Activated dynamics in a two-dimensional Ising spin glass: ${\mathrm{Rb}}_2$${\mathrm{Cu}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$${\mathrm{F}}_4$

The complex ac susceptibility of the two-dimensional Ising spin glass ${\mathrm{Rb}}_2$ ${\mathrm{Cu}}_{0.782}$ ${\mathrm{Co}}_{0.218}$ ${\mathrm{F}}_4$ is measured as a function of the temperature and frequency. The results are first analyzed in terms of the Cole-Cole formalism. It is found that the median relaxation time ${\tau }_c$ diverges towards $T_c$=0 K over as many as 16 decades according to activated dynamics, i.e., ${\tau }_c$∝exp[(b/T${)}^{1+\psi \nu }$]. The distribution of relaxation times appears to be nearly symmetric in lnτ, covering at least 10 decades in width. These findings are corroborated with analyses based on exponential-logarithmic and stretched-exponential forms of the time decay of the spin-spin correlation function. The out-of-phase susceptibility is further found to be in conformity with activated dynamic scaling over a wide range of temperatures and frequencies. For the critical exponents the various methods of analysis yield, on the average, γ=4.4±0.2, ν=2.4±0.3, θ=-0.41±0.05, and ψ=0.8±0.2.