  We report size-dependent melting of spherical copper nanoparticles embedded
into silica matrix. Based on the temperature dependence of the surface plasmon
resonance energy and its width we observe two distinct melting regimes. For
particles smaller than 20 nm the absorption spectrum changes monotonically with
the temperature, and this allows us to assume the gradual solid-liquid phase
transition (melting) of the nanoparticles or existence of superheated solid
nanoparticles. In contrast, for nanoparticles larger than 20 nm, we observe a
jump-like increase of the bandwidth and non-monotonic dependence of surface
plasmon energy at the temperatures below the bulk melting point. This indicates
that the melting of large nanoparticles is a first-order phase transition
similar to the melting of bulk copper.
