Heavy fermion quantum criticality at dilute carrier limit in CeNi_2-δ(As_1-xP_x)₂

We study the quantum phase transitions in the nickel pnctides, CeNi_2-δ(As_1-xP_x)₂ (δ ≈ 0.07-0.22) polycrystalline samples. This series displays the distinct heavy fermion behavior in the rarely studied parameter regime of dilute carrier limit. We systematically investigate the magnetization, specific heat and electrical transport down to low temperatures. Upon increasing the P-content, the antiferromagnetic order of the Ce-4f moment is suppressed continuously and vanishes at x_c ~ 0.55. At this doping, the temperature dependences of the specific heat and longitudinal resistivity display non-Fermi liquid behavior. Both the residual resistivity ρ₀ and the Sommerfeld coefficient γ₀ are sharply peaked around x_c. When the P-content reaches close to 100%, we observe a clear low-temperature crossover into the Fermi liquid regime. In contrast to what happens in the parent compound x = 0.0 as a function of pressure, we find a surprising result that the non-Fermi liquid behavior persists over a nonzero range of doping concentration, x_c < x < 0.9. In this doping range, at the lowest measured temperatures, the temperature dependence of the specific-heat coefficient is logarithmically divergent and that of the electrical resistivity is linear. We discuss the properties of CeNi_2-δ(As_1-xP_x)₂ in comparison with those of its 1111 counterpart, CeNi(As_1-xP_x)O. Our results indicate a non-Fermi liquid phase in the global phase diagram of heavy fermion metals.