Fragment-orbital-dependent spin fluctuations in the single-component molecular conductor [Ni(dmdt)_2]

Motivated by recent nuclear magnetic resonance experiments, we calculated the spin susceptibility, Knight shift, and spin-lattice relaxation rate (1/T_1T) of the single-component molecular conductor [Ni(dmdt)_2] using the random phase approximation in a multi-orbital Hubbard model describing the Dirac nodal line electronic system in this compound. This Hubbard model is composed of three fragment orbitals and on-site repulsive interactions obtained using ab initio many-body perturbation theory calculations. We found fragment-orbital-dependent spin fluctuations with the momentum q=0 and an incommensurate value of the wavenumber q=Q at which a diagonal element of the spin susceptibility is maximum. The q=0 and Q responses become dominant at low and high temperatures, respectively, with the Fermi-pocket energy scale as the boundary. We show that 1/T_1T decreases with decreasing temperature but starts to increase at low temperature owing to the q=0 spin fluctuations, while the Knight shift keeps monotonically decreasing. These properties are due to the intra-molecular antiferromagnetic fluctuations caused by the characteristic wave functions of this Dirac nodal line system, which is described by an n-band (n≥ 3) model. We show that the fragment orbitals play important roles in the magnetic properties of [Ni(dmdt)_2].