A first principles many-body approach is employed to calculate the band structure and optical response of nanometer sized ribbons of SiC. Many-body effects are incorporated using the GW approximation, and excitonic effects are included using the Bethe-Salpeter equation. Both unpassivated and hydrogen passivated armchair SiC nanoribbons are studied. As a consequence of low dimensionality, large quasiparticle corrections are seen to the Kohn-Sham energy gaps. In both cases quasiparticle band gaps are increased by up to 2 eV, as compared to their Kohn-Sham energy values. Inclusion of electron-hole interactions modifies the absorption spectra significantly, giving rise to strongly bound excitonic peaks in these systems.The results suggest that hydrogen-passivated armchair SiC nanoribbons have the potential to be used in optoelectronic devices operating in the UV-Vis region of the spectrum. We also compute the formation energies of these nanoribbons as a function of their widths, and conclude that hydrogen-saturated ribbons will be much more stable as compared to the bare ones.PACS numbers: Insert suggested PACS numbers here. arXiv:1506.07103v3 [cond-mat.mtrl-sci]

We performed electronic structure calculations based on the first-principles manybody theory approach in order to study quasiparticle band gaps, and optical absorption spectra of hydrogen-passivated zigzag SiC nanoribbons. Self-energy corrections are included using the GW approximation, and excitonic effects are included using the Bethe-Salpeter equation. We have systematically studied nanoribbons that have widths between 0.6 nm and 2.2 nm. Quasiparticle corrections widened the Kohn-Sham band gaps because of enhanced interaction effects, caused by reduced dimensionality. Zigzag SiC nanoribbons with widths larger than 1 nm, exhibit halfmetallicity at the mean-field level. The self-energy corrections increased band gaps substantially, thereby transforming the half-metallic zigzag SiC nanoribbons, to narrow gap spin-polarized semiconductors. Optical absorption spectra of these nanoribbons get dramatically modified upon inclusion of electron-hole interactions, and the narrowest ribbon exhibits strongly bound excitons, with binding energy of 2.1 eV.Thus, the narrowest zigzag SiC nanoribbon has the potential to be used in optoelectronic devices operating in the IR region of the spectrum, while the broader ones, exhibiting spin polarization, can be utilized in spintronic applications.arXiv:1701.05971v2 [cond-mat.mes-hall]

Kondo-based semimetals and semiconductors are of extensive current interest as a viable platform for strongly correlated states in the dilute carrier limit. It is thus important to understand the routes to understand such systems. One established pathway is through Kondo effect in metallic non-magnetic analogues, in the so called half-filling case of one conduction electron and one 4f electron per site. Here we advance a new mechanism, through which Kondo-based semimetals develop out of conduction electrons with a low-carrier density in the presence of an even number of rare-earth sites. We demonstrate this effect by studying the Kondo material Yb3Ir4Ge13 along with its closed-4f -shell counterpart, Lu3Ir4Ge13. Through magnetotransport, optical conductivity and thermodynamic measurements, we establish that the correlated semimetallic state of Yb3Ir4Ge13 below its Kondo temperature originates from the Kondo effect of a low-carrier conduction-electron background. In addition, it displays fragile magnetism at very low temperatures, which, in turn, can be tuned to a non Fermi liquid regime through Lu-for-Yb substitution. These findings are connected with recent theoretical studies in simplified models. Our results open an entirely new venue to explore the strong correlation physics in a semimetallic environment.

Single crystals of Sr2Mn3As2O2 have been grown for the first time, for which we show a possible layer-selective Mott insulator behavior. This compound stands out as a hybrid structure of MnO2 and MnAs layers, analogous to the active CuO2 and FeAs layers respectively, in the cuprate and iron-based high temperature superconductors. Electrical transport, neutron diffraction measurements, together with DFT calculations on Sr2Mn3As2O2 single crystals converge towards a picture of independent magnetic order at T1 ∼ 79 K and T2 ∼ 360 K for the two Mn sublattices, with insulating behavior at odds with the metallic behaviour predicted by calculations. Furthermore, our inelastic neutron scattering studies of spin wave dispersions for the Mn(1) sublattice reveal an effective magnetic exchange coupling of SJ ∼ 3.7 meV. This is much smaller than those for the Mn(2) sublattice.

Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb4, arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin–orbit coupling (SOC), the SOC, in this case, plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb4 to be a material system with narrow crossings approximately protected by nonsymmorphic crystalline symmetries.