Ultraweak electron-phonon coupling strength in cubic boron arsenide unveiled by ultrafast dynamics

We report a time-resolved ultrafast quasiparticle dynamics investigation of cubic boron arsenide ($c$-BAs), which is a recently discovered highly thermally conducting material. The excited-state ultrafast relaxation channels dictated by the electron-phonon coupling (EPC), phonon-phonon scattering, and radiative electron-hole recombination have been unambiguously identified, along with their typical interaction times. Significantly, the EPC strength is obtained from the dynamics, with a value of ${\ensuremath{\lambda}}_{{T}_{2}}=0.008$ (corresponding to $\ensuremath{\lambda}\ensuremath{\langle}{\mathrm{\ensuremath{\Omega}}}^{2}\ensuremath{\rangle}=1.18\ifmmode\pm\else\textpm\fi{}0.08\phantom{\rule{0.16em}{0ex}}\mathrm{p}{\mathrm{s}}^{--2}$), demonstrating an unusually weak coupling between the electrons and phonons. As a comparison, an ultraweak EPC strength for graphene is also expected. We propose that preserving an ultrasmall EPC strength may be a prerequisite for exhibiting an ultrahigh thermal conductivity. Our investigation provides insight for searching and designing ultrahigh thermal conductivity materials. Notably, during our analysis we have generalized the fluence-dependence method for obtaining the EPC strength to room temperature, which can be applied to many other types of quantum materials in the future.