Correlation-enhanced electron-phonon coupling for accurate evaluation of the superconducting transition temperature in bulk FeSe.

It has been widely recognized that, based on standard density functional theory calculations of the electron-phonon coupling, the superconducting transition temperature (T_c) in bulk FeSe is exceptionally low (almost 0 K) within the Bardeen-Cooper-Schrieffer formalism. Yet the experimentally observed T_c is much higher (∼10 K), and the underlying physical origin remains to be fully explored, especially at the quantitative level. Here we present the first accurate determination of T_c in FeSe where the correlation-enhanced electron-phonon coupling is treated within first-principles dynamical mean-field theory. Our studies treat both the multiple electronic bands across the Fermi level and phononic bands, and reveal that all the optical phonon modes are effectively coupled with the conduction electrons, including the important contributions of a single breathing mode as established by previous experiments. Accordingly, each of those phonon modes contributes pronouncedly to the electron pairing, and the resultant T_c is drastically enhanced to the experimentally observed range. The approach developed here should be broadly applicable to other superconducting systems where correlation-enhanced electron-phonon coupling plays an important role. [ABSTRACT FROM AUTHOR]