Fano-Kondo Interplay in a Side-Coupled Double Quantum Dot

We investigate low-temperature transport characteristics of a side-coupled double quantum dot where only one of the dots is directly connected to the leads. We observe Fano resonances, which arise from interference between discrete levels in one dot and the Kondo effect, or cotunneling in general, in the other dot, playing the role of a continuum. The Kondo resonance is partially suppressed by destructive Fano interference, reflecting novel Fano-Kondo competition. We also present a theoretical calculation based on the tight-binding model with the slave boson mean field approximation, which qualitatively reproduces the experimental findings.

Figure 1

Color-scale plot of the observed conductance as functions of the plunger gate voltages, $V_1$ and $V_2$, at 41 mK. Blue (dark gray) corresponds to zero conductance, white to $20\mu \mathrm{S}$, and red (light gray) to $40\mu \mathrm{S}$. A Kondo valley for QD1 is marked with a triangle. Solid and dotted lines denote charge state transitions on QD1 and QD2, respectively. The upper inset shows a scanning electron micrograph of the DQD device together with a schematic of the measurement setup. Current, $I$, is measured across QD1. The lower inset shows a schematic of side-coupled DQD geometry.

Figure 2

(a) The observed mid-valley conductance profile across resonance $\alpha$ in the non-Kondo valley (dots) fitted with the Fano theory (solid line). The dashed line denotes the nonresonant channel conductance, $G_N^{\alpha }$, and the peak amplitude measured from the dashed line is the resonant channel conductance, $G_R^{\alpha }$. (b) Plot similar to (a) for resonance $\beta$ in the Kondo valley. (c) Temperature dependence of $G_N^{\beta }$, $G_R^{\alpha }$, and $G_R^{\beta }$. (d) $G_R^{\alpha }$ and $G_R^{\beta }$ plotted as a function of inverse temperature.

Figure 3

(a) The tight-binding model for the side-coupled DQD. (b) Color-scale plot of the calculated conductance as functions of $E_F-{\epsilon }_1$ and $E_F-{\epsilon }_2$. $\alpha$ and $\beta$ correspond to the same symbols in Fig. 1. (c) Calculated conductance profile at $E_F-{\epsilon }_1=6$. (d) Calculated conductance profile at $E_F-{\epsilon }_1=2$. Since $\Gamma$ is energy dependent, its value far from QD2 resonance is used for normalization.

Figure 4

(a) Color-scale plot of the observed conductance as functions of $V_1$ and $V_2$ at $B=0.15\mathrm{T}$. Color scale is same as in Fig. 1. (b) Mid-valley conductance profiles across resonance $\alpha$ (dots) fitted with the Fano theory (solid line) for various magnetic fields. (c) Magnetic field dependence of $q$ and $G_N^{\alpha }$. (d) A Kondo zero-bias peak at $B=0.15\mathrm{T}$ near the resonance minima denoted by an arrow in (b). The vertical axis is common with (b).