Resonant and Inelastic Andreev Tunneling Observed on a Carbon Nanotube Quantum Dot

Abstract

We report the observation of two fundamental subgap transport processes through a quantum dot (QD) with a superconducting contact. The device consists of a carbon nanotube contacted by a Nb superconducting and a normal metal contact. First, we find a single resonance with position, shape, and amplitude consistent with the theoretically predicted resonant Andreev tunneling (AT) through a single QD level. Second, we observe a series of discrete replicas of resonant AT at a separation of ~145 μeV, with a gate, bias, and temperature dependence characteristic for boson-assisted, inelastic AT, in which energy is exchanged between a bosonic bath and the electrons. The magnetic field dependence of the replica's amplitudes and energies suggest that two different bosons couple to the tunnel process.

Charge-vibration interaction effects in normal-superconductor quantum dots

“…Finally, we remark that experiments with U as in Refs. [145,146,170] focused on devices with large asymmetry S N the normal lead basically acts as a spectroscopic probe of the Andreev spectra. In some ranges of parameters, the experimental results have been qualitatively explained in terms of a noninteracting model, viz.…”

Inelastic Kondo-Andreev tunneling in a vibrating quantum dot

“…In recent years, phonon-assisted inelastic AT in an N-QD-S system also leads to interesting physics on, for example, the electronic transport [25][26][27] , the heat generation 28 , the ground-state cooling 29 , the steadystate shot noise 30 , as well as the transient dynamics under a step bias 31 . More interestingly, the phonon-assisted AT can lead to resonant peaks every time the bias voltage changes by one phonon energy or the gate voltage changes by half a phonon energy 32,33 , which has been unambiguously observed in a recent experiment 34 . This is somewhat reminiscent of normal systems where phonon sidebands of Kondo cotunnelings [35][36][37][38][39][40] and single-electron tunnelings [41][42][43] are also separated by one phonon energy in the bias voltage.…”

“…Conclusions.-We have predicted in N-QD-S systems a series of differential conductance subpeaks developed at V = nε ph /2 and resulting from phonon-assisted inelastic Kondo-Andreev cotunnelings. These structure are truly remarkable when compared with the transport characteristics of i) the conventional inelastic AT in N-QD-S systems [32][33][34] and ii) the inelastic Kondo cotunneling in the N-QD-N systems [35][36][37][38][39][40] . Our prediction might be observed in the carbon nanotube device fabricated by J. Gramich et al 34 as long as the Kondo regime is achieved at low temperatures.…”

“…Our investigation reveals that the interplay of the Kondo correlations, the superconductivity, and the mechanical vibrations of the QD gives rise to distinct transport characteristics, as compared with those arising from the conventional phonon-assisted ATs [32][33][34] . The main physical scenario is illustrated in Fig.…”

“…This is somewhat reminiscent of normal systems where phonon sidebands of Kondo cotunnelings [35][36][37][38][39][40] and single-electron tunnelings [41][42][43] are also separated by one phonon energy in the bias voltage. Since the N-QD-S setup fabricated in the experiment 34 is indeed an ideal platform to explore the Kondo physics, it is our aim in this paper to provide a theoretical study of the Kondo transport in such a device.…”