Quantum oscillations in the parent magnetic phase of an iron arsenide high temperature superconductor*

We report quantum oscillation measurements in SrFe$_2$As$_2$, which is known to become superconducting under doping and pressure. The magnetic field and temperature dependences of the oscillations between 20 and 55~T suggest that the electronic excitations are those of a Fermi liquid. We show that the observed Fermi surface comprising small pockets is consistent with the formation of a spin-density wave. Our measurements thus demonstrate that high $T_{\rm c}$ superconductivity can occur on doping or pressurizing a conventional metallic spin-density wave state.