Ultrafast transport mediated homogenization of photoexcited electrons governs the softening of the $A_{1g}$ phonon in bismuth

Abstract

In order to determine the role of non-thermal 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 $7-197\mathrm{nm}$ 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 function of the incident fluence, film thickness, and probe wavelength in the range of $580-700\mathrm{nm}$. 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 $50\mathrm{nm}$ thickness a homogeneous excitation due to the ultrafast transport of hot charge carriers is observed, limited by a carrier penetration depth of $60\mathrm{nm}$ independent of the totally deposited laser energy.

arXiv comment7 pages, 2 figures

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• Photoinduced Interlayer Dynamics in T${}_d$-MoTe${}_2$: A Broadband Pump-Probe Study