Superposition principle and nonlinear response in spin glasses

摘要: The extended principle of superposition has been a touchstone of spin-glass dynamics for almost 30 years. The Uppsala group has demonstrated its validity for the metallic spin glass, CuMn, for magnetic fields H up to 10 Oe at the reduced temperature T r= T/T g= 0.95, where T g is the spin-glass condensation temperature. For H> 10 Oe, they observe a departure from linear response which they ascribe to the development of nonlinear dynamics. The thrust of this paper is to develop a microscopic origin for this behavior by focusing on the time development of the spin-glass correlation length, ξ (t, t w; H). Here, t is the time after H changes, and t w is the time from the quench for T> T g to the working temperature T until H changes. We connect the growth of ξ (t, t w; H) to the barrier heights Δ (t w) that set the dynamics. The effect of H on the magnitude of Δ (t w) is responsible for affecting differently the two dynamical …