Electronic structures of Bi 2 Se 3 and Ag x Bi 2 Se 3 under pressure studied by high-resolution x-ray absorption spectroscopy and density functional theory calculations

The pressure-induced change in the electronic structures of the superconductors ${\\mathrm{Bi}}_{2}{\\mathrm{Se}}_{3}$ and ${\\mathrm{Ag}}_{x}{\\mathrm{Bi}}_{2}{\\mathrm{Se}}_{3}$ has been measured with high-resolution x-ray absorption spectroscopy. As a common feature for these compounds, we find that pressure causes the broadening of the Se $4p$ band and an energy shift of the Bi $6s$ band above the Fermi level up to the pressure of the first structural transition. These results, corroborated by density functional theory calculations, correlate with an increase of the carrier density, the disappearance of the band gap, and the emergence of superconductivity. The electronic structure changes significantly at the pressure of the first structural transition, which may be a trigger of the emergence of superconductivity, while above the pressure of the first phase transition it does not change much even around the second phase transition pressure, corresponding to the nearly constant ${T}_{\\mathrm{c}}$ above the pressure of the second structural transition.