Spectrum of Curvature Perturbation of Multi-field Inflation with   Small-Field Potential
  In this paper, we have studied the spectrum of curvature perturbation of multi-field inflation with general small-field potential. We assume that the isocurvature perturbation may be neglected, and by using the Sasaki-Stewart formalism, we found that the spectrum may be redder or bluer than of its corresponding single field. The result depends upon the values of fields and their effective masses at the horizon-crossing time. We discuss the relevant cases. 
Iftikhar Ahmad, Yun-Song Piao, Cong-Feng Qiao