Quantum critical point followed by Kondo-like behavior due to Cu substitution in itinerant, antiferromagnet ${ ext{La}_{2} ext{(Cu}_{x} ext {Ni}_{1-x})_7}$

$\text{La}2 \text{Ni}7$ is an itinerant magnetic system with a small ordered moment of $\sim$ 0.1 $μ{B}/\text{Ni}$ and a series of antiferromagnetic (AFM) transitions at $T_1$ = 61.0 K, $T_2$ = 56.5 K and $T_3$ = 42.2 K. $M(H)$, and $ρ(H)$ isotherms as well as constant field $M(T)$ and $ρ(T)$ measurements on single crystalline samples manifest a complex, anisotropic $H-T$ phase diagram with multiple phase lines. Here we present the growth and characterization of single crystals of the ${\text{La}{2}\text{(Cu}{x}\text {Ni}{1-x})7}$ series for 0 $\leq x \leq$ 0.181. We measured powder x-ray diffraction, and composition, as well as anisotropic $R(H,T)$, $M(H,T)$, and $C_p(T)$ on these single crystals. Using the measured data, we infer a $(T-x)$ phase diagram to study the evolution of the AFM ordering upon Cu substitution. For ${0 \leq x \leq 0.097}$, the system remains magnetically ordered at base temperature with $x \leq$ 0.012, showing signs of multiple AFM ordering temperatures. For the higher substitution levels, ${0.125 \leq x \leq 0.181}$, there are no signatures of magnetic ordering, but anomalous features in $R(T)$ and $C_p(T)$ data are observed which are consistent with the Kondo effect in this system. The intermediate $x$ = 0.105 sample lies between the magnetic ordered and the Kondo regime and is in the vicinity of the AFM-quantum critical point (QCP). Thus, ${\text{La}{2}\text{(Cu}{x}\text {Ni}{1-x})7}$ is an example of a small moment system that can be tuned through a QCP. Given these data combined with the fact that the $\text{La}2 \text{Ni}7$ structure has kagome-like, Ni-sublattices running perpendicular to the crystallographic $c$ axis, and a predicted $3d$-electron flat band that contributes to the density of states near the Fermi energy, ${\text{La}{2}\text{(Cu}{x}\text {Ni}{1-x})_7}$ becomes a promising system to host and study exotic physics.

💡 Research Summary

The authors report a systematic study of the La₂(Ni₁₋ₓCuₓ)₇ series (0 ≤ x ≤ 0.181), focusing on how Cu substitution tunes the itinerant antiferromagnetism of La₂Ni₇ and drives the system through a quantum critical point (QCP) into a Kondo‑like regime. High‑quality single crystals were grown by a self‑flux method, and the actual Cu content (x_EDS) was verified by energy‑dispersive spectroscopy to follow the nominal composition linearly. Powder X‑ray diffraction and Rietveld refinement show a monotonic contraction of both a (=b) and c lattice parameters with increasing x, reflecting electron doping and modest chemical pressure from Cu substitution.

Magnetization measurements (M(T) and M(H)) were performed with the magnetic field parallel and perpendicular to the crystallographic c‑axis. For x ≤ 0.012 the three well‑known antiferromagnetic (AFM) transitions of the parent compound (T₁ ≈ 61 K, T₂ ≈ 56.5 K, T₃ ≈ 42.2 K) remain clearly visible, although their temperatures decrease gradually as Cu content rises. In the intermediate range 0.02 ≤ x ≤ 0.097 the transition peaks broaden and shift to lower temperatures, indicating a weakening of the ordered moment and increased disorder. At x = 0.105 the signatures of long‑range AFM order disappear; instead, the susceptibility shows a nearly linear temperature dependence and a reduced effective moment, consistent with proximity to a continuous suppression of the order parameter.

Resistivity ρ(T) measured in the basal plane (I ⊥ c) mirrors the magnetic evolution. For low Cu concentrations the metallic behavior is punctuated by sharp drops at the AFM transitions. At x = 0.105 the low‑temperature resistivity follows a non‑Fermi‑liquid (NFL) power law ρ ∝ Tⁿ with 0 < n < 1, and the temperature derivative dρ/dT exhibits a logarithmic divergence. This NFL behavior is a hallmark of quantum critical fluctuations. For higher Cu levels (x ≥ 0.125) the resistivity develops an upturn at the lowest temperatures, and the heat capacity divided by temperature, Cₚ/T, shows a -ln T increase, both characteristic of Kondo‑type spin‑screening. The estimated Kondo temperature is on the order of 5 K.

Heat‑capacity measurements confirm the magnetic phase diagram. The three AFM peaks in Cₚ/T vanish as x approaches 0.105, where instead a logarithmic temperature dependence emerges. In the Kondo regime the low‑temperature Cₚ/T continues to rise, indicating a large quasiparticle mass enhancement due to spin‑screening.

Combining the transition temperatures with Cu concentration yields a T‑x phase diagram in which the Néel temperature extrapolates to zero at x ≈ 0.105, defining a quantum critical point. The vicinity of this point is marked by NFL signatures in both transport and thermodynamics, while beyond it the system crosses over to a Kondo‑like heavy‑fermion state.

Structurally, La₂Ni₇ crystallizes in the hexagonal P6/mmm space group, featuring kagome‑like Ni₄/Ni₅ layers stacked perpendicular to the c‑axis. Density‑functional calculations on the parent compound predict a flat Ni‑3d band near the Fermi level, which can enhance electronic correlations and magnetic instability. Cu substitution modifies the band filling and the Stoner factor, thereby moving the system from a weak‑itinerant antiferromagnet toward a regime where local moments are screened by conduction electrons.

In summary, La₂(Ni₁₋ₓCuₓ)₇ provides a rare example of a small‑moment itinerant antiferromagnet that can be tuned continuously from long‑range order through a quantum critical point into a Kondo‑dominated heavy‑fermion state. The coexistence of kagome geometry, flat‑band electronic structure, and controllable electron doping makes this family an attractive platform for exploring exotic phenomena such as unconventional superconductivity, topological quantum states, or spin‑liquid behavior. Future work involving neutron diffraction, angle‑resolved photoemission spectroscopy, high‑pressure studies, and detailed theoretical modeling will be essential to elucidate the interplay between spin fluctuations, band topology, and Kondo physics in this system.