Via angular Shubnikov-de Hass (SdH) quantum oscillations measurements, we determine the Fermi surface topology of NbAs, a Weyl semimetal candidate. The SdH oscillations consist of two frequencies, corresponding to two Fermi surface extrema: 20.8 T ($
alpha$-pocket) and 15.6 T ($beta$-pocket). The analysis, including a Landau fan plot, shows that the $beta$-pocket has a Berry phase of $pi$ and a small effective mass $sim$0.033 $m_0$, indicative of a nontrivial topology in momentum space; whereas the $alpha$-pocket has a trivial Berry phase of 0 and a heavier effective mass $sim$0.066 $m_0$. From the effective mass and the $beta$-pocket frequency we determine that the Weyl node is 110.5 meV from the chemical potential. A novel electron-hole compensation effect is discussed in this system, and its impact on magneto-transport properties is addressed. The difference between NbAs and other monopnictide Weyl semimetals is also discussed.
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 correlat
ed 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.
A quantum critical point arises at a continuous transformation between distinct phases of matter at zero temperature. Studies in antiferromagnetic heavy fermion materials have revealed that quantum criticality has several classes, with an unconventio
nal type that involves a critical destruction of the Kondo entanglement. In order to understand such varieties, it is important to extend the materials basis beyond the usual setting of intermetallic compounds. Here we show that a nickel oxypnictide, CeNiAsO, displays a heavy-fermion antiferromagnetic quantum critical point as a function of either pressure or P/As substitution. At the quantum critical point, non-Fermi liquid behavior appears, which is accompanied by a divergent effective carrier mass. Across the quantum critical point, the low-temperature Hall coefficient undergoes a rapid sign change, suggesting a sudden jump of the Fermi surface and a destruction of the Kondo effect. Our results imply that the enormous materials basis for the oxypnictides, which has been so crucial to the search for high temperature superconductivity, will also play a vital role in the effort to establish the universality classes of quantum criticality in strongly correlated electron systems.
