Single-component molecular material hosting antiferromagnetic and spin-gapped Mott subsystems

We investigated a system based solely on a single molecular species, Cu(tmdt)$_{2}$, accommodating $d$ and $π$ orbitals within the molecule. $^{13}$C nuclear magnetic resonance measurements captured singlet-triplet excitations of $π$ spins indicating the existence of a $π$-electron-based spin-gapped Mott insulating subsystem, which has been hidden by the large magnetic susceptibility exhibited by the $d$ spins forming antiferromagnetic chains. The present results demonstrate a unique hybrid Mott insulator composed of antiferromagnetic and spin-singlet Mott subsystems with distinctive dimensionalities.