Quantum criticality in electron-doped BaFe_2-xNi _xAs₂

A quantum critical point is a point in a system's phase diagram at which an order is completely suppressed at absolute zero temperature (T). The presence of a quantum critical point manifests itself in the finite-T physical properties, and often gives rise to new states of matter. Superconductivity in the cuprates and in heavy fermion materials is believed by many to be mediated by fluctuations associated with a quantum critical point. In the recently discovered iron-pnictide superconductors, we report transport and NMR measurements on BaFe_2-xNi_xAs₂ (0≤x≤0.17). We find two critical points at x_c1 =0.10 and x_c2 =0.14. The electrical resistivity follows ρ=ρ 0 +ATⁿ, with n=1 around x_c1 and another minimal n=1.1 at x_c2. By NMR measurements, we identity x_c1 to be a magnetic quantum critical point and suggest that x_c2 is a new type of quantum critical point associated with a nematic structural phase transition. Our results suggest that the superconductivity in carrier-doped pnictides is closely linked to the quantum criticality.