Tunneling spectroscopy of a single quantum dot coupled to a superconductor: From Kondo ridge to Andreev bound states

Abstract

International audienceWe performed tunneling spectroscopy of a carbon nanotube quantum dot (QD) coupled to a metallic reservoir either in the normal or in the superconducting state. We explore how the Kondo resonance, observed when the QD's occupancy is odd and the reservoir is normal, evolves towards Andreev bound states (ABS) in the superconducting state. Within this regime, the ABS spectrum observed is consistent with a quantum phase transition from a singlet to a degenerate magnetic doublet ground state, in quantitative agreement with a single-level Anderson model with superconducting leads

Nonlocal spectroscopy of Andreev bound states

“…Its slope is given by the capacitive crosstalk from the source contact. However, the reason for the conductance difference between ζ and −ζ, also observed in [4][5][6][7][8], remains unclear. One possible explanation might be a soft superconducting gap for which quasiparticle states at energies E < ∆ are available [23].…”

“…A natural experiment to measure the Andreev addition energy ζ = |E − − E ↑,↓ |, defined as the energy difference between ABS and magnetic doublet, uses a normal conducting tunnel probe (N) in a N-QD-S geometry. If the tunnel coupling between N and the QD, Γ N , is sufficiently weak, the influence of the tunnel probe on the QD-S excitation spectrum is negligible and the differential conductance across the device, G = ∂I/∂V SD , shows a peak for |eV SD | = ζ [4][5][6][7][8][9].…”

“…We interpret these sub-gap features as Andreev resonances at ±ζ. The crossing of two Andreev resonances at zero energy is associated with a quantum phase transition in which the GS of the QD changes from the |− singlet to the magnetic doublet, or vice versa [2, 7,8]. For odd occupation numbers the Coulomb repulsion, which favours the doublet GS, can prevail over the superconducting pairing, which favours the ABS as GS.…”

Subgap spectroscopy of thermally excited quasiparticles in a Nb-contacted carbon nanotube quantum dot

“…3,[5][6][7] The supercurrent is carried by Andreev bound states, whose presence is revealed by peculiar subgap features. [8][9][10][11][12] By fabricating the contacts from sputtered Nb, they can remain superconducting up to a critical temperature T c = 8.5 K and a correspondingly large critical magnetic field B c .…”

Enhancements of the Andreev conductance due to emission/absorption of bosonic quanta

“…In various experimental realizations of the quantum dots (such as self-assembled InAs islands [25], carbon nanotubes [26,27] or semiconducting nanowires [28,29]) attached to the superconducting leads the energy gap ∆ was safely smaller than the repulsion potential U . For this reason, in the subgap Andreev spectroscopy the correlations hardly contributed any Coulomb blockade.…”