Classical Néel-ordered and quantum spin singlet ground states of MoI₃ and MoBr₃

The properties of quasi-one-dimensional (q-1D) materials can span dimensionalities ranging from one-dimensional (1D) to three-dimensional (3D) depending on the strength of the interchain coupling. Two of these materials that exhibit properties of both 1D and 3D systems are MoI₃ and MoBr₃. We observe classical Néel order supported by weak interchain coupling at low temperature and quantum spin-singlet ground states of single-chains at higher temperatures. These materials consist of strongly dimerized single chains weakly coupled to neighboring chains. Using exchange parameters extracted from density functional theory calculations in Monte Carlo simulations, we explicitly show how weak interchain coupling stabilizes an antiferromagnetic (AFM) ground state and show the dependence of the Néel temperature (T_N) on the interchain coupling. The weak interchain coupling results in a low T_N of 40 K. We also perform exact diagonalization (ED) and density matrix renormalization group (DMRG) calculations on single dimerized chains and find a large singlet-triplet gap  of 21-25 meV, consistent with those extracted from experimental susceptibility measurements. Above T_N, the ground state is consistent with a quantum spin-singlet with a large singlet-triplet gap (Δ) resulting from exchange dimerization that gives rise to a large broad peak in the susceptibility with a maximum at T ≈ Δ.