Ph.D. Thesis Defense: Manipulating Superconductivity at the Nanoscale through Magnetism and Proximity Effects

CIC nanoGUNE Seminars

Stefano Trivini
Pre-doctoral Researcher, Nanoimaging Group
Carlos Santamaria Center Auditorium
José Ignacio Pascual
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Ph.D. Thesis Defense: Manipulating Superconductivity at the Nanoscale through Magnetism and Proximity Effects

Superconductivity is an intriguing phenomenon that crosses many fields of science with still very high interest in fundamental understanding and technology applications. Electron-electron interactions and correlations are a basic interest of condensed matter physics, in fact, they describe many phenomena including magnetism and superconductivity. In this thesis, with Ultra-High-Vacuum material synthesis we couple normal metals, superconductors, 2D materials, magnetic atoms, and molecules to study by Scanning Tunneling Microscopy the interplay of different properties transferred by proximity at the atomic scale described within various theoretical frameworks.

In the first part, we build 2D diluted atomic lattices of Mn on the ?-Bi2Pd superconductor by means of scanning tunneling microscope (STM) atomic manipulation. The anisotropic Fermi surface of ?-Bi2Pd implies a strong dependence of the coupling on the orientation of the built structures, affecting the Yu-Shiba-Rusinov resonance and coupling.

In the second part we show how tunneling a single electron can excite a pair-breaking excitation in a proximitized gold film in the presence of magnetic impurities. Combining STM with theoretical modeling, we map the excitation spectrum of an Fe-porphyrin molecule on the proximitized surface into a manifold of entangled Yu-Shiba-Rusinov and spin excitations.

Then, in the third part, we study Pb islands on SiC graphene, that induce superconductivity by the proximity effect. These islands can slide on the surface of graphene, pushed by the STM tip without being damaged critically. This manipulation tool allows the building of superconducting nanostructures that can confine superconductivity in graphene in virtually infinite geometries.

Finally, in the fourth part, we study small Pb islands on graphene, that shows a competition between conventional BCS superconducting gap and a Coulomb blockade gap. This induce a characteristic asymmetry of the coherence peaks, that is switchable by island manipulation on the surface. We correlate the asymmetry with the presence of an excess charge on the islands.