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A nanowire connected to two electrodes and cooled down to very low temperature (T = 25 mK) forms a Quantum Dot where the number of electrons can be controlled precisely with the gate voltage applied on the substrate, panel b. When an odd number of electrons are present in the Quantum Dot, it behaves as a paramagnetic impurity.
When the two electrodes are superconducting, this leads to the formation of a superconducting quantum dot constituted of a discrete spectrum of Andreev bound states.
When an unpaired electron (paramagnetic impurity) is present in this quantum dot, the consequences on the spectrum of Andreev bound states depends on the intensity of coupling between the electronic states of the Quantum Dot and the superconducting electrodes, as shown by the theoretical phase diagram, panel d. In a doped semiconducting nanowire, the spatial spread of electronic wave functions and so the coupling to the electrodes depends on the energy location of the electronic states with respect to the mobility edge.
In this experiment, we have studied the evolution of the Andreev spectrum and the formation of the Shiba states as the Fermi level is tuned across the mobility edge with a gate voltage. The spectrum displays a superconducting gap at strong coupling (panel a) while it displays a zero-energy Shiba state at weak coupling (panel c).
This work highlights the role of electron localization in the formation of Shiba states, which is of interest for the fabrication of future electronic components based on hybrid superconducting-semiconducting structures.
Ref:
A. Assouline, C. Feuillet-Palma, A. Zimmers, H. Aubin, M. Aprili, and J.-C. Harmand
Phys. Rev. Lett. 119, 097701 (2017).
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