Quantum Phase Transition of a Molecular Radical Pair

Quantum phase transitions lie at the heart of condensed matter physics. Observing these transitions, however, requires the careful engineering of materials to realize specific, finely tuned systems. In this work, we demonstrate a quantum phase transition within artificial molecular spin pairs, where the ground state can be controllably switched between the antiferromagnetic phase and ferromagnetic phase by an external magnetic field. The combination of scanning tunneling microscopy experiments and theoretical calculations not only reveals the microscopic details of this phase transition but also establishes that its critical behavior is tunable through precise modulation of the intermolecular interactions. These results establish a practical platform for exploring quantum phase transitions in molecular systems.