Revealing the electronic structure of a carbon nanotube carrying a supercurrent

TL;DR

This paper presents the first tunneling spectroscopy of Andreev Bound States in a carbon nanotube-superconductor device, revealing detailed electronic structure information and demonstrating potential for quantum and superconducting applications.

Contribution

It introduces a novel spectroscopic technique to analyze ABS in CNT devices, linking ABS spectra to electronic levels, spin, and lead coupling, with implications for quantum technology.

Findings

Resolved ABS spectra reveal electronic level energies

ABS spectra depend on gate voltage and spin orientation

Device functions as a new type of dc-measurable SQUID

Abstract

Carbon nanotubes (CNTs) are not intrinsically superconducting but they can carry a supercurrent when connected to superconducting electrodes. This supercurrent is mainly transmitted by discrete entangled electron-hole states confined to the nanotube, called Andreev Bound States (ABS). These states are a key concept in mesoscopic superconductivity as they provide a universal description of Josephson-like effects in quantum-coherent nanostructures (e.g. molecules, nanowires, magnetic or normal metallic layers) connected to superconducting leads. We report here the first tunneling spectroscopy of individually resolved ABS, in a nanotube-superconductor device. Analyzing the evolution of the ABS spectrum with a gate voltage, we show that the ABS arise from the discrete electronic levels of the molecule and that they reveal detailed information about the energies of these levels, their relative spin orientation and the coupling to the leads. Such measurements hence constitute a powerful new spectroscopic technique capable of elucidating the electronic structure of CNT-based devices, including those with well-coupled leads. This is relevant for conventional applications (e.g. superconducting or normal transistors, SQUIDs) and quantum information processing (e.g. entangled electron pairs generation, ABS-based qubits). Finally, our device is a new type of dc-measurable SQUID.