Bulk Superconductivity at 2.6 K in Undoped RbFe₂As₂

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The iron arsenide RbFe₂As₂ with the ThCr₂Si₂-type structure is found to be a bulk superconductor with Tc=2.6 K. The onset of diamagnetism was used to estimate the upper critical field Hc2(T), resulting in dHc2/dT=-1.4 T/K and an extrapolated Hc2(0)=2.5 T. As a new representative of iron pnictide superconductors, superconducting RbFe₂As₂ contrasts with BaFe₂As₂, where the Fermi level is higher and a magnetic instability is observed. Thus, the solid solution series (Rb,Ba)Fe₂As₂ is a promising system to study the crossover from superconductivity to magnetism.