Selective electron-phonon coupling strength from nonequilibrium optical spectroscopy: The case of MgB₂

The coupling between quasiparticles and bosonic excitations rules the energy transfer pathways in condensed matter systems. The possibility of inferring the strength of specific coupling channels from their characteristic time scales measured in nonequilibrium experiments is still an open question. Here, we investigate MgB₂, in which conventional superconductivity at temperatures as high as 39 K is mediated by the strong coupling between the conduction electrons and the E₂₆ phonon mode. By means of broadband time-resolved optical spectroscopy, we show that this selective electron-phonon coupling dictates the nonequilibrium optical response of MgB₂ at early times (<100 fs) after photoexcitation. Furthermore, based on an effective temperature model analysis, we estimate its contribution to the total electron-boson coupling function extracted from complementary equilibrium spectroscopy approaches, namely optical reflectivity and ARPES. The coupling strength with the E₂₆ phonon modes is thus estimated to be ~ 0. 56, which is approximately half of the total coupling constant, in agreement with ab-initio calculations from the literature. As a benchmark, broadband time-resolved optical spectroscopy is performed also on the isostructural and non-superconducting compound AlB₂, showing that the nonequilibrium optical response relaxes on a slower timescale due to the lack of strongly-coupled phonon modes. Our findings demonstrate the possibility to resolve and quantify selective electron-phonon coupling from nonequilibrium optical spectroscopy.