Change of the charge modulation during superconducting transition in SmFeAsO0.91F0.09 seen by 57Fe Mössbauer spectroscopy

Iron-based superconductor SmFeAsO0.91F0.09 has been investigated by the 57Fe Mössbauer spectroscopy versus temperature with the special attention paid to the region of the superconducting transition at about 47 K. Modulation of the electron charge density was found. It leads to the development of the charge density wave (CDW) and electric field gradient wave (EFGW). The modulation of CDW is enhanced in the temperature region of the superconducting gap opening, while the amplitude of EFGW is partly suppressed within this temperature region. This effect is exactly opposite to the similar effect in Ba0.6K0.4Fe2As2 superconductor. Hence, it seems that d electrons contribute significantly to the Cooper pair formation in both compounds as EFGW is perturbed within the temperature region of the superconducting gap formation.