Valence Transition of YbInCu4 Observed in Hard X-Ray Photoemission Spectra

Yb 3d and valence-band photoemission spectra of the first-order valence-transition compound YbInCu4 have been measured with hard x ray at an excitation energy of 5.95 keV. Abrupt changes are clearly observed in both spectra around the transition temperature TV = 42 K, in comparison with ultraviolet and soft x-ray photoemission (VUV-PES and SX-PES) spectra. From the Yb 3d spectra, the Yb valence has been estimated to be ~2.90 from 220 down to 50 K and ~2.74 at 30–10 K. We propose that Yb 3d hard x-ray photoemission spectroscopy is a very powerful method to estimate the valence of Yb with high accuracy. On the other hand, the Yb2+ 4f-derived peaks in the valence-band spectra exhibit a remarkable enhancement below TV. The shape of the valence-band spectra is different from those of the VUV-PES and SX-PES spectra above TV, reflecting the In 5s and 5p contributions