  Germanium is the detector material of choice in many rare-event searches
looking for low-energy nuclear recoils induced by dark matter particles or
neutrinos. We perform a systematic exploration of its quenching factor for
sub-keV nuclear recoils, using multiple techniques: photo-neutron sources,
recoils from gamma-emission following thermal neutron capture, and a
monochromatic filtered neutron beam. Our results point to a marked deviation
from the predictions of the Lindhard model in this mostly unexplored energy
range. We comment on the compatibility of our data with low-energy processes
such as the Migdal effect, and on the impact of our measurements on upcoming
searches.
