Change of the effective spin degeneracy in CeNi _{9− x} Cu _x Ge ₄ due to the interplay between Kondo and crystal field effects

Abstract

Elastic and inelastic neutron scattering experiments were carried out on the heavyfermion systems CeNi8.6Cu0.4Ge4 and CeNi8CuGe4 to study i) the influences of Ni/Cu substitution on the crystal field parameters and to identify ii) the driving forces of quantum criticality in CeNi9−xCuxGe4. The relevance of competing RKKY and Kondo interactions and changes of the crystal field parameters is discussed. The crystallographic site where the Ni replacement by copper atoms takes place is identified by neutron powder diffraction studies. Furthermore, quasielastic and inelastic neutron scattering studies provide detailed information regarding the Kondo properties and the changes of the crystal field parameters resulting from the Ni/Cu replacement. Hence, a reduction of the effective spin degeneracy of the crystal field ground state with increasing Cu concentration is identified as one important control parameter of quantum criticality in CeNi9−xCuxGe4. The results of these experiments are complemented by measurements of the thermopower.

Evolution of quantum criticality in the system CeNi9Ge4

“…The quasi-elastic neutron data, thus, reveal that Ni/Cu substitution causes a significant reduction of the Kondo energy scale and also a reduction of the effective ground state degeneracy towards a doublet in the case of CeNi 8 CuGe 4 . The latter is also confirmed by inelastic neutron data measured recently with an incident neutron energy of E i = 20.5 meV at FRMII where an approximate CF scheme with doublets at ground state, 5 meV and 12 meV is observed for CeNi 8 CuGe 4 [16].…”

Influence of competing energy scales on the effective spin degeneracy in CeNi9−xFexGe4

“…mm point symmetry, respectively) [12]. This result was later confirmed by neutron powder diffraction experiments and subsequent Rietveld refinements which revealed that "on average each fourth Ni atom at the 16k sites is substituted by copper" [15]. In case of the CeNi 9−x Fe x Ge 4 solid solution series, our calculations suggest that substitution occurs preferentially at the 16l sites.…”

“…The latter observation is counter-intuitive in the framework of the standard Doniach picture [14]. These results clearly suggest that it is rather the reduction of the effective spin degeneracy N [12,15] away from the paramagnetic quasi-quartet ground state in CeNi 9 Ge 4 which controls the formation of AF order in both systems. To gain further inside, whether (i) the interplay between Kondo effect and RKKY interaction or (ii) a change of the effective spin degeneracy is the most relevant control parameter in fine-tuning the CeNi 9−x T x Ge 4 systems (T: Co, Cu), we performed specific heat studies of CeNi 9−x Fe x Ge 4 (x ≤ 1).…”

“…At least the AF order in the Cu series also suggests a reduction of the effective spin degeneracy. In this case a decreasing Kondo energy is attended by a strong (1) 7 - (2) 7 -splitting energy [15]. Summarizing, the Fe substitution series CeNi 9−x Fe x Ge 4 is one of the rare examples where a substitution driven increase of Fig.…”

Interplay between crystal field splitting and Kondo effect in CeNi9Ge4−xSix

“…Coleman pointed out that a large effective spin degeneracy N tends to stabilize the Kondo lattice ground state relative to a magnetically ordered one [6]. This means that, apart from the common tuning parameters such as (i) hydrostatic pressure [7], (ii) chemical pressure [8], or (iii) external magnetic field [9], also the change in effective spin degeneracy N by shifting the CF levels on an energy scale comparable to the Kondo energy might enforce a system towards a quantum critical point (QCP), as recently reported for CeNi 9−x Cu x Ge 4 [5,10,11]. This series of solid solutions represents a first benchmark system where a substitution driven quantum phase transition is not only linked with the changes of the relative strength of the Kondo effect and RKKY interaction, but also with a reduction of the effective crystal field ground state degeneracy.…”