  The accurate determination of electronic temperatures in metallic
nanostructures is essential for many technological applications, like
plasmon-enhanced catalysis or lithographic nanofabrication procedures. In this
Letter we demonstrate that the electronic temperature can be accurately
measured by the shape of the tunnel electroluminescence emission edge in tunnel
plasmonic nanocavities, which follows a universal thermal distribution with the
bias voltage as the chemical potential of the photon population. A significant
deviation between electronic and lattice temperatures is found below 30 K for
tunnel currents larger than 15 nA. This deviation is rationalized as the result
of a two-electron process in which the second electron excites plasmon modes
with an energy distribution that reflects the higher temperature following the
first tunneling event. These results dispel a long-standing controversy on the
nature of overbias emission in tunnel junctions and adds a new method for the
determination of electronic temperatures and quasiparticle dynamics.
