Charge density wave in a SnSe2 layer on and the effect of surface hydrogenation

We have carried out an investigation using density functional theory (DFT) of the atomic and electronic structures of SnSe₂ layers on the surface and hydrogenation of this surface. We have considered a (2 × 2) SnSe₂ superstructure oriented along the diagonal direction of the surface periodicity, for which scanning tunneling microscopy (STM) measurements have recently been reported. In the band structure calculations, while the s-p character surface state originating from each SnSe₂ layer is determined, there is an additional half-filled surface state in the fundamental band gap region due to the Sn adatom. At the M̄ point in the Brillouin zone, a charge density wave (CDW) partial gap opening of ∼0.1 eV occurs between these surface states close to the Fermi level. Here, the CDW gap is caused by two reasons; (i) Fermi surface nesting, due to the inequivalent electron pockets at the M̄ point, and (ii) the out of plane weak electron-phonon coupling regime due to the mean-field (MF) theory (2Δ/k_BT^MF = 3.52). Upon hydrogen adsorption on the surface, we have obtained a β-phase SnSe layer and SeH₂ molecule with a bond angle of ∼90°. The hydrogenated surface pushes the surface state associated with the SnSe₂ layer into the Si projected bulk band continuum. After SeH₂ desorption, the work function drops from 5.20 eV to 4.39 eV.

Electronic, phononic and superconducting properties of trigonal Li2MSi2(MIr, Rh)

We have usedab initiodensity functional theory to study electronic, mechanical, phononic, and superconducting properties of Li2MSi2(M= Ir, Rh), which has recently been produced as a new type of transition metal-based ternary compound in the trigonal structure (Horiganeet al2019New J. Phys.21093056). The calculated electronic band structure and the density of states indicate that the Li2IrSi2and Li2RhSi2compounds are in metallic character. Mechanical properties such as elastic constants, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature were calculated for these compounds. The calculated results suggest that the compounds are mechanically stable and behave in a ductile manner. The phonon spectra have no imaginary frequency, which proves that these compounds are dynamically stable. Electron-phonon coupling parameters confirm that they are weak-coupling superconductors. Although the influence of spin-orbit coupling in superconductivity is not significant for these compounds, it has a very small influence on electronic structure for Li2IrSi2. The calculated critical temperature (Tcμ⋆=0.11) values of 3.29 K for Li2IrSi2and 2.82 K for Li2RhSi2agree well with experimental estimates.

Melt Spinning Effects on Optical and Mechanical Properties of Poly(Aryl Sulfone)

Poly(aryl sulfone) belongs to a group of polymers with a very low crystallizabilities which include polycarbonates, polyether imide and polyarylate. When subjected to high temperature gradients in such processes as injection molding and melt spinning, this class of polymers vitrify into glassy form. However their processing-property relationships are quite different from those conventional amorphous materials such as atactic polystyrene (a-PS) and atactic poly (methyl methacrylate) (a-PMMA). These new polymers being already highly ductile materials do not undergo brittle-ductile transition when oriented beyond a critical orientation levels which is typical of a-PS and a-PMMA. The chemical structure of poly(aryl sulfone) chain is mainly composed of para linked phenyl groups and this gives rise to high optical anisotropy in the chain. With small levels of orientation large optical birefringences are attained in the fibers. This polymer was also found to obey the stress optical law which states that there is a linear relationship between the stress at the vitrification and the birefringences developed in the fibers. Beyond a critical stress level around 108 dynes/cm2 deviation from this linearity is observed. This deviation from linearity may possibly be related to onset of structural ordering that takes place at high orientation levels. The mechanical properties were found to correlate quite well with the spinline stress and modulus, yield stress and tensile stress were found to increase and elongation at break was found to decrease with increasing orientation.