Evolution of quantum criticality in CeNi_9−xCu_xGe₄

Abstract

Crystal structure, specific heat, thermal expansion, magnetic susceptibility and electrical resistivity studies of the heavy fermion system CeNi(9-x)Cu(x)Ge(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(9)Ge(4) (with pronounced non-Fermi-liquid features) towards a magnetically ordered, effectively twofold-degenerate ground state in CeNi(8)CuGe(4) with T(N) = 175 ± 5 mK. Quantum critical behavior, [Formula: see text], is observed for [Formula: see text]. Hitherto, CeNi(9-x)Cu(x)Ge(4) represents the first system where a substitution-driven quantum phase transition is connected not only with 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.

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

“…For CeNi 8.8 Fe 0.2 Ge 4 the upper maximum increases and remains at the same temperature while the Kondo-type anomaly decreases as it is also observed in the homologous CeNi 8.8 T 0.2 Ge 4 (T: Co, Cu) [12,13]. This common behavior at lowest level of substitution is most likely a consequence of disorder introduced in the pseudo-ternary solid solution.…”

“…m and m . 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].…”

“…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).…”

“…Indeed, no magnetic order is observed in CeNi 9 Ge 4 down to the lowest experimentally reached temperatures (35 mK). Subsequent substitution of Ni by the neighboring transition metals, Cu and Co, causes in both cases a reduction of the effectively four-fold degeneracy of the Kondo ground state toward an effectively two-fold one and long range antiferromagnetic (AF) order is observed with T N = 0.2 K and 1 K for CeNi 8 CuGe 4 and CeNi 8 CoGe 4 , respectively [12,13]. In case of Cu doping a tendency toward AF order is indeed expected due to an increase of the 3d-electron count.…”

“…6. The entropy temperature ratio ˛ = T R ln 4 /T R ln 2 with respect to the Fe, Co [13] and Cu [12] amount x.…”