Evolution of Quantum Criticality in CeNi_{9-x}Cu_xGe_4

TL;DR

This study investigates how chemical substitution in CeNi_{9-x}Cu_xGe_4 tunes its ground state from non-magnetic to magnetically ordered, revealing a quantum critical point linked to changes in crystal field degeneracy and Kondo-RKKY balance.

Contribution

It demonstrates a substitution-driven quantum phase transition involving both Kondo-RKKY competition and crystal field degeneracy reduction, a novel finding in heavy fermion systems.

Findings

Quantum critical behavior observed at x ~ 0.4.

Ground state evolves from non-magnetic to magnetic with Cu substitution.

First system linking substitution-driven quantum phase transition with crystal field degeneracy change.

Abstract

Crystal structure, specific heat, thermal expansion, magnetic susceptibility and electrical resistivity studies of the heavy fermion system CeNi_{9-x}Cu_xGe_4 (0 <= x <= 1) reveal a continuous tuning of the ground state by Ni/Cu substitution from an effectively fourfold degenerate non-magnetic Kondo ground state of CeNi_9Ge_4 (with pronounced non-Fermi-liquid features) towards a magnetically ordered, effectively twofold degenerate ground state in CeNi_8CuGe_4 with T_N = 175 +- 5 mK. Quantum critical behavior, C/T ~ \chi ~ -ln(T), is observed for x about 0.4. Hitherto, CeNi_{9-x}Cu_xGe_4 represents the first system where a substitution-driven quantum phase transition is connected not only with changes of the relative strength of Kondo effect and RKKY interaction, but also with a reduction of the effective crystal field ground state degeneracy.