Electronic density of state in valence fluctuating kondo lattice systems studied by point-contact spectroscopy (Review article)

Abstract

This article reviews our recent point-contact spectroscopy (PCS) investigation in valence fluctuating Kondo lattice systems, EuNi2P2 and Ge-substituted EuNi2(P0.8Ge0.2)2, and YbPd. A heavy fermion (HF) compound EuNi2P2 exhibits a significant valence fluctuation owing to the intermediate Eu valence of 2.5 + at low temperatures, leading to a strong hybridization between the conduction and f electrons (c–f hybridization). The electronic density of state measurements on EuNi2P2 by using PCS technique reveal that the HF behavior arises from the formation of the indirect hybridization gap with a double-peak structure near the Fermi level due to c–f hybridization. In EuNi2(P0.8Ge0.2)2, the valence fluctuation is suppressed by the Ge substitution, and the separation of the double-peak structure decreases. These suggest that the HF behavior is induced by the valence fluctuation. In YbPd, where two-dimensional (2D) layers of Yb3+ and Yb2.6+ are alternately aligned, a HF-like behavior with a large Sommerfeld coefficient appears at low temperatures in the specific heat. The PCS spectra represent a zero-bias dip structure with an asymmetric background, reproduced by the summation of two Fano curves with two different characteristic temperatures, meaning the formation of two Kondo resonance states for each Yb3+ and Yb2.6+ site. These suggest that a local coherence causes the large Sommerfeld coefficient in the 2D layers of Yb3+, which are screened by the conduction electrons provided by the valence fluctuating Yb2.6+ sites. The observed results indicate that PCS is a powerful technique for examining the variation of electronic DOS in valence fluctuating Kondo lattice systems.