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 $\pi$ orbitals within the molecule. ${}^{13}$C nuclear magnetic resonance measurements captured singlet-triplet excitations of $\pi$ spins indicating the existence of a $\pi$-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.