Heterogeneous Yb³⁺-Yb²⁺ mixed valency and unusual Kondo ground state in Yb₄As₃

Abstract

We present 170 Yb Mossbauer spectroscopy data in the temperature range 0.045-50 R and magnetic susceptibility and isothermal magnetisation measurements in the temperature range 0.6-50 R in the heavy fermion compound Yb 4 As 3 . The presence of ∼20% of Yb 3+ ions is detected in the Mossbauer spectra with an applied magnetic field of 7 T, yielding a microscopic proof of the existence of a heterogeneous mixed valence state containing Yb 3+ and Yb 2+ ions. No magnetic ordering of the Yb 3+ sublattice (with moments bigger than 0.15 μ B ) is detected down to 0.045 R. The low temperature upturn of the magnetic susceptibility is shown to be an intrinsic feature of Yb 4 As 3 , suggesting that the ground state is of unconventional Rondo lattice type