Temperatur Dependence of the Upper Critical Field of Superconducting Eu_0.5K_0.5Fe₂As₂

The discovery of superconductivity (SC) in FeAs-layered systems has triggered enormous activities on this field of solid state physics. Up to now a variety of compounds based on the two families RFeOAs (R = rare-earth element) and AFe₂As₂ (A = Ba, Sr, ...) were found with transition temperatures up to 57 K. A main feature of these compounds is, that SC occurs in close proximity to magnetic order, which suggests, that magnetic fluctuations play a crucial role in the formation of the superconducting ground state. In the system EuFe₂As₂ even two different types of magnetic order have been found. At 190 K a Fe spin density wave (SDW) forms, which is accompanied by a structural phase-transition. Below 19 K a second magnetic phase transition is observed, where Eu²⁺-irons order antiferromagnetically. Band structure calculation suggest, that both magnetic lattices are predominantly decoupled. A detailed study of Eu_1-xK_xFe₂As₂ indicates the complete suppression of the SDW order for x = 0.5 and the simultaneous development of SC below 32 K. In the contribution, we present magnetoresistance measurements carried out at the Hochfeldmagnetlabor Dresden on polycrystalline samples of Eu_0.5K_0.5Fe₂As₂ in pulsed magnetic fields up to 60 T. We could determine the complete temperature dependence of the upper critical field H_c2(T), at temperatures down to 1.5 K. The initial slope µ₀dH_c2/dT in the proximity of T_c(H = 0) varies between -1.60 T/K (0.1 Rn) and -1.75 T/K (0.9 Rn), dependent on the used criterion, and is rather small compared to other pnictides of the same family. A comparison of our results with the Werthamer-Helfand-Hohenberg (WHH) model indicates that the upper critical field in Eu_0.5K_0.5Fe₂As₂ is not Pauli limited. Moreover, the experimentally estimated critical field µ₀H_c2(T → 0) of 45 T is significant higher than the value of 38 T expected for orbital limiting in the WHH model.