  For the Sr2-xIrO4 system, recent work pointed out the abnormal electronic and
magnetic properties around the critical point. Here, to understand Sr-vacancy
effect on spin-orbit coupled Mott insulator Sr2IrO4, the electronic structure
and local structure distortion for Sr2-xIrO4 system have been investigated by
x-ray absorption spectroscopy (XAS). By comparing the intensity of white-line
features at the IrL2,3-edge x-ray absorption near-edge spectroscopy (XANES), we
observe a sudden rise of the branching ratio in the vicinity of the critical
point.Further analysis on theIrL3 -edge extended x-ray absorption fine
structure (EXAFS) and calculated data demonstrated that the abrupt enhancement
of the branching ratio is intimately related to the less distorted IrO6
octahedra induced by deep Sr-vacancies, which in turn alters the relatively
strength of the spin-orbit coupling (SOC) and crystal electric field (CEF)that
would dictate the abnormal behavior of electronic and magnetic structure near
the critical point.
