  Ultrafast perturbations offer a unique tool to manipulate correlated systems
due to their ability to promote transient behaviors with no equilibrium
counterpart. A widely employed strategy is the excitation of coherent optical
phonons, as they can cause significant changes in the electronic structure and
interactions on short time scales. Here, we explore a promising alternative
route: the non-equilibrium excitation of acoustic phonons. We demonstrate that
it leads to the remarkable phenomenon of a momentum-dependent temperature, by
which electronic states at different regions of the Fermi surface are subject
to distinct local temperatures. Such an anisotropic electronic temperature can
have a profound effect on the delicate balance between competing ordered states
in unconventional superconductors, opening a novel avenue to control correlated
phases.
