13C NMR evidence for strong electron correlation and antiferromagnetic order in the single-component molecular material Pd(tmdt)2

We performed C-13 NMR measurements on the single-component molecular material Pd(tmdt)(2) to reveal the nature of the electronic phase. For the conducting state above 200 K, the analyses using the paramagnetic shift and the nuclear spin-lattice relaxation rate find large values of the Korringa ratio indicative of highly enhanced antiferromagnetic fluctuations. A clear NMR line broadening, indicative of an antiferromagnetic order, occurs at approximately 80 K with a traceable broadening extending up to 140 K, which possibly signifies a minor fraction of a higher-temperature magnetic phase in conjunction with a change in the NMR relaxation curve below 140 K. Comparing the observed spectral profile at 5 K and simulations, the magnetic moment is estimated to be similar to 0.18 mu(B)/tmdt. The present results show that Pd(tmdt)(2) is a Mott insulator situated near the Mott transition and hosts strong exchange interactions that give an antiferromagnetic order at an extremely high transition temperature among molecular materials.