In the framework of the ab initio random structure search method,we show that elemental Se and Te undergo pressure-induced structural transition from the bcc to fcc phase,in agreement with the theoretical results previously reported.By means of the pseudopotential plane-wave method based on density functional perturbation theory,the fcc structure for both elements is found to be another phonon-mediated superconducting phase of these materials.With a reasonable value for the Coulomb pseudopotentialμ^(*)=0.12,the maximum superconducting transition temperature Tc in the fcc phase of Se and Te is estimated to be about 5.7 K and 4.6 K,respectively.Furthermore,we show that in the entire fcc phase for Se and Te,the superconducting transition temperature decreases together with the increase in pressure,leading to the final suppression of the superconductivity.It is suggested that such behavior is mainly caused by the rapid increase in the mean-square phonon frequency(ω^(2))with pressure.Finally,a very strong electron-phonon coupling value,for both Se and Te in the fcc phase,is found along the G-K high symmetry lines.
Electron-phonon coupling (EPC) in the three high-pressure phases of Ba is investigated using a pseudopotential plane-wave method based on density functional perturbation theory. The calculated values of superconducting critical temperature T c of Ba-I and Ba-II under pressure are consistent well with the trends observed experimentally. Moreover, Ba-V is found to be superconducting with a maximum T c exceeding 7.8 K at 45 GPa. With the increase of pressure, the values of T c increase in Ba I and Ba-II but the value of T c decreases in Ba-V. For Ba-I at pressures below 2 GPa, the increases of logarithmic average frequency ω log and electron-phonon coupling parameters λ both contribute to the enhancement of T c . For all the three phases at pressures above 2 GPa, T c is found to be primarily determined by λ . Further investigation reveals that for all the three phases, the change in λ with pressure can be explained mainly by change in the phonon frequency. Thus for Ba-II and Ba-V, although they exhibit completely different superconducting behaviors, their superconductivities have the same origin; the pressure dependence of T c is determined finally by the pressure dependence of phonon frequency.
