Phonon softening and higher-order anharmonic effect in the superconducting topological insulator Sr_xBi₂Se₃

We report the anharmonic effect of the Raman-scattering spectrum in the low-carrier density superconductor Sr_xBi₂Se₃, which is dominated by the quartic term. Compared to the parent Bi₂Se₃, the superconducting Sr_xBi₂Se₃crystals show obvious phonon softening in the three optical phonon modesA1g1,Eg2, andA1g2. Based on the simulations by the Fano function, we present compelling evidence of the enhanced electron-phonon coupling in Sr_xBi₂Se₃for its smaller asymmetric parameterq. Moreover, an anomalous broadening and intensity antiresonance in the characteristic Raman peaks were observed as the temperature decreased to around 160 K. Thus, the superconducting topological material Sr_xBi₂Se₃represents a counterintuitive example where the electron-phonon interaction is strengthened by the accumulation of electron carriers.