The electron spin resonance (ESR) absorption spectrum of ${\text{Eu}}^{2+}$ ions serves as a probe of the normal and superconducting state in ${\text{Eu}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_2{\text{As}}_2$. The spin-lattice relaxation rate $1/T_1^{\text{ESR}}$ obtained from the ESR linewidth exhibits a Korringa-type linear increase with increasing temperature above $T_{\text{c}}$ evidencing a normal Fermi-liquid behavior. Below 45 K deviations from the Korringa law occur which are ascribed to enhanced magnetic fluctuations within the FeAs layers upon approaching the superconducting transition. Below $T_{\text{c}}$ the spin lattice relaxation rate $1/T_1^{\text{ESR}}$ follows a $T^{1.5}$ behavior without any appearance of a coherence peak.

• Received 22 December 2009

DOI:https://doi.org/10.1103/PhysRevB.82.054525