Warm dense matter (WDM) represents a highly excited state that lies at the intersection 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 copper nanoparticles with a femtosecond laser below the ablation threshold, we create uniformly excited WDM. Using photoelectron spectroscopy, we measure the instantaneous electron temperature and extract the electron-ion coupling of the nanoparticle as it undergoes a solid-to-WDM phase transition. By 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 a fast energy loss of electrons to ions, and a strong modulation of the electron temperature induced by strong acoustic breathing modes that change the nanoparticle volume. This work demonstrates a new route for experimental exploration of the exotic properties of WDM.

1JILA, Department of Physics, University of Colorado and NIST, Boulder, Colorado 80309, USA 2Theoretical Division, Los Alamos National Laboratory, NM 87545, USA 3Kapteyn-Murnane Laboratories Inc., 4775 Walnut St #102, Boulder, CO 80301, USA 4Department of Chemistry, University of Colorado Boulder, CO 80309, USA 5Renewable and Sustainable Energy Institute, University of Colorado Boulder, CO 80309, USA 6EaStCHEM, School of Chemistry, Edinburgh University, David Brewster Road, Edinburgh EH9 3FJ, U.K. 7Molecular Foundary, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. †Present address: Stanford PULSE Institute, SLAC National Accelerator Laboratory and Stanford University, Menlo Park, CA 94025, USA. *Email:Quynh.L.Nguyen@colorado.edu