Engineering quantum states in radical molecules on superconducting surfaces

A study published in ACS Nano, explores the manipulation of spinful, radical molecules on superconducting surfaces, revealing interesting insights into quantum spin behavior and charge rearrangement in various molecular assemblies ranging from single molecules to pentamer chains. This research opens new avenues for applications in superconducting molecular spintronics.

Description

Differential conductance measured on a single TBTAP molecule using tunneling spectroscopy (left) and the corresponding density of states obtained using deconvolution technique (center) revealing a pair of YSR states that cross at the center, marking a quantum phase transition. The results of the theoretical model (right) are in agreement with the experiment and provide deep insight into the properties of molecules on superconducting surfaces.

Key Findings

1. Molecular Manipulation and YSR spectroscopy: The experimental team employed high-precision techniques to manipulate the relative position and distance between molecules on Pb(111) superconducting surface, specifically focusing on tetrabromo-tetraazapyrene (TBTAP) molecules. This manipulation allowed for precise control of the quantum state of the system. The study observed multiple Yu-Shiba-Rusinov (YSR) peaks in tunneling spectroscopy, which can be manipulated with varying distances between the molecules and their relative orientation.
2. Quantum Phase Transition (QPT): By adjusting the distance between the scanning tunneling microscope (STM) tip and the single TBTAP molecule, the researchers induced a QPT from a singlet to a doublet ground state, indicated by a crossing of the YSR peaks. The experiment was described using advanced theoretical methods including the numerical renormalization group (NRG), providing deep insight which was crucial for understanding the behavior of more complex molecular assemblies.
3. Magnetic coupling in molecular dimers: Intermolecular coupling, either direct or via the superconducting substrate, leads to complex behavior of the YSR states that deviate from classical spin models and can be tuned by intermolecular distance and relative orientation. This behavior was again well described using theoretical models, revealing the weak ferromagnetic coupling between the spins, which allows us to understand the behavior of longer chains.
4. Charge Redistribution and Information Encoding: The researchers demonstrated that intermolecular coupling in longer molecular chains leads to a periodic pattern of charged or neutral molecules, with edge molecules consistently hosting charge/spin, showcasing the effects of electrostatic coupling. This finding is pivotal for possible applications in molecular electronics. Notably, tetramers chains could be switched repetitively and non-destructively between two equivalent states using the external field induced by STM tip, indicating potential for using these structures as information units in superconducting molecular circuits.

Implications

This research not only enhances our understanding of electron spin interactions on superconducting surfaces but also paves the way for innovative applications in quantum technologies and molecular electronics. The ability to manipulate molecular states at such a precise level could lead to advancements in data storage and processing technologies that leverage quantum properties. By bridging molecular manipulation with quantum state control, this work lays a foundation for future explorations into spin-based technologies and their applications in next-generation electronic devices.

Contact person: Vladislav Pokorný