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The physics of quantum critical phase transitions connects to some of the most difficult problems in condensed matter physics, including metal-insulator transitions, frustrated magnetism and high-temperature superconductivity. Near a quantum critical point, a new kind of metal emerges, the thermodynamic and transport properties of which do not fit into the unified phenomenology for conventional metals-the Landau Fermi-liquid theory-characterized by a low-temperature limiting T-linear specific heat and a T 2 resistivity 1 . Studying the evolution of the temperature dependence of these observables as a function of a control parameter leads to the identification of both the presence and the nature of the quantum phase transition in candidate systems. In this study we measure the transport properties of BaFe 2 (As 1−x P x ) 2 below the critical temperature T c by suppressing superconductivity with high magnetic fields. At sufficiently low temperatures, the resistivity of all compositions (x 0.31) crosses over from a linear to a quadratic temperature dependence, consistent with a low-temperature Fermi-liquid ground state. As compositions with optimal T c are approached from the overdoped side, this crossover becomes steeper, consistent with models of quantum criticality where the effective Fermi temperature T F goes to zero.The iron-based superconductors are part of a family of unconventional superconductors that exhibit several competing orders. The parent material BaFe 2 As 2 is a tetragonal paramagnet at high temperature and becomes an orthorhombic metallic antiferromagnet at ∼140 K (ref. 2). As the material is electron doped, hole doped or isovalently substituted this transition is rapidly suppressed, giving rise to superconductivity. In this work, we attempt to understand the nature of the low-T metallic state of the Fe-based superconductor BaFe 2 (As 1−x P x ) 2 by suppressing the superconductivity in a high magnetic field. Even though BaFe 2 (As 1−x P x ) 2 is isovalently substituted, we will describe the chemical composition-temperature (x-T ) phase diagram using language commonly applied to electron/hole-doped compounds, namely 'underdoped' refers to materials that exhibit a structural/magnetic instability, and 'overdoped' for paramagnetic compounds that do not. For this material, the maximum T c (optimal doping) occurs at x = 0.30.BaFe 2 (As 1−x P x ) 2 is a multi-band compound with both electronand hole-like carriers and the magnetoresistance is therefore a sum of contributions from all Fermi surfaces. In Fig. 1 we illustrate the magnetoresistance as a function of temperature and field for a range of compositions from x = 0.31 to x = 0.73 and T c spanning 29.5 K to 0 K. For all temperatures measured, a quadratic magnetoresistance fit captures most of the data and the intercept ρ 0,T is extrapolated (shown by black lines in Fig. 1). At low fields, this fit deviates from the quadratic dependence in the near-optimally doped samples, even at temperatures T > T c , although the deviation ostensibly disappears a...

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 (α-pocket) and 15.6 T (β-pocket). The analysis, including a Landau fan plot, shows that the β-pocket has a Berry phase of π and a small effective mass ∼0.033 m0, indicative of a nontrivial topology in momentum space; whereas the α-pocket has a trivial Berry phase of 0 and a heavier effective mass ∼0.066 m0. From the effective mass and the β-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.