).

Figure 6

(a) Structure 1 and (b) structure 2 can be cast into two different models of molecular states. The flux through the loop of structure 1 differs from that of structure 2 by a phase of $\pi$.

Figure 7

The evolution of transmission spectrum with total magnetic flux $\varphi$ tuned for three asymmetric configurations.

Figure 8

(Color online) The evolution of transmission spectrum with phase difference $\theta$ in three asymmetric configurations.

Figure 9

Illustration for two different periods of conductance. Multipathway interference as functions of (a) the total flux $\varphi$ and (b) the flux difference $\theta$.

Figure 10

Continuously modulated conductance by the magnetic flux. (a) The conductance is tuned by $\theta$, when $\varphi$ is fixed at $(2n+1)\pi$. (b) The conductance is tuned by $\varphi$, when $\theta =(2n+1)\pi ∕2$.