We present optical transmission measurements that reveal a charge gap of 0.86 eV in the local moment antiferromagnetic insulator BaMn2As2 , an order of magnitude larger than previously reported. Density functional theory plus dynamical mean field theory (DFT+DMFT) calculations correctly reproduce this charge gap only when a strong Hund's coupling is considered. Thus, BaMn2As2 is a member of a wider class of Mn pnictide compounds that are Mott-Hund's insulators. We also present optical reflectance for metallic 2% K doped BaMn2As2 that we use to extract the optical conductivity at different temperatures. The optical conductivity σ1(ω) exhibits a metallic response that is well described by a simple Drude term. Both σ(ω→0, T) and ρ(T) exhibit Fermi liquid temperature dependencies. From these measurements, we argue that a more strongly correlated Hund's metal version of the parent compounds of the iron pnictide superconductors has not yet been realized by doping this class of Hund's insulators.

We report the discovery of CaMn2Al10, a metal with strong magnetic anisotropy and moderate electronic correlations. Magnetization measurements find a Curie-Weiss moment of 0.83 µB/Mn, significantly reduced from the Hund's rule value, and the magnetic entropy obtained from specific heat measurements is correspondingly small, only ≈ 9 % of Rln 2. These results imply that the Mn magnetism is highly itinerant, a conclusion supported by density functional theory calculations that find strong Mn-Al hybridization. Consistent with the layered nature of the crystal structure, the magnetic susceptibility χ is anisotropic below 20 K, with a maximum ratio of χ [010] /χ [001] ≈ 3.5. A strong power-law divergence χ(T ) ∼ T −1.2 below 20 K implies incipient ferromagnetic order, and an Arrott plot analysis of the magnetization suggests a vanishingly low Curie temperature TC ∼ 0. Our experiments indicate that CaMn2Al10 is a rare example of a Mn-based weak itinerant magnet that is poised on the verge of ferromagnetic order.

The lack of a mechanistic framework for chemical reactions forming inorganic extended solids presents a challenge to accelerated materials discovery. We demonstrate here a combined computational and experimental methodology to tackle this problem, in which in situ X-ray diffraction measurements monitor solid-state reactions and deduce reaction pathways, while theoretical computations rationalize reaction energetics. The method has been applied to the LaCuO S (0 ≤ ≤ 4) quaternary system, following an earlier prediction that enhanced superconductivity could be found in these new lanthanum copper(II) oxysulfide compounds. In situ diffraction measurements show that reactants containing Cu(II) and S(2-) ions undergo redox reactions, leaving their ions in oxidation states that are incompatible with forming the desired new compounds. Computations of the reaction energies confirm that the observed synthetic pathways are indeed favored over those that would hypothetically form the suggested compounds. The consistency between computation and experiment in the LaCuO S system suggests a role for predictive theory: to identify and to explicate new synthetic routes for forming predicted compounds.

We spectroscopically investigated the energy gap of the correlated antiferromagnetic insulator LaMnPO1−xFx (x=0.0 and 0.04) as a function of temperature and pressure, separately, in conjunction with many body electronic structure calculations. These results show that the electronic structure in all measured regimes is well described by a model that includes both Mott-Hubbard interactions and Hund's rule coupling. Moreover, we find that by appying external pressure, thereby reducing the effective Mott-Hubbard interaction and Hund's coupling, the energy gap in LaMnPO1−xFx can be fully closed, yielding a metallic state.