Kondo Lattice Behavior in the Valence Fluctuating Systems α- and β-YbAlB4

Abstract

We have measured specific heat and magnetization of the Yb-based heavy Fermion systems α-YbAlB 4 and β-YbAlB 4 . α-YbAlB 4 is a locally isostructural polymorph of the first example of an Yb-based heavy Fermion superconductor β-YbAlB 4 , which exhibits pronounced non-Fermi-liquid behavior above the superconducting transition temperature T c = 80 mK. The results reveal a Fermi liquid ground state for α-YbAlB 4 in contrast to the quantum criticality found for β-YbAlB 4 . Interestingly, both systems exhibit Kondo lattice behavior with its characteristic temperature scale T * ∼8 K, despite of their strongly intermediate valence with the temperature scale of ∼ 200 K. We discuss the origin of this unusual behavior, making a comparison with the typical mixed valent compound YbAl 3 . Large Wilson ratios indicate the existence of ferromagnetic correlation as a possible origin for the Kondo lattice behavior.