Yu-Shiba-Rusinov states in superconductor-quantum dot transistors made by electromigration

Resumen

A magnetic impurity embedded in a superconductor (SC) can have a disruptive effect on superconductivity, locally reducing the pairing energy of Cooper pairs and leading to the generation of Yu-Shiba-Rusinov (YSR) sub-gap states around it. The exchange interaction between the spin at the impurity and the electrons at the medium favors a Kondo-screening singlet configuration, while superconducting correlations oppose to the formation of a screening cloud, leaving the impurity unscreened in a doublet state. The local ground state (GS) of the system is then determined by the complex interplay between both phenomena, giving rise to intriguing properties that have been subject of great interest over the last decade.Hybrid devices contacting quantum-dot (QD) structures to superconducting leads offer an ideal platform for the study of such competition, as QDs can also display a magnetic character for odd occupancies. In this thesis, we investigate the transport properties of colloidal QDs (metallic nanoparticles and fullerenes) individually contacted to bare aluminum leads through tunnel barriers made by electromigration, in a transistor configuration. The combination of a strongly discretized density of states in colloidal QDs with the sharp gap present in superconducting aluminum allows to measure the YSR spectra, appearing in SC-QD-SC junctions displaying a large asymmetry in the tunnel coupling, with an unprecedented resolution. The response of the SC-QD system against gate voltage, external magnetic field and temperature has been characterized, identifying the transition point (QPT) from the singlet-GS phase to the doublet-GS. A striking upturn of the QPT gate position is revealed at low magnetic fields, attributed to the Zeeman-splitting of the doublet state.