Susceptibility represents the linear response to the applied external force. It diverges at the critical point of a second order phase transition, and the divergence is characterized by the critical exponent. The susceptibility and the critical exponent are computed by statistical mechanics if we focus on thermal equilibrium. However, a long-range Hamiltonian system is trapped in a long-lasting quasi- stationary state after applying the external force, and, in a simple model, it has been reported that the critical exponent in such a state may have strange value. We extend this result to a Hamiltonian system which has two order parameters. We show some suppressions of divergence and existence of negative susceptibility.
This work is in a collaboration with D. Sawai.