Andreev bound states in a superconductor coupled CNT-quantum dot

Recent spectroscopy of individual Andreev bound states (ABS) in superconductors coupled nanostructures devices received a huge amount of experimental/theoretical interest and opened a new area of research to address the fundamental quantum effects in electronic devices. In this talk, I will begin with an overview of discrete charge transport through quantum dot, quantum phenomena at superconductor/normal conductor interfaces and then discuss our recent low temperature (30 mK) experimental results on the observation of individual ABS in a single wall carbon nanotube (CNT) quantum dot device coupled to superconductors [1]. The CNT is strongly coupled to niobium superconducting leads at two ends and weakly coupled to a tunnel probe (aluminium) in the middle, serves as a tunnel probe Gate dependent differential conductance measurements show the formation of individual ABS inside the superconducting gap. By virtue of the larger superconducting gap of the Niobium, we observe more than one pair of ABS. In addition, a weak replica of the ABS next to the main resonance lines but at lower bias voltage is also observed, which is generated by higher order Andreev reflection from the bound states to the tunnel probe. Gate tunable ABS display a level crossing of two bound states, which is understood as a 0-1 quantum phase transition of the quantum dot Josephson junction. Finally, I will discuss temperature dependent measurements, reveal the thermal occupation of the upper bound state level and show an opposite curvature at the centre of a Coulomb blocked valley at higher temperatures (> 400mK).