Studying phase transitions is a central issue in many-body Hamiltonian systems. The phase transitions are classified into some universality classes and the classes are characterized by the critical exponents. In this talk, we focus on the critical exponents defined by applying an external force. The critical exponents can be computed by using statistical mechanics, if we are interested in thermal equilibrium states, to which the systems are believed to relax after a long time. However, in the so-called mean-field models, the systems are trapped in nonequilibrium states even in the long time limit. As a result, we have different critical exponents for response from the ones obtained by statistical mechanics [1,2]. We investigate the universality of such strange critical exponents by introducing classical spin models.
This talk is based on a collaboration with D. Das and S. Gupta.
 Non-mean-field critical exponent in a mean-field model: Dynamics versus statistical mechanics, S. Ogawa, A. Patelli, and Y. Y. Yamaguchi, Phys. Rev. E 89, 032131 (2014).
 Landau-like theory for universality of critical exponents in quasistationary states of isolated mean-field systems, S. Ogawa and Y. Y. Yamaguchi, Phys. Rev. E 91, 062108 (2015).