Tunable Kondo Physics in a Carbon Nanotube Double Quantum Dot

We investigate a tunable two-impurity Kondo system in a strongly correlated carbon nanotube double quantum dot, accessing the full range of charge regimes. In the regime where both dots contain an unpaired electron, the system approaches the two-impurity Kondo model. At zero magnetic field the interdot coupling disrupts the Kondo physics and a local singlet state arises, but we are able to tune the crossover to a Kondo screened phase by application of a magnetic field. All results show good agreement with a numerical renormalization group study of the device.

Figure 1

(a) Schematic of the carbon nanotube DQD device. (b) Stability diagram of the DQD measured at $T\sim 60\mathrm{mK}$. The Si backgate voltage $V_{\mathrm{bg}}=-1.7\mathrm{V}$. The barrier gate voltage $V_{\mathrm{bar}}=0\mathrm{mV}$. (c) Differential conductance in the centers of the various charge regimes of panel (b) as indicated by ($n$, $m$). The curves in the rightmost panel are offset in steps of 0.03 $e^2/h$. (d) Differential conductance along the lines indicated in panel (a).

Figure 2

(a) NRG calculation of the stability diagram. (b) Differential conductance in the centers of the various charge regimes of panel (a) as indicated by ($n$, $m$). (c) Differential conductance along the lines indicated in panel (a).

Figure 3

(a) Magnetic field dependence of the differential conductance in the (1,1) charge regime for various values of the detuning as indicated by the black dots along line 3 in Fig. 1b, with the left plot for the center of the (1,1) regime. (b) NRG results corresponding to the left and right panels in (a). Dashed lines show the threefold splitting of one zero-field peak. (c) Schematic of crossover between LS and KS phases, see text, either by varying $B$ (solid arrowed line) or $J$ (dashed arrowed line). (d) Zero-bias conductance vs $B$ for different detuning, along dashed white lines in panel (a); both measured (left) and calculated (right). Individual curves from left to right correspond to (a) from left to right.

Figure 4

(a) Observed stability diagrams of the DQD for different dot-lead coupling strengths as varied by $V_{\mathrm{bar}}$. (b) Left: differential conductance in the center of the (1,1) charge regime for different dot-lead couplings. The curves are offset in steps of 0.05 $e^2/h$. Right: NRG calculations, with the ratio ${\Gamma }_R/{\Gamma }_L=2$ fixed and ${\Gamma }_R$ as indicated (curves again offset in steps of 0.05 $e^2/h$).