The normal-state charge transport is studied systematically in high-quality single crystals of ${\text{BaFe}}_2{\left({\text{As}}_{1-x}{\text{P}}_x\right)}_2$ $\left(0\le x\le 0.71\right)$. By substituting isovalent P for As, the spin-density-wave (SDW) state is suppressed and the dome-shaped superconducting phase $\left(T_c\lesssim 31\text{K}\right)$ appears. Near the SDW end point $\left(x\approx 0.3\right)$, we observe striking linear temperature $\left(T\right)$ dependence of resistivity in a wide $T$ range, and remarkable low-$T$ enhancement of Hall-coefficient magnitude from the carrier number estimates. We also find that the magnetoresistance apparently violates the Kohler’s rule and is well scaled by the Hall angle ${\Theta }_H$ as $\Delta {\rho }_{xx}/{\rho }_{xx}\propto {\text{tan}}^2{\Theta }_H$. These non-Fermi-liquid transport anomalies cannot be attributed to the simple multiband effects. These results capture universal features of correlated electron systems in the presence of strong antiferromagnetic fluctuations.

A Spike Inside the Dome: The transition temperature T _c of iron-based superconductors has a dome-shaped dependence on chemical doping, and the superconductivity that develops underneath may obscure a potential quantum critical point (QCP) residing at absolute zero. With the aim of detecting signatures of this quantum criticality, Hashimoto et al. (p 1554 ; see the Perspective by Sachdev ) measured the penetration depth of the pnictide series BaFe ₂ (As _{1− x} P _x ) ₂ as a function of x. A sharp peak right around the point where T _c has a maximum ( x = 0.30) was observed, implying that the superfluid density diminishes sharply where one would expect it to be the most robust. This unusual finding is interpreted as a sign of a QCP at x = 0.30.

An Eight-Noded Monster: In superconductors, electrons are bound into pairs, and the exact form of that pairing and the resulting energy gap can vary, depending on the details of the electron-electron interaction and the band structure of the material. The energy gaps of the recently discovered iron-based superconductors exhibit a variety of pairing functions. KFe ₂ As ₂ has been suggested to have a d -wave gap, similar to cuprate superconductors. Okazaki et al. (p. 1314 ) use laser-based angle-resolved photoemission spectroscopy (ARPES) to map out the superconducting gap on three Fermi surfaces (FS) of the compound. They find a different gap structure on each, with the middle FS gap vanishing at eight distinct positions (nodes). It appears that the gap respects the tetragonal symmetry of the crystal, indicating (although the details may vary) the all iron-based superconductors have an extended s -wave–symmetric pairing—a finding that will help understanding of unconventional superconductivity.

We report magnetic penetration depth and thermal-conductivity data for high-quality single crystals of ${\text{BaFe}}_2{\left({\text{As}}_{1-x}{\text{P}}_x\right)}_2\left(T_c=30\text{K}\right)$ which provide strong evidence that this material has line nodes in its energy gap. This is distinctly different from the nodeless gap found for $\left(\text{Ba},\text{K}\right){\text{Fe}}_2{\text{As}}_2$ which has similar $T_c$ and phase diagram. Our results indicate that repulsive electronic interactions play an essential role for Fe-based high-$T_c$ superconductivity but that uniquely there are distinctly different pairing states, with and without nodes, which have comparable $T_c$.

Using the de Haas–van Alphen effect we have measured the evolution of the Fermi surface of ${\mathrm{BaFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ as a function of isoelectric substitution ($\mathrm{As}/\mathrm{P}$) for $0.41<x<1$ ($T_c$ up to 25 K). We find that the volumes of electron and hole Fermi surfaces shrink linearly with decreasing $x$. This shrinking is accompanied by a strong increase in the quasiparticle effective mass as $x$ is tuned toward the maximum $T_c$. These results are not explained by simple band structure calculations, and it is likely that these trends originate from the same many-body interactions which give rise to superconductivity.