Abstract
Single crystals of Ba${}_{1\ensuremath{-}x}$Eu${}_{x}$Fe${}_{2}$As${}_{2}$ were studied by magnetic susceptibility, heat capacity, resistivity, and electron spin resonance (ESR) measurements. Spin-density wave (at ${T}_{\text{SDW}}$) and antiferromagnetic (at ${T}_{N}$) phase transitions were mapped as a function of $x$. For $x\ensuremath{\ge}0.2$, we found a single Eu${}^{2+}$ ESR Dysonian line that presents an isotropic linear increase (Korringa) of its linewidth ($\ensuremath{\Delta}H$) above ${T}_{\text{SDW}}$ which systematically decreases with decreasing $x$. In contrast, for a critical concentration ${x}_{c}$ ($0.10l{x}_{c}l0.20$), $\ensuremath{\Delta}H$ decreases with increasing $T$, suggesting a distinct relaxation process that we associate with a Eu${}^{2+}$ Kondo single impurity regime. The Korringa rate suppression towards the Ba-rich compounds is claimed to be due to the reduction of the $q$-dependent exchange interaction between the Eu${}^{2+}$ $f$ electrons and the conduction electrons, which is likely associated with an increasing of localization of Fe $d$ electrons. This result may help the understanding of the SDW phase suppression (that can lead to superconductivity) in this class of materials.
Highlights
INTRODUCTIONInteresting is the fact that superconductivity (SC) can be found in both FeAs-based systems with comparable critical temperatures despite the fact that one class is an oxide family and the other is an intermetallic system
The latter crystallizes in the tetragonal ThCr2Si2-type structure (I 4/mmm) and exhibits a structural distortion accompanied by a spin-density wave (SDW) phase transition (100 K TSDW 200 K)
Single crystals of Ba1−xEuxFe2As2 were characterized by transport, magnetic susceptibility, heat capacity, and electron spin resonance experiments
Summary
Interesting is the fact that superconductivity (SC) can be found in both FeAs-based systems with comparable critical temperatures despite the fact that one class is an oxide family and the other is an intermetallic system.. Interesting is the fact that superconductivity (SC) can be found in both FeAs-based systems with comparable critical temperatures despite the fact that one class is an oxide family and the other is an intermetallic system.3 The latter crystallizes in the tetragonal ThCr2Si2-type structure (I 4/mmm) and exhibits a structural distortion accompanied by a spin-density wave (SDW) phase transition (100 K TSDW 200 K). A microscopic understanding of the interplay between SDW and SC is still an open question that strongly motivates further research on these materials In this regard, microscopic spin probes that can directly study the spin dynamics of SDW and SC phases are highly desirable. The highest SC Tc within the AFe2As2 series is found in (Ba,K)Fe2As2 (38 K).
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