Electron-phonon coupling in Au depends on electron and phonon temperatures: Congrats Ash!

Ash Giri’s paper, “Experimental evidence of excited electron number density and temperature effects on electron-phonon coupling in gold films,” was recently published in Journal of Applied Physics (J. Appl. Phys. 117, 044305 (2015)). In this work, we studied the electron-phonon coupling processes in Au films after short pulsed laser heating using a range of laser fluences to control electron temperature while varying the lattice temperature via a LN2-cooled cryostat.  We provide quantitative experimental evidence of the transition from nonequilibrium-to-equilibrium dominated electron-phonon coupling relaxation in thin films. We show that the temperature dependence of the electron-phonon coupling factor in Au varies based on both the lattice and electron temperatures, in contrast to the two temperature model. This paper focuses on answering the fundamental question of scattering mechanisms and their relation to the experimental conditions that lead to different energy relaxation rates between the electronic and vibrational states in metals. In doing so, our measurements provide experimental evidence that elucidate novel transport processes in thin metal films along with resolving the discrepancies in nearly 20 years of experimental data.  Congrats Ash!!!

Abstract

The electronic transport properties of metals with weak electron-phonon coupling can be influenced by non-thermal electrons. Relaxation processes involving non-thermal electrons competing with the thermalized electron system have led to inconsistencies in the understanding of how electrons scatter and relax with the less energetic lattice. Recent theoretical and computational works have shown that the rate of energy relaxation with the metallic lattice will change depending on the thermalization state of the electrons. Even though 20 years of experimental works have focused on understanding and isolating these electronic relaxation mechanisms with short pulsed irradiation, discrepancies between these existing works have not clearly answered the fundamental question of the competing effects between non-thermal and thermal electrons losing energy to the lattice. In this work, we demonstrate the ability to measure the electron relaxation for varying degrees of both electron-electron and electron-phonon thermalization. This series of measurements of electronic relaxation over a predicted effective electron temperature range up to 􏰤3500 K and minimum lattice temperatures of 77 K validate recent computational and theoretical works that theorize how a nonequilibrium distribution of electrons transfers energy to the lattice. Utilizing this wide temperature range during pump-probe measurements of electron-phonon relaxation, we explain discrepancies in the past two decades of literature of electronic relaxation rates. We experimentally demonstrate that the electron-phonon coupling factor in gold increases with increasing lattice tem- perature and laser fluences. Specifically, we show that at low laser fluences corresponding to small electron perturbations, energy relaxation between electrons and phonons is mainly governed by non-thermal electrons, while at higher laser fluences, non-thermal electron scattering with the lattice is less influential on the energy relaxation mechanisms.

 

This material is based upon work supported by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-13-1-0067 (P.E.H—AFOSR Young Investigator Program).

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