Antiferromagnetic Mott insulating state in the single-component molecular material Pd(tmdt) 2

A family of compounds built by a single molecular species, M(tmdt)2, with a metal ion, M, and organic ligands, tmdt, affords diverse electronic phases due to M-dependent interplays between d electrons in M, and π electrons in tmdt. We investigated the spin state in Pd(tmdt)2, a π-electron system without a d-electron contribution, through H1 nuclear magnetic resonance (NMR) and muon-spin resonance experiments. The temperature profiles of the NMR linewidth, relaxation rate, and asymmetry parameter in muon decay show an inhomogeneous antiferromagnetic order with moments distributed around ∼0.1μB that onsets at above 100 K. This result provides an example of the antiferromagnetic order in a pure π-electron system in M(tmdt)2, and it demonstrates that correlation among the π electrons is so strong as to give the Néel temperature over 100 K. The small and inhomogeneous moments are understandable as the crucial disorder effect in correlated electrons situated near the Mott transition.