  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.
