  The presence of an electrical transport current in a material is one of the
simplest and most important realisations of non-equilibrium physics. The
current density breaks the crystalline symmetry and can give rise to dramatic
phenomena, such as sliding charge density waves [1], insulator-to-metal
transitions [2,3] or gap openings in topologically protected states [4]. Almost
nothing is known about how a current influences the electron spectral function,
which characterizes most of the solid's electronic, optical and chemical
properties. Here we show that angle-resolved photoemission spectroscopy with a
nano-scale light spot (nanoARPES) provides not only a wealth of information on
local equilibrium properties, but also opens the possibility to access the
local non-equilibrium spectral function in the presence of a transport current.
Unifying spectroscopic and transport measurements in this way allows
non-invasive local measurements of the composition, structure, many-body
effects and carrier mobility in the presence of high current densities.
