  We used transport and inelastic neutron scattering to study the optimally
phosphorus-doped BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ superconductor ($T_c = 30$
K). In the normal state, we find that the previously reported linear
temperature dependence of the resistivity below room temperature extends to
$\sim$ 500 K. Our analysis of the temperature and energy ($E=\hbar\omega$)
dependence of spin dynamical susceptibility at the antiferromagnetic (AF)
ordering wave vector $\chi^{\prime\prime}({\bf Q}_{\rm AF},\omega)$ reveal an
$\omega / T$ scaling within $1.1<E/k_BT<110$. These results suggest that the
linear temperature dependence of the resistivity is due to the presence of a
magnetic quantum critical point in the cleanest iron pnictides near optimal
superconductivity. Moreover, the results reconcile the strange-metal
temperature dependences with the weakly first-order nature of the quantum
transition out of the AF and nematic orders.

Superconductivity