Electronic Structure and Optical Properties of Cu₂ZnSnS₄ under Stress Effect

By using the pseudopotential plane-wave method of first principles based on density functional theory, the band structure, density of states and optical properties of Cu2ZnSnS4 under isotropic stress are calculated and analyzed. The results show that Cu2ZnSnS4 is a direct band gap semiconductor under isotropic stress, the lattice is tetragonal, and the band gap of Cu2ZnSnS4 is 0.16 eV at 0 GPa. Stretching the lattice causes the bottom of the conduction band of Cu2ZnSnS4 to move toward lower energies, while the top of the valence band remains unchanged and the band gap gradually narrows. Squeezing the lattice causes the bottom of the conduction band to move toward the high-energy direction, while the top of the valence band moves downward toward the low-energy direction, and the Cu2ZnSnS4 band gap becomes larger. The static permittivity, absorption coefficient, reflectivity, refractive index, electrical conductivity, and energy loss function all decrease when the lattice is stretched, and the above optical parameters increase when the lattice is compressed. When the lattice is stretched, the optical characteristic peaks such as the dielectric function shift to the lower-energy direction, while the optical characteristic peak position shifts to the higher-energy direction when the lattice is compressed

Half-Metallicity and Magnetism of the Quaternary Heusler Compound TiZrCoIn_1−xGe_x from the First-Principles Calculations

The effects of doping on the electronic and magnetic properties of the quaternary Heusler alloy TiZrCoIn were investigated by first-principles calculations. Results showed that the appearance of half-metallicity and negative formation energies are associated in all of the TiZrCoIn1−xGex compounds, indicating that Ge doping at Z-site increases the stability without damaging the half-metallicity of the compounds. Formation energy gradually decreased with doping concentration, and the width of the spin-down gap increased with a change in Fermi level. TiZrCoIn0.25Ge0.75 was found to be the most stable half-metal. Its Fermi level was in the middle of the broadened gap, and a peak at the Fermi level was detected in the spin majority channel of the compound. The large gaps of the compounds were primarily dominated by the intense d-d hybridization between Ti, Zr, and Co. The substitution of In by Ge increased the number of sp valence electrons in the system and thereby enhanced RKKY exchange interaction and increased splitting. Moreover, the total spin magnetic moments of the doped compounds followed the Slater⁻Pauling rule of Mt = Zt − 18 and increased from 2 μB to 3 μB linearly with concentration