Uncovering and Engineering Mixed-Valence States in Blatter-Type Radicals on Au(111).

The interplay between localized magnetic moments and conduction electrons gives rise to a rich quantum phase diagram that underpins a wide range of exotic quantum phenomena. Within this framework, the mixed-valence regime where charge and spin fluctuations are entangled represents a crucial frontier for discovering novel quantum phenomena. However, such behavior has so far been largely confined to bulk intermetallic compounds containing rare-earth or actinide elements. Herein, we report the observation and precise tuning of the mixed valence regime in stable, pure organic Blatter-type radicals synthesized on a Au(111) surface. Using scanning tunneling microscopy and spectroscopy, we identify a characteristic asymmetric double-peak spectral line shape, in which the splitting of the resonance peaks arises from the hybridization of many-body states, while the asymmetry is intrinsic to the mixed-valence regime. Theoretical simulations based on an effective quasiparticle Hamiltonian well reproduce the experimental findings, validating this line shape as a spectroscopic fingerprint of mixed valence states. Furthermore, by engineering the local adsorption geometry, molecular configuration, and supramolecular assembly, we demonstrate in situ crossover between a Kondo-like state and a mixed-valence state in Blatter radical dimers on Au(111). This work establishes purely organic radicals as a new system for accessing the mixed-valence regime and its crossover with the Kondo regime, providing a well-defined and tunable chemical platform for exploring emergent quantum phenomena and their underlying physical mechanism at the single-molecule level.