First-principles study on the structural, electronic and optical properties in bulk and surfaces of full-Heusler alloy X2CoGa (X = Ti, Hf)

The structural, electronic and optical properties in bulk and (100) as well as (110) surfaces of full-Heusler alloys X2CoGa (X = Ti, Hf) have been investigated by spin-polarized first-principles calculations. In spin-down channel of bulk X2CoGa (X = Ti, Hf) alloys, the conduction band minimum is contributed by X-A-3d(t(2g)) and X-B-3d(t(2g)), while the valence band maximum is contributed by X-B-3d(t(2g)) and Co-3d(t(2g)) states or bulk X2CoGa (X = Ti, Hf) alloys, the peak values of real part epsilon 1(omega) of dielectric constant epsilon(omega) all present the blue shift characteristics with decreasing lattice constant induced by increasing pressure. The ideal TiGa (HfGa) termination of Ti2CoGa (Hf2CoGa) (100) surface and TiCoGa termination of Ti2CoGa (110) surface with the much lower surface energies have higher spin-polarizations of 97.9% (84.9%) and 94.0%, respectively, but the Ti2CoGa alloy has the superiority using for spintronic devices compared to the Hf2CoGa alloy in thin film form. Compared to the Hf2CoGa alloy, the ideal TiCoGa termination of Ti2CoGa (110) surface with much superiority optical property has the largest absorption coefficient alpha(omega)\ along c axis at about 7.5 eV and the alpha(omega) is almost zero when the energy is larger than 22.5 eV in all the six surface cases which is much different from the bulk alloy case.