Aging in ferromagnetic systems at criticality near four dimensions
  We study the off-equilibrium response and correlation functions and the corresponding fluctuation-dissipation ratio for a purely dissipative relaxation of an O(N) symmetric vector model (Model A) below its upper critical dimension. The scaling behavior of these quantities is analyzed and the associated universal functions are determined at first order in epsilon expansion in the high-temperature phase and at criticality. A non trivial limit of the fluctuation-dissipation ratio is found in the aging regime. 
Pasquale Calabrese and Andrea Gambassi