In situ temperature-dependent x-ray-absorption spectroscopic studies for the mercury-based superconductors

In situ temperature-dependent x-ray-absorption spectroscopic studies have been carried out for the mercury-based cuprate superconductor (Formula presented) in order to examine the evolution of electronic and geometric structures below and above the superconducting transition temperature (Formula presented) According to the in situ Cu (Formula presented)-edge x-ray-absorption near-edge structure (XANES) analyses, it is found that the copper valence is remarkably enhanced with a partial formation of Cu(III) ions in the superconducting state, reflecting a lattice instability around copper upon the superconducting transition, which is also confirmed by the in situ Hg (Formula presented)- and Tl (Formula presented)-edge XANES analyses. Such a finding underlines that the phonon-mediated pairing mechanism makes a significant contribution to the superconductivity of the Hg-based cuprates. On the other hand, the systematic Cu (Formula presented)-edge XANES spectroscopic studies for a series of (Formula presented) ((Formula presented) and 0.5; (Formula presented) 2, and 3) superconductors have also been performed, which allows us to determine the effects of the Tl substitution and the number of (Cu-O) sheet per formula unit (Formula presented) on the hole concentration of the copper-oxygen layer.