Direct observation of enhanced electron-phonon coupling in copper nanoparticles in the warm-dense matter regime

TL;DR

This study creates warm-dense matter in copper nanoparticles using femtosecond laser excitation and directly measures the strongest electron-ion coupling observed, providing new insights into matter at the intersection of solids, plasmas, and liquids.

Contribution

It introduces a novel experimental method to create and probe warm-dense matter in nanoparticles, enabling direct measurement of electron-ion coupling in this regime.

Findings

Observed the strongest electron-ion coupling in copper nanoparticles.

Confirmed nanoparticles are at the boundary between hot solids and plasmas.

Detected rapid energy transfer and temperature modulation via acoustic oscillations.

Abstract

Warm-dense matter (WDM) is a highly-excited state that lies at the confluence of solids, plasmas, and liquids and that cannot be described by equilibrium theories. The transient nature of this state when created in a laboratory, as well as the difficulties in probing the strongly-coupled interactions between the electrons and the ions, make it challenging to develop a complete understanding of matter in this regime. In this work, by exciting isolated ~8 nm nanoparticles with a femtosecond laser below the ablation threshold, we create uniformly-excited WDM. We then use photoelectron spectroscopy to track the instantaneous electron temperature and directly extract the strongest electron-ion coupling observed experimentally to date. By directly comparing with state-of-the-art theories, we confirm that the superheated nanoparticles lie at the boundary between hot solids and plasmas, with associated strong electron-ion coupling. This is evidenced both by the fast energy loss of electrons to ions, as well as a strong modulation of the electron temperature by acoustic oscillations in the nanoparticle. This work demonstrates a new route for experimental exploration and theoretical validation of the exotic properties of WDM.