Nonequilibrium aging effects in the critical behavior of structurally disordered planar magnets

Nonequilibrium critical behavior of structurally disordered planar magnets described by the two-dimensional $XY$ model has been studied by Monte Carlo methods. For systems evolving from a high-temperature nonequilibrium initial state, the influence of defects of the structure on aging effects, as well as the violation of the fluctuation-dissipation theorem for “freezing” temperatures in the low-temperature phase, has been analyzed. Power-law temperature dependences of the limiting values of the fluctuation-dissipation ratio have been revealed for the first time.