Magnetic fluctuations and superconductivity in iron pnictides as probed by electron spin resonance

Abstract

The electron spin resonance (ESR) absorption spectrum of Eu2+ ions serves as a probe of the normal and superconducting state in Eu0.5 K0.5 Fe2 As2. The spin-lattice relaxation rate 1/ T1 ESR obtained from the ESR linewidth exhibits a Korringa-type linear increase with increasing temperature above Tc 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 Tc the spin lattice relaxation rate 1/ T1 ESR follows a T1.5 behavior without any appearance of a coherence peak. © 2010 The American Physical Society.