Physical properties of the ferromagnetic quantum critical system YbNi$_4$(P$_{1-x}$As$_{x}$)$_2$

TL;DR

This study investigates the evolution of magnetic, electronic, and structural properties in the ferromagnetic quantum critical system YbNi$_4$(P$_{1-x}$As$_{x}$)$_2$ across arsenic substitution, revealing a crossover from non-Fermi liquid to Fermi liquid behavior and potential quantum critical region.

Contribution

First comprehensive analysis of physical properties across the entire substitution range in YbNi$_4$(P$_{1-x}$As$_{x}$)$_2$, highlighting the nature of quantum criticality and valence changes.

Findings

Quantum critical point at x ≈ 0.1.

Transition from non-Fermi liquid to Fermi liquid behavior.

Evidence for a quantum critical region rather than a point.

Abstract

We report on single crystal growth and physical properties of the quantum critical Kondo-lattice system YbNi$_4$(P$_{1-x}$As$_{x}$)$_2$ with $0\leq x\leq 1$ which hosts a ferromagnetic quantum critical point at $x \approx 0.1$. We performed measurements of the magnetization, electrical resistivity, thermopower, heat capacity, and resonant X-ray emission spectroscopy. Arsenic substitution leads to a homogeneous increase of the unit-cell volume, with well-defined As-concentrations in large parts of the single crystals. All data consistently show that with increasing $x$ the Kondo temperature increases, while the magnetic anisotropy observed at low $x$ fully vanishes towards $x=1$. Consequently, at low temperatures, the system shows a crossover from pronounced non-Fermi liquid behaviour for $x \leq 0.2$ to a Fermi liquid behavior for $x > 0.2$ with weak correlations. There is a continuous change in Yb valence from nearly trivalent at low $x$ to a slightly lower value for $x = 0.6$, which correlates with the Kondo temperature. Interestingly, specific heat measurements at very low temperatures show that $C/T$ strongly increases towards lower $T$ for $x = 0.13$ and $x = 0.2$ with a very similar power law. This suggests that in YbNi$_4$(P$_{1-x}$As$_{x}$)$_2$ a quantum critical region rather than a quantum critical point might exist.