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A quantum critical point (QCP) is a point in a system's phase diagram atwhich an order is completely suppressed at absolute zero temperature (T). Thepresence of a quantum critical point manifests itself in the finite-T physicalproperties, and often gives rise to new states of matter. Superconductivity inthe cuprates and in heavy fermion materials is believed by many to be mediatedby fluctuations associated with a quantum critical point. In therecently-discovered iron-pnictide high temperature superconductors, it isunknown whether a QCP exists or not in a carrier-doped system. Here we reporttransport and nuclear magnetic resonance (NMR) measurements onBaFe_{2-x}Ni_xAs_2 (0 =< x =< 0.17). We find two critical points at x_{c1} =0.10 and x_{c2} = 0.14. The electrical resistivity follows \rho = \rho_0 +A*T^n, with n = 1 around x_{c1} and another minimal n = 1.1 at x_{c2}. By NMRmeasurements, we identity x_{c1} to be a magnetic QCP and suggest that x_{c2}is a new type of QCP associated with a nematic structural phase transition. Ourresults suggest that the superconductivity in carrier-doped pnictides isclosely linked to the quantum criticality.

Quantum Criticality in Electron-doped BaFe_{2-x}Ni_xAs_2