We have attained an experimental realization of the two-impurity Kondo system, allowing gate-tunable suppression of the Kondo effect by a non-local RKKY-like interaction. The spin "impurities" consist of two quantum dots coupled through an open conducting region. We have observed splitting and suppression of the zero-bias Kondo resonance as the dots are coupled, allowing the strength of the effective exchange interaction to be extracted. These results suggest a novel method of non-local spin control potentially relevant to quantum information processing.
FIG. 1 (a) Scanning electron micrograph of a device identical in design to one measured, with schematic ovals indicating locations of dots upon gate depletion. Gate voltages VgL and VgR change the energies and occupancies of the left and right dots; Vgc tunes the coupling of the right dot to the central region. (b) Differential conductance dI/dVL of the left dot for an odd number of electrons, N. When the right dot contains even number of electrons, (M±1), a zero-bias peak in dI/dVL is seen, indicating a Kondo state. When the right dot contains an odd number (M) of electrons, the Kondo state in the left dot is suppressed. The states M-1, M, M+1 for the right dot are consecutive Coulomb blockade valleys.
FIG. 2 (a) Differential conductance dI/dVR of the right dot as a function of both VgR and VR shows a zero bias feature for odd occupancy, M. Here, the left dot contains an even number (N-1) of electrons. (b) Slices taken mid-valley from (a) show a zero-bias peak only for odd occupancy M of the right dot. (c) dI/dVR of the right dot as a function of both VgR and VR, now with an odd number (N) of electrons in the left dot. Suppression of the zero-bias peak in the middle valley is evident. (d) Slices taken mid-valley from (c) show the suppression of the zero-bias peak for the odd-odd (two-impurity) case.
FIG. 3 (a) Differential conductance through the left dot for various values of the coupling between the right dot and the center region. The left dot and right dot both contain odd numbers of electrons (N and M, respectively). For strong couplings, the zero-bias resonance in the left dot is fully suppressed; suppression decreases as the coupling is decreased, so that the zero-bias resonance is fully evident for weak coupling. Notice the splitting of suppressed peaks, which is consistent across a range of couplings. (b) Differential conductance through the left dot for various couplings between the right dot and center region, with an even number of electrons (M-1) in the right dot. Traces exhibit a strong zero-bias resonance across all values of the coupling.
For more information contact Nathaniel Craig (ncraig@fas)