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Layered material $\beta$-IrSn$_4$ ($I4_1/acd$, $D^{20}_{4h}$, #142), whose electron bands have symmetry-enforced Dirac points, was investigated using high-quality single crystals. It exhibits a pronounced linear field-dependence of magnetoresistance (LMR), which cannot be explained by currently existing models. Structures in the field-angle dependence of magnetoresistance and Hall resistivity are attributable to the Fermi surface topology; the presence of open orbits is inferred. At the superconducting (SC) transition, the specific-heat jump exhibits a significant increase in applied fields, revealing the type-I SC nature. This feature is attributable to the high Fermi velocity of linearly dispersive multibands. To clarify the mechanism of the puzzling LMR, investigations into the topological nature of those multibands in applied fields are highly desired.

In this study, using the density functional theory, we have investigated the bulk physical properties like structural, electronic band structure, elastic properties, thermal properties, optical properties and bonding features of AlPO4 compound in the hexagonal form. The values of our optimized structural parameters are very close to the previous results. Most of the results presented in this work are novel. The elastic constants indicate that AlPO4 is mechanically stable and brittle in nature. The compound is moderately hard and possesses low machinability index. AlPO4 contains significant elastic anisotropy. The charge density distribution , bond population analysis, Vickers hardness, thermo-mechanical properties, and optical properties have been investigated for the first time. The electronic band structure calculations reveal clear insulating behavior with a band gap of 6.0 eV. Band structure calculations were carried out without and with spin-orbit coupling (SOC) to explore possible topological signature. The energy dependent optical properties conform to the electronic band structure calculations. Major optical properties like dielectric functions, refractive index, photoconductivity, absorption coefficient, loss function and reflectivity are calculated and discussed in detail in this study. The compound is optically anisotropic. It is an efficient absorber and reflector of the ultraviolet light.

Density functional theory based study of the pressure dependent physical properties of binary SnS compound has been carried out. The computed elastic constants reveal that SnS is mechanically stable and brittle under ambient conditions. With increasing pressure, the compound becomes ductile. The Poisson's ratio also indicates brittle-ductile transition with increasing pressure. The hardness of SnS increases significantly with pressure. The compound possesses elastic anisotropy. The ground state electronic band structure is semiconducting with a small band gap which becomes metallic under pressure. The band becomes more and more dispersive with the increase in pressure while the electronic correlations decrease as pressure is raised. Both the Debye temperature and the phonon thermal conductivity of SnS increase sharply with pressure. The Melting temperature of the compound is low. Mixed bonding characteristics are found with ionic and covalent contributions. SnS is a good absorber of ultraviolet light. The reflectivity of the material increases with the increase in pressure. The reflectivity is nonselective over a wide spectral range. The low energy refractive index is high. All these optical characteristics are useful for prospective optoelectronic device applications. The optical anisotropy is low.

Binary metallic carbides belong to a technologically prominent class of materials. We have explored the structural, mechanical, electronic, optical, and some thermophysical properties of XC (X = Nb, Ta, Ti) binary metallic carbides in details employing first-principles method. Some of the results obtained are novel. A comparative analysis has been made.

Transition metal dichalcogenides (TMDCs) belong to technologically important compounds. We have explored the structural, elastic, bonding, optoelectronic and some thermo-physical properties of ZrX2 (X = S, Se, Te) TMDCs in details via ab-initio technique in this study. Elastic anisotropy indices, atomic bonding character, optoelectronic properties and thermo-physical parameters including melting temperature and minimum phonon thermal conductivity are investigated for the first time. All the TMDCs under investigation possess significant elastic anisotropy and layered structural features. ZrX2 (X = S, Se, Te) compounds are fairly machinable, and ZrS2 and ZrSe2 are moderately hard. ZrTe2, on the other hand, is significantly softer. Both covalent and ionic bondings contribute in the crystals. Electronic band structure calculations display semiconducting behavior for ZrS2 and ZrSe2 and metallic behavior for ZrTe2. Energy dependent optoelectronic parameters exhibit good correspondence with the underlying electronic energy density of states features. ZrX2 (X = S, Se, Te) compounds absorb ultraviolet radiation effectively. The reflectivity spectrum, R(w), remains over 50% in the energy range from 0 eV to 20 eV for ZrTe2. Therefore, this TMDC has wide band and nonselective high reflectivity and can be used as an efficient reflector to reduce solar heating. Debye temperature, melting point and minimum phonon thermal conductivity of the compounds under study are low and show excellent correspondence with each other and also with the elastic and bonding characteristics.

A topological insulator possesses a bulk energy gap splitting the lowest empty band from the highest occupied electronic band. The electronic states at the surface (or edge in two dimensions), on the other hand, of a topological insulator are gapless and are protected by the time reversal symmetry. Such systems are promising for variety of optoelectronic, superconducting, thermoelectric and quantum computation related applications. We have studied elastic, mechanical, electronic, optical properties, bonding character and the electronic charge density distribution of ternary Bi2Te2Se topological insulator. The compound under study is mechanically stable and elastically anisotropic. The electronic band structure calculations reveal high degree of anisotropy in the energy dispersion. Electronic effective mass is high in the c-direction compared to that in the ab-plane. The optical constants show moderate level of variation with respect to the polarization of the electric field of the incident radiation. The optical spectra are consistent with the electronic band structure and electronic density of states features. Both electronic band structure and optical constants show clear indications of a direct band gap of 0.610 eV for Bi2Te2Se. It is also found that Bi2Te2Se possesses high refractive index at low photon energies in the infrared and visible region. It has low reflectivity in the ultraviolet region. Bi2Te2Se absorbs photons strongly in the ultraviolet energies. All these features make Bi2Te2Se suitable for diverse class of optoelectronic device applications., Comment: Submitted for publication