Resonant excitation of the axion field during the QCD phase transition
  We find that the adiabatic fluctuations produced in the primordial plasma by
cosmological inflation resonantly excite the axion field during the QCD phase
transition by pumping axions from low momentum modes to modes with momentum up
to of order $\sqrt{3} m$ where $m$ is the axion mass. We derive the momentum
distribution of the excited axions. The fraction of cold axions that get
excited is of order one if the axion mass is larger than a few $\mu$eV. The
effect occurs whether inflation happens before or after the Peccei-Quinn phase
transition.
