We have examined the electronic structure evolution in transition metal dichalcogenides MX2 where M = Mo, W and X = S, Se, and Te. These are generally referred to as van der Waals materials on the one hand, yet one has band gap changes as large as 0.6 eV with thickness in some instances. This does not seem to be consistent with a description where the dominant interactions are van der Waals interactions. Mapping onto a tight binding model allows us to quantify the electronic structure changes, which are found to be dictated solely by interlayer hopping interactions. Different environments that an atom encounters could change the Madelung potential and therefore the onsite energies. This could happen while going from the monolayer to the bilayer as well as in cases where the stackings are different from what is found in 2H structures. These effects are quantitatively found to be negligible, enabling us to quantify the thickness-dependent electronic structure changes as arising from interlayer interactions alone.

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Hybrid organic–inorganic lead halide perovskites are projected as new generation photovoltaic and optoelectronic materials with improved efficiencies. However, their electronic structure so far remains poorly understood, particularly in the orientationally disordered cubic phase. We performed electronic structure investigations using angle-resolved photoemission spectroscopy on two prototypical samples (MAPbBr3 and MAPbCl3) in their cubic phase, and the results are compared with the calculations within two theoretical models where MA+ is orientationally (1) disordered (MA+ ion is replaced by spherically symmetric Cs+ ion) and (2) ordered (MA oriented along (100) direction) but keeping the symmetry of the unit cell cubic. Degeneracy of the valence bands and behavior of constant energy contours are consistent with model 1, which supports strongly the disordered nature of the orientation of the MA+ ions in the cubic phase. Band structure calculations also reveal that spin–orbit coupling induced Rashba splitting is suppressed by the orientational disorder.

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We have explored doping electrons into an antiferromagnetic (AFM) insulator as a route to realizing an AFM metal within a multiband Hubbard model. Considering parameters relevant for a 5d transition metal oxide with a half-filled t2g band we find that an AFM metallic phase is stabilized for occupancies up to 3.375 electrons per transition metal site for values of U up to 1.75 eV. At higher values of U, one has a charge-ordered insulating state. In contrast, the large Hund coupling associated with the 3d transition metal oxides does not allow for an AFM metallic phase for the concentrations examined. One has a ferromagnetic metallic phase for the 3d oxide for small values of U at 25% doping, however, at large values, one again finds charge ordering. Orbital degeneracy is found to play an important role, introducing the charge-ordered insulating phase into the phase diagram.