Magnetic properties of graphenic carbon nanostructures, which are relevant for future spintronic applications, depend crucially on doping and on the presence of defects. In this paper we study the magnetism of the recently detected substitutional Ni $\left({\text{Ni}}_{\text{sub}}\right)$ impurities. ${\text{Ni}}_{\text{sub}}$ defects are nonmagnetic in flat graphene and develop a nonzero-spin moment only in metallic nanotubes. This surprising behavior stems from the peculiar curvature dependence of the electronic structure of ${\text{Ni}}_{\text{sub}}$. A similar magnetic-nonmagnetic transition of ${\text{Ni}}_{\text{sub}}$ can be expected by applying anisotropic strain to a flat graphene layer.

The electronic band structure, density of states, dielectric function, optical absorption, and infrared spectrum of cubic ${\text{BaSnO}}_3$ were simulated using density functional theory, within both the local density and generalized gradient approximations, LDA and GGA, respectively. Dielectric optical permittivities and polarizabilities at $\omega =0$ and $\omega =\infty$ were also estimated. Indirect band gaps E($\mathrm{R}\to \Gamma$ ) of 1.01 eV (LDA) and 0.74 eV (GGA) were found, which are smaller than the experimental one ($\approx$ 3.1 eV). A comparison of the calculated cubic ${\text{BaSnO}}_3$ band gap with those of others stannates ${\text{ASnO}}_3$ (A = Ca, Sr, Cd) already published highlights their dependence on each crystal profile. The cubic ${\text{BaSnO}}_3$ effective masses of electrons and holes were computed by parabolic fittings along different directions at the conduction band minimum and valence band maximum, being anisotropic for both electrons and holes. The experimental band gap and calculated effective masses confirm the semiconductor character of cubic ${\text{BaSnO}}_3$ . Finally, the vibrational normal modes and the infrared spectrum of cubic ${\text{BaSnO}}_3$ were obtained and assigned.

Tight-binding (TB) models exploited in calculating band structure of monolayer transition-metal dichalcogenides (TMDCs), namely ${MX}_2$ ($M=$ Mo and W; $X=$ S, Se and Te), can be divided into two groups: one is based on group theory and the other uses Slater-Koster (SK) method. The former in general is lack of flexibility to be extended to confined finite systems with lower symmetry, e.g., nanoribbons (NRs) and quantum dots. Unlike ubiquitous TB models, here we present an improved scheme of the flexible SK TB method in which the second-nearest-neighbor $M\text{−}M$ and $X\text{−}X$ hopping terms are included. Its improvement, being of comparable accuracy to first-principles calculations, is clearly elucidated through a comprehensive comparison between our results and those produced by widely accepted TB models in literature for monolayer ${\mathrm{MoS}}_2$. Besides, its high flexibility allows us to successfully extend our TB model from monolayer TMDCs to the $\mathrm{Mo}{X}_2$ ($X=$ S, Se, and Te) and $\mathrm{W}{X}_2$ ($X=$ S and Se) NRs of both zigzag and armchair types. We find that the zigzag NR could be either metallic or semiconducting, depending on the spin-orbit strength and band gap of its parent two-dimensional bulk TMDC, which is in contradiction to the usual consensus concerning TMDC NRs which exhibit the metallic behavior only. For a certain Fermi level, remarkably, we discovered that the ${\mathrm{MoS}}_2$ NRs demonstrate quantum valley Hall effect, while others only present topological insulator phase.