Quantum Noise in Carbon Nanotubes as a Probe of Correlations in the Kondo Regime

Most of the time, electronic excitations in mesoscopic conductors are well described, around equilibrium, by non-interacting Landau quasi-particles. This allows a good understanding of the transport properties in the linear regime. However, the role of interaction in the non-equilibrium properties beyond this regime has still to be established. A paradigmatic example is the Kondo many-body state, which can be realized in a carbon nanotube (CNT) quantum dot for temperatures below the Kondo temperature T_K . As CNT possesses spin and orbital quantum numbers, it is possible to investigate the twofold degenerate SU(2) Kondo effect as well as the fourfold degenerate SU(4) state by tuning the degeneracies and filling factor. Our article aims at providing a comprehensive review on our recent works on the Kondo correlations probed by quantum noise measurement at both low and high frequencies. At low frequency, combining transport and current noise measurements in such a dot, we have identified the SU(2) and SU(4) Kondo states. Our experiment shows that a two-particle scattering process due to residual interaction emerges in the non-equilibrium regime. The effective charge e^* , which characterizes this peculiar scattering, is determined to be e^*/e =1.7 ± 0.1 for SU(2) and e^*/e =1.45 ± 0.1 for SU(4), in perfect agreement with theory. This result demonstrates that current noise can detect unambiguously the many-particle scattering induced by the residual interaction and the symmetry of the ground state. In addition, we have measured the high-frequency emission noise of a similar carbon nanotube QD in the Kondo regime, at frequencies of the order of k_B T_K/h . At the lowest measured frequencies, the derivative of the noise exhibits an expected Kondo peak. However, this peak is strongly suppressed at higher frequency, pointing towards the existence of a high-frequency cut-off of the electronic emission noise at a Kondo resonance. This leads us to postulate that a new timescale, related to the Kondo energy k_B T_K , emerges besides the timescale associated with the transport of electrons through the dot, given by the coupling to the reservoirs. The goal of this overview article on our recent research is to demonstrate how current noise measurements yield new insight on interaction effects and dynamics of a Kondo correlated state.