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 second-order phase transition is characterized by spontaneous symmetry breaking. The nature of the broken symmetry in the so-called “hidden-order” phase transition in the heavy-fermion compound URu₂Si₂, at transition temperature T_h = 17.5 K, has posed a long-standing mystery. We report the emergence of an in-plane anisotropy of the magnetic susceptibility below T_h, which breaks the four-fold rotational symmetry of the tetragonal URu₂Si₂. Two-fold oscillations in the magnetic torque under in-plane field rotation were sensitively detected in small pure crystals. Our findings suggest that the hidden-order phase is an electronic “nematic” phase, a translationally invariant metallic phase with spontaneous breaking of rotational symmetry.

In-plane microwave penetration depth ${\lambda }_{ab}$ and quasiparticle conductivity at 28 GHz are measured in underdoped single crystals of the Fe-based superconductor ${\mathrm{PrFeAsO}}_{1-y}$ ($T_c\approx 35\mathrm{K}$) by using a sensitive superconducting cavity resonator. ${\lambda }_{ab}(T)$ shows flat dependence at low temperatures, which is incompatible with the presence of nodes in the superconducting gap $\Delta (\mathbf{k})$. The temperature dependence of the superfluid density demonstrates that the gap is nonzero ($\Delta /k_B{T}_c\gtrsim 1.6$) all over the Fermi surface. The microwave conductivity below $T_c$ exhibits an enhancement larger than the coherence peak, reminiscent of high-$T_c$ cuprate superconductors.

Among the iron-based pnictide superconductors the material ${\text{KFe}}_2{\text{As}}_2$ is unusual in that its Fermi surface does not consist of quasinested electron and hole pockets. Here we report measurements of the temperature dependent London penetration depth of very clean crystals of this compound with residual resistivity ratio $>1200$. We show that the superfluid density at low temperatures exhibits a strong linear-in-temperature dependence which implies that there are line nodes in the energy gap on the large zone-centered hole sheets. The results indicate that ${\text{KFe}}_2{\text{As}}_2$ is an unconventional superconductor with strong electron correlations.

We report high-sensitivity microwave measurements of the in-plane penetration depth ${\lambda }_{ab}$ and quasiparticle scattering rate $1/\tau$ in several single crystals of the hole-doped Fe-based superconductor ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($x\approx 0.55$). While a power-law temperature dependence of ${\lambda }_{ab}$ with a power $\sim 2$ is found in crystals with large $1/\tau$, we observe an exponential temperature dependence of the superfluid density consistent with the existence of fully opened two gaps in the cleanest crystal we studied. The difference may be a consequence of different levels of disorder inherent in the crystals. We also find a linear relation between the low-temperature scattering rate and the density of quasiparticles, which shows a clear contrast to the case of $d$-wave cuprate superconductors with nodes in the gap. These results demonstrate intrinsically nodeless order parameters in the Fe arsenides.

Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in ${\text{PrFeAsO}}_{1-y}$ and NdFeAs(O,F) superconducting single crystals. In spite of spatial fluctuations of the critical current density on the macroscopic scale, it is shown that the major contribution comes from collective pinning of vortex lines by microscopic defects by the mean-free-path fluctuation mechanism. The defect density extracted from experiment corresponds to the dopant atom density, which means that dopant atoms play an important role both in vortex pinning and in quasiparticle scattering. In the studied underdoped ${\text{PrFeAsO}}_{1-y}$ and NdFeAs(O,F) crystals, there is a background of strong pinning, which we attribute to spatial variations in the dopant atom density on the scale of a few dozen to 100 nm. These variations do not go beyond 5%—we therefore do not find any evidence for coexistence of the superconducting and the antiferromagnetic phase. The critical current density in sub-tesla fields is characterized by the presence of a peak effect, the location of which in the $\left(B,T\right)$ plane is consistent with an order-disorder transition of the vortex lattice.