Prolonged photo-carriers generated in Black Phosphorus

Transient electron-hole pairs generated in semiconductors can exhibit unconventional excitonic condensation. Anisotropy in the carrier mass is considered as the key to elongate the life time of the pairs, and hence to stabilize the condensation. By optical pumping, photoexcited carriers facing across the band gap form excitons due to the Coulomb interaction between electrons and holes. As being a direct band gap and highly anisotropic semiconductor, black phosphorus (BP) can be expected to form excitons due to the nonequilibrium electron and hole populations after optical pumping. However, the electrons excited in the conduction band and their carrier dynamics which can offer hints on e-h pair condensation have not yet been clarified.
Here we employ time-resolved ARPES to explore the dynamics of photo-generated carriers in BP. The electronic structure above the Fermi level has been clearly observed, where a massive-and-anisotropic Dirac-type dispersions are confirmed. Importantly, we have directly observe that the photo-carriers generated across the direct band gap have the life time exceeding 400 ps. Such a long duration can be attributed to the stabilized e-h pairs between conduction and valence bands and confirms that the BP is a suitable platform for excitonic condensations.