Strong valence fluctuation in the quantum critical heavy fermion superconductor beta-YbAlB4: A hard x-ray photoemission study

TL;DR

This study uses hard x-ray photoemission spectroscopy to reveal strong valence fluctuations in the quantum critical heavy fermion superconductor beta-YbAlB4, indicating a need for new quantum critical models.

Contribution

It provides the first direct evidence of strong mixed valence at quantum criticality in beta-YbAlB4, challenging traditional models.

Findings

Yb valence ~2.73 in alpha-YbAlB4 and ~2.75 in beta-YbAlB4

Presence of Yb2+ peaks at the Fermi level

Establishment of strong valence fluctuations at quantum criticality

Abstract

Electronic structures of the quantum critical superconductor beta-YbAlB4 and its polymorph alpha-YbAlB4 are investigated by using bulk-sensitive hard x-ray photoemission spectroscopy. From the Yb 3d core level spectra, the values of the Yb valence are estimated to be ~2.73 and ~2.75 for alpha- and beta-YbAlB4, respectively, thus providing clear evidence for valence fluctuations. The valence band spectra of these compounds also show Yb2+ peaks at the Fermi level. These observations establish an unambiguous case of a strong mixed valence at quantum criticality for the first time among heavy fermion systems, calling for a novel scheme for a quantum critical model beyond the conventional Doniach picture in beta-YbAlB4.