Quantum critical regime in the phase diagram of K$_x$Sr$_{1-x}$Fe$_2$As$_2$*

The electrical and thermoelectric properties of K$_x$Sr$_{1-x}$Fe$_2$As$_2$ are investigated. While the temperature dependence of the resistivity of SrFe$_2$As$_2$ (x=0) and KFe$_2$As$_2$ (x=1) is strongly nonlinear over a large temperature range it becomes surprisingly linear for x close to x$_c$ = 0.4 above the superconducting transition. This apparent deviation from the Fermi liquid behavior is similar to the high-T$_c$ cuprate superconductors and may indicate the existence of a quantum critical regime above the superconducting dome. We show that the temperature dependence of the thermoelectric power S follows a logarithmic scaling, S/T = const.*log(T) at the critical value x$_c$. The experimental results are consistent with a Ginzburg-Landau model for FeAs compounds predicting quantum critical scaling with a dynamical exponent z=2 and an effective dimension d+z=4.