Ultrafast transport-mediated homogenization of photoexcited electrons governs the softening of the A1g phonon in bismuth

In order to determine the role of nonthermal transport of hot carriers, which is decisive for the dissipation of energy in condensed matter, we performed time-resolved broadband femtosecond transient reflectivity measurements on 7-197-nm-thick Bi(111) films epitaxially grown on Si(111). We monitored the behavior of the Fourier amplitude and the central frequency of the coherent A(1g) phonon mode as a function of the incident fluence, film thickness, and probe wavelength in the range 580-700 nm. The frequency redshift that follows photoexcitation was used as a robust quantity to determine the effective distribution of excited carriers that governs the displacive excitation mechanism of coherent A(1g) phonons in Bi. For Bi films up to 50 nm thickness a homogeneous excitation due to the ultrafast transport of hot charge carriers is observed, limited by a carrier penetration depth of 60 nm independent of the totally deposited laser energy.

Automated summary

In this study, we investigated the role of nonthermal transport in the dissipation of energy in condensed matter. Our results showed that in Bi films with a thickness of up to 50 nm, a homogeneous excitation caused by the ultrafast transport of hot charge carriers was observed. This study is of significance for understanding the energy dissipation mechanism in condensed matter.