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

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 $0\le x\le 1$ 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 $x\le 0.2$ 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.