Structural anomaly in superconductivity of CaFe2As2 class of materials
Quantum transitions in Fe-based systems are believed to involve spin, charge and nematic fluctuations. Complex structural phase diagram in these materials often emphasizes importance of covalency in their exotic properties, which is directly linked to the local structural network and barely understood. In order to address this outstanding issue, we investigate the evolution of structural parameters and their implication in unconventional superconductivity of 122 class of materials employing extended x-ray absorption fine structure studies. The spectral functions near the Fe K- and As K-absorption edge of CaFe2As2 and its superconducting composition, CaFe1.9Co0.1As2 (Tc = 12 K) exhibit evidence of enhancement of Fe contribution with Co-substitution near the Fermi level. As-Fe and Fe-Fe bondlengths derived from the experimental data exhibit interesting changes with temperature across the magneto-structural transition. Curiously, the evolution in Co-doped composition is similar to its parent compound despite absence of magneto-structural transition. In addition, we discover anomalous change of Ca-X (X = Fe, As) bondlengths with temperature in the vicinity of magneto-structural transition and disorder appears to be less important presumably due to screening by the charge reservoir layer. These results reveal evidence of doping induced evolution to the proximity to critical behavior presumably leading to superconductivity in the system.