Discovery, Evidence and Properties for Superconducting Fe-Based Compounds Superconducting (A _{1− x} Sr _x) Fe ₂ As ₂ with A= K and Cs with T_c up to 37 K.

In superconductivity (SC) discovery also understanding provide principal archetype, recently in 2008 discovery of iron-pnictide with several families, including spontaneous gauge symmetry breaking and Anderson-Higgs mechanism remain outstanding issue and opened new avenue of research in condensed matter physics. T_c raised in excess of 55ºK, bringing insights into bad-metal behavior, magnetism, and their striking interplay with unconventional SC. New high-T_c Fe-based-122 compounds, AFe₂As₂ with A = K, Cs, K/Sr, and Cs/Sr, were synthesized. T_c of KFe₂As₂ and CsFe₂As₂ is 3.8K and 2.6 K, respectively, T_c rises with partial substitution of Sr for K and Cs, T_c peaks at 37 K for 50%–60% Sr substitution, and compounds enter a spin-density-wave state with increasing electron number (Sr content). Compounds represent p-type analogs of the n-doped rare-earth oxypnictide SC. In cuprates correlations Hubbard-dominated, Fe-Pn involves multiorbital (multiband) physics and correlation effects. Electronic and structural behavior demonstrate the crucial role of (Fe₂As₂) layers in SC of Fe-based layered systems, and special feature of having elemental A layers provides new avenues to SC at higher T_c. Developments and intense activity ensued commonalities and differences between high-T_c Fe-based and Cuprate, Heavy-fermion and Organic SC. And I will discuss fundamental issues, such as symmetry of SC state, pairing mechanism, magnetic properties, electronic correlations, and role of orbital degrees of freedom.