Quantum criticality in NaFe_1-xCo_xAs

In iron-based superconductors, superconductivity emerges in the vicinity of magnetic and nematic orders whose fluctuations related to the quantum critical points (QCPs) are thought to be the medium for superconducting electron pairing. While the presence of magnetic QCP has been suggested by extensive studies, the direct evidence for a nematic QCP is lacked. Here we report transport and nuclear magnetic resonance (NMR) measurements on NaFe_1-xCo_xAs where the magnetic and nematic transitions are well separated. Non-Fermi liquid behavior is observed at x_M = 0.027 and x_c = 0.032 where the resistivity shows T-linear behavior. Furthermore, we observe two peaks in the doping dependence of zero-temperature London penetration depth at x_M and x_c. These results indicate two separate QCPs lie beneath the superconducting dome. By NMR measurements, we identify x_M to be a magnetic QCP and suggest x_c is a nematic QCP. Our results suggest the important role of quantum nematic fluctuations on various properties and superconductivity itself. (author)