Probing spins in molecular structures on superconducting surfaces

A quantum computer is considered to overtake classical computers from its ability of parallel information processing. However, qubits fabricated from quantum states are easily perturbed by their environment, leading to short decoherence time and limited gate operations. To extend decoherence time, qubits based on topological superconductivity are promising candidates thanks to their immunity against external perturbations. This dissertation aims on the fundamental understanding of condensed matter systems to realize topological superconductivity by targeting the observation of Majorana zero modes using low-temperature scanning probe microscopy. To accomplish this goal, we fabricate spin lattices via on-surface reactions on superconducting substrates (Nb and Pb). We first demonstrate the synthesis of atomically precise nanographenes on the superconducting Ag/Nb(110), and characterize their structures with atomic force microscopy at low temperatures. Our approach opens the route to couple π-magnetism to superconducting states. We then compare the spin signature of Fe atoms in the coordinated organometallic frameworks on Pb(111) and on Ag(111). On both substrates, we observe spin-flip excitations due to magnetocrystalline anisotropy. We last investigate an electron-spin superlattice fabricated by self-assembly of radical molecules on Pb(111). Using superconducting tips, we probe by tunneling spectroscopy Yu-Shiba-Rusinov states arising from the coupling of such spin-1/2 system with the superconductor. The observation of low-energy modes near boundaries of this two-dimensional magnetic island is consistent with the signature of Majorana zero modes.