Electron and lattice dynamics of transition metal thin films observed by ultrafast electron diffraction and transient optical measurements.

A Nakamura,K Ishizaka,T Shimojima,Y Iwasa,M Nakano

doi:10.1063/1.4971210

A Nakamura, K Ishizaka + Show 3 more

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https://doi.org/10.1063/1.4971210

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Abstract

We report the ultrafast dynamics of electrons and lattice in transition metal thin films (Au, Cu, and Mo) investigated by a combination of ultrafast electron diffraction (UED) and pump-probe optical methods. For a single-crystalline Au thin film, we observe the suppression of the diffraction intensity occuring in 10 ps, which direcly reflects the lattice thermalization via the electron-phonon interaction. By using the two-temperature model, the electron-phonon coupling constant (g) and the electron and lattice temperatures (Te, Tl) are evaluated from UED, with which we simulate the transient optical transmittance. The simulation well agrees with the experimentally obtained transmittance data, except for the slight deviations at the initial photoexcitation and the relaxed quasi-equilibrium state. We also present the results similarly obtained for polycrystalline Au, Cu, and Mo thin films and demonstrate the electron and lattice dynamics occurring in metals with different electron-phonon coupling strengths.

Highlights

Investigation of the nonequilibrium states in solids induced by the photoexcitation has been one of the most challenging problems in condensed matter physics
We report the ultrafast dynamics of electrons and lattice in transition metal thin films (Au, Cu, and Mo) investigated by a combination of ultrafast electron diffraction (UED) and pump-probe optical methods
We present the results obtained for polycrystalline Au, Cu, and Mo thin films and demonstrate the electron and lattice dynamics occurring in metals with different electron-phonon coupling strengths

Summary

Introduction

Investigation of the nonequilibrium states in solids induced by the photoexcitation has been one of the most challenging problems in condensed matter physics. In some simple systems such as noble metals, a common approach to describing the electron and lattice dynamics is the so-called two-temperature model, which assumes that electrons and phonons are in thermal quasi-equilibriums with two different temperatures. When a solid is irradiated by a femtosecond laser pulse, the electrons are immediately (non-thermally) excited and quickly transferred into deeper parts of the sample.. The electron subsystem is quasi-thermalized via electronelectron interactions and starts to follow the Fermi-Dirac statistics characterized by the electron temperature (Te). The excess energy in the electron subsystem is redistributed to the lattice through the electron-phonon interactions usually within a few picoseconds, raising the lattice temperature (Tl).. The excess energy in the electron subsystem is redistributed to the lattice through the electron-phonon interactions usually within a few picoseconds, raising the lattice temperature (Tl). The two-temperature model was originally proposed by Anisimov et al. The time-dependent relation between Te and Tl can be written in the form cTe

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