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

TL;DR

The paper investigates Cu substitution in the itinerant antiferromagnet La2Ni7 to tune magnetic order toward a quantum critical point (QCP). Through synthesis of single crystals (0 ≤ x ≤ 0.181) and comprehensive measurements of M(T,H), ρ(T), and Cp(T), a T–x phase diagram is constructed, revealing AFM order persisting up to x ≈ 0.097 and disappearing at higher x, while Kondo-like behavior emerges for x ≥ 0.125; notably, x ≈ 0.105 lies near the AFM-QCP exhibiting non-Fermi-liquid transport signatures. The results show diverging $A$ and enhanced $C_p/T$ near the QCP, with Kadowaki–Woods scaling linking transport and thermodynamics, indicating a rich interplay between itinerant magnetism, low-dimensional kagome physics, and Kondo phenomena in a 3d system. The authors propose a double kagome-layer framework and discuss flat-band contributions from Ni-derived bands as a basis for local moment formation and exotic ground states, highlighting La2(Cu_x Ni_1-x)_7 as a promising platform for studying d-electron quantum criticality and kagome-band driven Kondo physics.

Abstract

$\text{La}_2 \text{Ni}_7$ 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.

Figures (29)

• Figure 1: (Color online) (a) The crystal structure of $\text{La}_2 \text{Ni}_7$ as generated by VESTA Momma2011iVESTA3/iData. The Ni4 and Ni5 planes are equivalent and have been marked by red arrows. (b) The Ni4/Ni5 plane along the $ab-$ plane which form a kagome lattice.
• Figure 2: (a) Measured Cu substitution level, ($x_{EDS}$) values, for ${\text{La}_{2}\text{(Cu}_{x}\text{Ni}_{1-x})_7}$ crystals versus $x_{nominal}$ values used in the growth solution. The black line corresponds to the linear fit across the data points with the intercept fixed to (0,0), and a slope of essentially 1, clearly showing that $x_{EDS}$$\sim$ with $x_{nominal}$. (Inset: typical crystals of ${\text{La}_{2}\text{(Cu}_{x}\text{Ni}_{1-x})_7}$ for $x_{EDS}$ = 0.105 on a mm grid.) (b) Powder X-Ray diffraction data for ${\text{La}_{2}\text{(Cu}_{x}\text{Ni}_{1-x})_7}$ for $x_{EDS} = 0.105$. The data for other $x$-values are qualitatively the same.
• Figure 3: Lattice parameters $a-$ on the top panel, and $c-$ on the bottom panel and the unit cell volume, $V$ (as an inset in the bottom panel) as a function of the Cu substitution level $(x_{EDS})$ in ${\text{La}_{2}\text{(Cu}_{x}\text{Ni}_{1-x})_7}$ single crystals obtained from the refinement of the powder x-ray diffraction data.
• Figure 4: (Color online) Anisotropic temperature dependent magnetization ($M(T)$) measured at $H$ = 100 Oe for ${\text{La}_{2}\text{(Cu}_{x}\text{Ni}_{1-x})_7}$ single crystals. $M(T)$ for each Cu substituted sample is plotted in separate panels for both $H||c$ and $H \perp c$. Although measurements were done up to $T = 300$ K, the data here is plotted only up to T$= 120$ K to highlight the transition related features. The samples were measured for both ZFC-FC mode for 0 $\leq x \leq$ 0.125 and only in ZFC mode for $x$ = 0.152 and 0.181. Some of the panels (0.097 $\leq x \leq$ 0.181) have insets to show the low temperature behavior in greater detail. [NOTE: The $y$ axis scale varies from panel to panel.]
• Figure 5: Temperature dependent, zero field, in-plane resistivity on the left axis and its corresponding temperature-derivative on the right axis of ${\text{La}_{2}\text{(Cu}_{x}\text{Ni}_{1-x})_7}$ single crystals plotted in separate panels. The colored curves show the $\rho(T)$ data and the black curves the corresponding $\frac{d \rho}{dT}$. $\rho(T)$ for $0 \leq x \leq 0.097$ are plotted starting above their respective magnetic ordering temperatures down to 1.8 K whereas for the higher doped samples, $0.105 \leq x \leq 0.181$, which do not show magnetic ordering, resistivity is plotted from 20 K down to 1.8 K. The magnetic ordering temperatures determined from maxima in the derivative data are shown in each panel using arrows.