Static critical behavior of the two-dimensional Ising spin glass ${\mathrm{Rb}}_2$${\mathrm{Cu}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$${\mathrm{F}}_4$

The temperature and field dependences of the nonlinear susceptibility are examined in ${\mathrm{Rb}}_2$ ${\mathrm{Cu}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Co}}_{\mathrm{x}}$ ${\mathrm{F}}_4$ with x=0.218, which is an experimental realization of the two-dimensional Edwards-Anderson Ising spin glass with random nearest-neighbor bonds. From the divergence of the nonlinear terms in the expansion of the magnetization in the field as well as from static scaling, the spin-glass transition is shown to occur at zero temperature, which implies the lower critical dimensionality for Ising spin glasses to exceed 2. The critical exponents γ=4.5±0.2 and β=0.0±0.1 agree with recent theoretical estimates for the ±J Ising spin glass in two dimensions.