Effects of anisotropy and Coulomb interactions on quantum transport in a quadruple quantum-dot structure

We present an analytical and numerical investigation of the spectral and transport properties of a quadruple quantum-dot (QQD) structure which is one of the popular low-dimensional systems in the context of fundamental quantum physics study, future electronic applications, and quantum calculations. The density of states, occupation numbers, and conductance of the structure were analyzed using the nonequilibrium Green's functions in the tight-binding approach and the equation-of-motion method. In particular the anisotropy of hopping integrals and on-site electron energies as well as the effects of the finite intra- and interdot Coulomb interactions were investigated. It was found out that the anisotropy of the kinetic processes in the system leads to the Fano-Feshbach asymmetrical peak. We demonstrated that the conductance of the QQD device has a wide insulating band with steep edges separating triple-peak structures if the intradot Coulomb interactions are taken into account. The interdot Coulomb correlations between the central QDs result in the broadening of this band and the occurrence of an additional band with low conductance due to the Fano antiresonances. It was shown that in this case the conductance of the anisotropic QQD device can be dramatically changed by tuning the anisotropy of on-site electron energies.

Figure 1

The QQD structure between metallic leads.

Figure 2

The conductance and TDOS of the isotropic QQD: (a) $t_0=\mathrm{\Delta }=0$; (b) $t_0=1,\mathrm{\Delta }=0$; (c) $t_0=0,\mathrm{\Delta }=1$; (d) $t_0=1,\mathrm{\Delta }=1$.

Figure 3

The conductance and TDOS of the anisotropic QQD, $t_2=0.1$: (a) $t_0=\mathrm{\Delta }=0$; (b) $t_0=0.2,\mathrm{\Delta }=0$; (c) $t_0=0,\mathrm{\Delta }=\pm 1$; (d) $t_0=0.2,\mathrm{\Delta }=\pm 1$; (e) $t_0=0,\mathrm{\Delta }=\pm 0.2$; (f) $t_0=0.2,\mathrm{\Delta }=\pm 0.2$.

Figure 4

The temperature effects on the conductance of the anisotropic QQD structure $t_2=0.1, t_0=0.2, \mathrm{\Delta }=-1$.

Figure 5

The influence of the Coulomb interactions in the QQD on the conductance: (a) the effect of the intradot interactions, $U$; (b) the effect of the interdot interaction, $V$; $k_{B}T=0.01$.

Figure 6

The conductance of the isotropic QQD, $k_{B}T=0.01$: (a) $t_0=\mathrm{\Delta }=0$; (b) $t_0=1,\mathrm{\Delta }=0$; (c) $t_0=0,\mathrm{\Delta }=1$; (d) $t_0=1,\mathrm{\Delta }=1$.

Figure 7

The gate-field dependence of occupation numbers of the isotropic QQD, $t_0=\mathrm{\Delta }=0, k_{B}T=0.01$: (a) $U=V=0$; (b) $U=5,V=0$; (c) $U=5,V=1$.

Figure 8

The conductance of the anisotropic QQD, $t_2=0.1, k_{B}T=0.01$: (a) $t_0=\mathrm{\Delta }=0$; (b) $t_0=0.2,\mathrm{\Delta }=0$; (c) $t_0=0,\mathrm{\Delta }=1$; (d) $t_0=0.2,\mathrm{\Delta }=1$.