Pressure-tuned quantum criticality in the antiferromagnetic Kondo semi-metal CeNi$_{2-\delta}$As$_2$

TL;DR

This study investigates how pressure and magnetic field induce quantum criticality in the semi-metallic Kondo-lattice compound CeNi$_{2- ext{ } ext{ extdelta}}$As$_2$, revealing unconventional behavior due to its low carrier density.

Contribution

It provides the first systematic transport and thermodynamic analysis of CeNi$_{2- ext{ extdelta}}$As$_2$ near quantum critical points, highlighting the role of low carrier density in quantum criticality.

Findings

Small carrier density leads to unique quantum critical signatures.

Delayed Kondo lattice coherence develops at low temperatures.

Unconventional, local-moment quantum criticality observed.

Abstract

The easily tuned balance among competing interactions in Kondo-lattice metals allows access to a zero-temperature, continuous transition between magnetically ordered and disordered phases, a quantum-critical point (QCP). Indeed, these highly correlated electron materials are prototypes for discovering and exploring quantum-critical states. Theoretical models proposed to account for the strange thermodynamic and electrical transport properties that emerge around the QCP of a Kondo lattice assume the presence of an indefinitely large number of itinerant charge carriers. Here, we report a systematic transport and thermodynamic investigation of the Kondo-lattice system CeNi$_{2-\delta}$As$_2$ ($\delta$$\thickapprox$0.28) as its antiferromagnetic order is tuned by pressure and magnetic field to zero-temperature boundaries. These experiments show that the very small but finite carrier density of $\sim$0.032 $e^-$/f.u. in CeNi$_{2-\delta}$As$_2$ leads to unexpected transport signatures of quantum criticality and the delayed development of a fully coherent Kondo lattice state with decreasing temperature. The small carrier density and associated semi-metallicity of this Kondo-lattice material favor an unconventional, local-moment type of quantum criticality and raise the specter of Nozi\`{e}res exhaustion idea that an insufficient number of conduction-electron spins to separately screen local moments requires collective Kondo screening.