Valence instability in YbPd2Si2: Magnetic susceptibility, x-ray absorption, and photoemission studies

Abstract

The bulk and surface valence states of Yb were determined for the intermetallic compound Yb${\mathrm{Pd}}_{2}$${\mathrm{Si}}_{2}$ by magnetic susceptibility ($\ensuremath{\chi}$), ${L}_{\mathrm{III}}$-edge x-ray absorption, and valence-band photoemission measurements. In the susceptibility data this compound exhibits the characteristic features of homogeneously mixed-valent systems: enhanced Pauli paramagnetism at low temperatures, a maximum in $\ensuremath{\chi}$ at about 30 K, and Curie-Weiss behavior at high temperatures. The observed temperature dependence of $\ensuremath{\chi}$ is discussed in light of interconfigurational fluctuation (ICF) as well as Fermi-liquid models. The mixed-valent state of Yb is also borne out in the ${L}_{\mathrm{III}}$-edge x-ray absorption spectra, which reveal a weak temperature dependence of the Yb mean valence $\overline{v}$ in agreement with the results from an analysis of the $\ensuremath{\chi}$ data in light of the ICF model. Valence-band photoemission spectra support our view of a homogeneously mixed-valent state of Yb; they further identify a valence transition to the divalent state in the outermost surface layer. $4f$ binding energies are found to be 0.80\ifmmode\pm\else\textpm\fi{}0.05 eV higher at the surface as compared to the bulk. All three methods suggest that---within the limits of accuracy---the room-temperature mean valence of Yb in Yb${\mathrm{Pd}}_{2}$${\mathrm{Si}}_{2}$ is close to the high-temperature limit given by the ionic ICF model.