Organic diamond lattice Mott insulator: three dimensional Dirac semimetal candidate*

A half-filled Diamond lattice is a promising candidate of three-dimensional (3D) topological Dirac semimetal. We report the model material in a molecular compound (ET)Ag₄(CN)₅ with a S=1/2 diamond lattice. The DFT calculation shows Dirac nodal lines with the vanishing density of states at the Fermi energy. Owing to the strong electron correlations, the system exhibits a Mott insulating ground state with antiferromagnet order below 102 K. The weak ferromagnetism indicate the significant spin-orbit coupling and inversioin symmetry breaking, which can lift the degeneracy of the nodal line. We report the possible Mott transition into 3D Dirac semimetal under high pressures.