Valence fluctuation in Yb3Si5 probed by synchrotron X-ray photoemission spectroscopy

We report the direct evidence of valence fluctuation in the thermoelectric material Yb3Si5 by using synchrotron X-ray photoemission spectroscopy. The Yb 3d core level photoemission spectrum of a Yb3Si5 single crystal shows clear separation of the Yb2+ peak and Yb3+ multiplet peaks. From the spectral weight of each component, the mean valence of Yb ions is estimated to be +2.67 at 20 K. Furthermore, a Kondo resonance peak just below the Fermi energy is also observed by high-resolution soft X-ray photoemission spectroscopy, suggesting the valence fluctuating state in Yb3Si5.