Ultrafast lattice dynamics and electron-phonon coupling in platinum extracted with a global fitting approach for time-resolved polycrystalline diffraction data

Abstract

Quantitative knowledge of electron-phonon coupling is important for many applications as well as for the fundamental understanding of nonequilibrium relaxation processes. Time-resolved diffraction provides direct access to this knowledge through its sensitivity to laser-induced lattice dynamics. Here, we present an approach for analyzing time-resolved polycrystalline diffraction data. A two-step routine is used to minimize the number of time-dependent fit parameters. The lattice dynamics are extracted by finding the best fit to the full transient diffraction pattern rather than by analyzing transient changes of individual Debye-Scherrer rings. We apply this approach to platinum, an important component of novel photocatalytic and spintronic applications, for which a large variation of literature values exists for the electron-phonon coupling parameter $G_{\mathrm{ep}}$. Based on the extracted evolution of the atomic mean squared displacement (MSD) and using a two-temperature model (TTM), we obtain $G_{\mathrm{ep}}=(3.9\pm 0.2)\cdot 10^{17}\frac{\mathrm{W}}{{\mathrm{m}}^3\mathrm{K}}$ (statistical error). We find that at least up to an absorbed energy density of $124\frac{\mathrm{J}}{{\mathrm{cm}}^3}$, $G_{\mathrm{ep}}$ is not fluence-dependent. Our results for the lattice dynamics of platinum provide insights into electron-phonon coupling and phonon thermalization and constitute a basis for quantitative descriptions of platinum-based heterostructures in nonequilibrium conditions.

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