On Cross-correlations between Curvature and Isocurvature Perturbations
  during Inflation
  We investigate the effects of couplings between curvature and isocurvature
perturbations before and around horizon-crossings during cosmological
inflation. We consider a generalized two-field inflation model, in which the
non-canonical kinetic term allows us arbitrary sound speeds of curvature and
isocurvature perturbations. By using the field-theoretical perturbative
analysis, we calculate the cross-spectrum between curvature and isocurvature
perturbations and the corrections to curvature and isocurvature power spectra
due to the presence of couplings between them. Our analysis confirms previous
results that the cross-correlations are generated and amplified when
perturbations cross the horizons. Moreover, we find the cross-correlation,
which was previously shown to be first-order in slow-roll parameter, can be
enhanced when the sound speed of isocurvature perturbation is much smaller than
that of the curvature perturbation. This is because in this case the
isocurvature perturbation exits its horizon much earlier than the curvature
perturbation and acts as a nearly constant source on the curvature
perturbation.
