Parametric Resonance of Entropy Perturbations in Massless Preheating
  Here, we revisit the question of possible preheating of entropy modes in a
two field model with a massless inflaton coupled to a matter scalar field.
Using a perturbative approximation to the covariant method we demonstrate that
there is indeed a parametric instability of the entropy mode which then at
second order leads to exponential growth of the curvature fluctuation on
super-Hubble scale. Back-reaction effects shut off the induced curvature
fluctuations, but possibly not early enough to prevent phenomenological
problems. This confirms previous results obtained using different methods and
resolves a controversy in the literature.
