Evolution of Eu valence and superconductivity in layered Eu_(0.5)La_(0.5)FBiS_(2_x)Se_x system

摘要

We have studied the effect of Se substitution on Eu valence in a layered Eu_(0.5)La_(0.5)FBiS_(2-c)Se_x superconductor using a combined analysis of x-ray absoφtion near-edge structure (XANES) and x-ray photoelectron spectroscopy (XPS) measurements. Eu L_3-edge XANES spectra reveal that Eu is in the mixed valence state with coexisting Eu~(2+) and Eu~(3+). The average Eu valence decreases sharply from ～ 2.3 for x = 0.0 to ～ 2.1 for x = 0.4. Consistently, Eu 3d XPS shows a clear decrease in the average valence by Se substitution. Bi 4/ XPS indicates that effective charge carriers in the BiCh_2 (Ch = S, Se) layers are slightly increased by Se substitution. On the basis of the present results it has been discussed that the metallic character induced by Se substitution in Eu_(0.5)La_(0.5)FBiS_(2-x)Se_x is likely to be due to increased in-plane orbital overlap driven by reduced in-plane disorder that affects the carrier mobility.