Superconductivity and electron-phonon interaction in SrxBi2Se3 under pressure

Pressure-induced superconductivity has been widely explored and observed in Bi2Se3-based topological materials to hunt for topological superconductors. Although their triggered superconductivity has a close connection to their pressure-induced structural phase transitions, the quest for the electron pairing mechanism of these superconducting semiconductors in both their initial rhombohedral and high-pressure phases remains unknown. In this work, we present a systematic study of the pressure effect on superconducting properties and lattice dynamics using a combination of electrical transport, Raman-scattering, and synchrotron x-ray diffraction measurements using diamond anvil cells. One key finding is our observation of a cooperative connection between the strength of the electron-phonon interaction (EPI) generated by optical branches and the pressure-tunable superconductivity in rhombohedral SrxBi2Se3 crystal. The underlying suppression mechanism of the T_c by pressure is ascribed to the weakening of the electrons' interaction with the optical phonon modes in the rhombohedral phase. In the intermediate monoclinic phase, the Tc value underwent a sharp increase with carrier density accumulation accompanying the concurrent enhanced EPI. This is intuitively unusual since it is expected that the EPI shall be weakened by inducing more conducting carriers in a normal metal. In the tetragonal phase, the superconductivity is interpreted within BCS theory, since it is fully metallized and obeys the adiabatic Born-Oppenheimer approximation well. Our findings are important to fully understand unconventional superconductivity and the unusual pairing mechanism in the layered rhombohedral Bi2Se3-based superconductors.