Engineering quantum states in radical molecules on superconducting Pb(111) surface

Abstract: 

We investigated properties of molecular assemblies of metal-free organic radical molecule tetrabromo-tetraazapyrene on a Pb(111) superconducting surface using a combination of scanning tunneling spectroscopy with theoretical calculations based on the numerically exact solution of the superconducting Anderson impurity model. Tunneling spectra of isolated molecules reveal the presence of a quantum phase transition from a singlet to a doublet ground state that can be induced by changing the scanning tip distance. Additionally, we show results on molecular dimers in which the energy as well as the number of in-gap, Yu-Shiba-Rusinov (YSR) states can be effectively tuned by changing the relative orientation or the  distance of the two molecules. Finally, constructing short molecular chains of odd and even length gives evidence of a periodic arrangement of charged and neutral molecules along the chains. The charge state of such chains can be manipulated by external electric fields, opening the possibility to store information in these structures. Together, we show that the different molecular assemblies can be utilized as highly tunable building blocks for superconducting molecular quantum technologies. 

C. Li, V. Pokorný, M. Žonda, J.-C. Liu, P. Zhou, O. Chahib, T. Glatzel, R. Häner, S. Decurtins, S.-X. Liu, R. Pawlak, E. Meyer, ACS Nano 19, 3403 (2025).