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relaxation time T$_{1}$ 26 and, hence, provides information complementary to NMR or NQR measurements. One can clearly identify a linear increase with temperature with a slope b = 5 . 1 Oe/K and a residual zero-temperature width Δ H$_{0}$ = 374 Oe. Upon entering the superconduct- ing state, a pronounced drop of the linewidth can be recognized in the inset of Fig. 4. Below about 20 K the linewidth increases again due to growing magnetic fluctu- ations of the Eu spins on approaching short-range order. 

The observed linear increase of the linewidth Δ H ∝ 〈 J 2 CE$_{−}$$_{Eu}$( q ) 〉 N $^{2}$( E$_{F}$ ) T (Korringa relaxation) is a typical signature of local moments in a Fermi liquid and depends on the conduction-electron density of states N ( E$_{F}$ ) at the Fermi energy E$_{F}$ and the exchange coupling J$_{CE}$$_{−}$$_{Eu}$ be- tween the conduction electrons and the Eu spins. The ob- served slope of 5.1 Oe/K is a usual value for S-state 4 f 7 local moments in metals and indicates a normal three- dimensional Fermi-liquid state. 26,32,33 In case of ferro- magnetic correlations between the Eu ions (reading of the Curie-Weiss temperature from the inset in the lower panel of Fig. 3 gives Θ CW ≃ 10K) the residual linewidth is expected to be negative Δ H$_{0}$ = − b Θ$_{CW}$ ≃ − 50 Oe with b being the derived Korringa slope. 26 The larger positive value Δ H$_{0}$ = 374 Oe is probably due to an in- homogeneous distribution of the Eu and K ions, a prob- lem which is well-known for the 122-systems. This may lead to strong fluctuations of the long-range dipolar fields while the narrowing effect of the short-range exchange narrowing is reduced. 

In the inset of Fig. 4 we show the temperature de- pendence of the reciprocal electron spin-lattice relaxation time given by 

where Δ H$_{0}$ = 374 Oe is the residual zero-temperature linewidth obtained from the linear fit in the normal regime and γ denotes the gyromagnetic ratio. The ob- tained power-law in the normal state clearly confirms the Korringa-law, but for the superconducting state we ob- serve a behavior ∝ T 1 . 5 without any indication of a co- herence (Hebel-Slichter) peak, ruling out a conventional BCS scenario with an isotropic gap. Due to the onset of magnetic fluctuations the ESR power-law can only be traced in a narrow temperature range below T$_{c}$ and we cannot exclude the possibility that 1 /T ESR 1 may be influ- enced by the vicinity of the short-range Eu interactions. To examine closer the vicinity of T$_{c}$ we plot 1 / ( T ESR 1 T ) as a function of temperature in Fig. 5. A clear increase from the constant high-temperature (Korringa) value oc- curs already below a temperature T ∗ ≃ 45 K and leads to a maximum and a sharp drop just below T$_{c}$ . A similar behavior of the spin-lattice relaxation time of $^{57}$Fe and $^{75}$As above T$_{c}$ has been reported by NMR in the related system Ba$_{0}$ . $_{6}$K$_{0}$ . $_{4}$Fe$_{2}$ As$_{2}$ and attributed to spin fluctua- tions in the FeAs layers due to interband nesting. 34 

A Korringa behavior in the normal state has also been found by ESR in pure and Co-doped EuFe$_{2}$As$_{2}$ 22,35 and by NMR and NQR studies on other 122-compounds, 34,36 

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but in the superconducting state of Fe-based supercon- ductors the temperature behavior of the nuclear spin- lattice relaxation time has revealed non-universal power- laws 1 /T$_{1}$ ∝ T α with α ranging from 2.5-6. 34,36–38 We want to point out that an NQR study of pure KFe$_{2}$As$_{2}$ revealed a power-law ∝ T 1 . 4 similar to our ESR results which was interpreted in terms of multi-gap supercon- ductivity with line nodes, 36 and a recent NMR study sug- gested that this exponent may be universal for overdoped Ba$_{1}$$_{−}$ x K x Fe$_{2}$As$_{2}$. 39 

Given the narrow temperature range of the ESR power-law, we refrain at the present stage from perform- ing a fit of the data, as the above mentioned NMR studies showed that very different power-laws can be obtained by varying the gaps’ sizes and symmetry. However, our data nicely show that local-moment ESR in iron pnictides is a highly efficient tool to shed light on the superconducting order parameters as a complementary method to nuclear spin resonance techniques. 

In summary, we show that the ESR signal of Eu 2+ spins gives direct access to the superconducting state of the 122-class of pnictides. We identify a normal Fermi- liquid behavior above T$_{c}$ from the Korringa-law of the ESR spin-lattice relaxation rate 1 /T ESR 1 . Just above T$_{c}$ we observe a deviation from the Korringa behavior which we assign to magnetic fluctuations in the FeAs lay- ers on approaching the superconducting state. Below T$_{c}$ no Hebel-Slichter peak is observed, ruling out a simple isotropic BCS scenario, and the spin-lattice relaxation rate follows 1 /T ESR 1 ∝ T 1 . $^{5}$. 

## IV. SUMMARY 

