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Séminaire matériaux 2D et confinement de l’INSP

The Kondo effect in carbon nanotube quantum dots : universal quantum fluctuations and competition with superconducting proximity effect - Richard Deblock - Jeudi 17 octobre à 11 h

INSP - Sorbonne Université - 4 place Jussieu - 75005 Paris - Barre 22-32, 2e étage, salle 201

Richard Deblock - Laboratoire de Physique des Solides, Université Paris-sud, Université Paris Saclay, Orsay

Abstract

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 and corresponds to the screening of the magnetic moment of the dot by the conduction electron of the reservoirs, below the Kondo temperature TK. As CNT possess spin and orbital quantum numbers, it is possible to investigate the usual twofold degenerate SU(2) Kondo effect as well as the four fold degenerate SU(4) state by tuning the degeneracies and filling factor.

We have probed this system by combining transport and current noise measurements. Our experiment shows that, a two-particle scattering process due to residual interaction emerges in the non-equilibrium regime [1]. This leads to an effective charge e*, which characterizes this peculiar scattering, of e*/e =1.7 for SU(2) and e*/e =1.45 for SU(4), in perfect agreement with theory. We have also probed the dynamics of this correlated state by high frequency current noise measurement and show the existence of a high frequency cut-off, related to the Kondo energy kBTK, of the electronic emission noise at a Kondo resonance [2].

When the CNT dot is connected to superconducting reservoirs, there is a competition between the superconducting proximity effect and the Kondo effect. This leads to a quantum transition which can be controlled by the filling factor but also by the superconducting phase difference. This transition has been probed in CNT based Josephson junctions thanks to current-phase measurement [3].

[1] Ferrier et al, Nat. Phys. 12, 230-235 (2016) ; Phys. Rev. Lett. 118, 196803 (2017).

[2] Delagrange et al, Phys. Rev. B 97, 041412 (2018).