We have fabricated $c$-axis Josephson junctions on single crystals of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ by using Pb as the counterelectrode in two geometries, planar and point contact. Junctions in both geometries show resistively shunted junction $I\mathrm{\text{−}}V$ curves below the $T_C$ of the counterelectrode. Microwave induced steps were observed in the $I\mathrm{\text{−}}V$ curves, and the critical currents are suppressed with an in-plane magnetic field with well-defined modulation periods indicating that the Josephson current is flowing in a manner consistent with the small to intermediate sized junction limit. $I_C{R}_N$ products of up to 0.3 mV have been observed in these junctions at 4.2 K. The observation of Josephson coupling along the $c$ axis between ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ and a conventional superconductor suggests the existence of an $s$-wave symmetry in this class of iron pnictide superconductors.

Seeing magnetism on an atomic level: Electrons tunneling from the magnetized tip of a scanning tunneling microscope into the surface of a material can reveal the material's magnetic structure. Although researchers have used the technique on simple nanostructures, they've had trouble preparing the tip in just the right way to visualize the magnetic order in more exotic materials. Enayat et al. used a tip with a magnetic cluster on its apex to reveal patterns of magnetic ordering in the material Fe _1+y Te, which becomes superconducting by substituting Te with Se atoms. The researchers prepared the tip by simply picking up atoms from the surface of the material. Science , this issue p. 653

We study the relationship between precipitate morphology and superconductivity in K${}_x$Fe${}_{1.6+y}$Se${}_2$ single crystals grown by self-flux method. Scanning electron microscopy (SEM) measurements revealed that the superconducting phase forms a network in the samples quenched above iron vacancy order-disorder transition temperature $T_s$, whereas it aggregates into micrometer-sized rectangular bars and aligns as disconnected chains in the furnace-cooled samples. Accompanying this change in morphology the superconducting shielding fraction is strongly reduced. By post-annealing above $T_s$ followed by quenching in room temperature water, the network recovers with a superconducting shielding fraction approaching 80$\%$ for the furnace-cooled samples. A reversible change from network to bar chains was realized by a secondary heat treatment in annealed samples showing a large shielding fraction, that is, heating above $T_s$ followed by slow cooling across $T_s$. The large shielding fraction observed in K${}_x$Fe${}_{1.6+y}$Se${}_2$ single crystals actually results from an uniform and contiguous distribution of superconducting phase. Through the measurements of temperature dependent x-ray diffraction, it is found that the superconducting phase precipitates while the iron vacancy ordered phase forms together by cooling across $T_s$ in K${}_x$Fe${}_{1.6+y}$Se${}_2$ single crystals. It is a solid solution above $T_s$, where iron atoms randomly occupy both Fe1 and Fe2 sites in the iron vacancy disordering status; and phase separation is driven by the iron vacancy order-disorder transition upon cooling. However, neither additional iron in the starting mixtures nor as-quenching at high temperatures can extend the miscibility gap to the KFe${}_2$Se${}_2$ side.

We report ${}^{75}$As nuclear magnetic resonance studies on an underdoped single-crystal Ba${}_{0.77}$K${}_{0.23}$Fe${}_2$As${}_2$ with $T_{\mathrm{c}}=16.5$ K. Below $T_{\mathrm{N}}=46$ K, the NMR peaks for $H\parallel c$ split and those for $H\parallel a$ shift to higher frequencies, which indicates that an internal magnetic field along the $c$ axis develops below $T_{\mathrm{N}}$. The spin-lattice relaxation rate $1/T_1$ measured at the shifted peak with $H\parallel a$, which experiences the internal field, shows a distinct decrease below $T_{\mathrm{c}}({\mu }_{0}H=12\mathrm{T})=16$ K, following a $T^3$ relation at low temperatures. Our results show unambiguously that antiferromagnetic order and superconductivity coexist microscopically. The unusual superconducting state with the coexisting magnetism is highlighted. DOI: http://dx.doi.org/10.1103/PhysRevB.86.180501 ©2012 American Physical Society

Large high-quality single crystals of hole-doped iron-based superconductor (Ba${}_{1$-${}x}$K${}_{x}$)Fe${}_{2}$As${}_{2}$ were grown over a broad composition range $0.22$\le${}x$\le${}1$ by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and an adjusted temperature window of the growth are necessary to obtain single crystals of heavily K-doped crystals ($0.65$\le${}x$\le${}0.92$) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the $c$-axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity $$\rho${}(T)$ found doping-independent, constant within error bars, resistivity at room temperature, $$\rho${}(300\phantom{\rule{0.28em}{0ex}}\mathrm{K})$, in sharp contrast with the significant doping dependence in electron and isovalent substituted BaFe${}_{2}$As${}_{2}$ based compositions. The shape of the temperature-dependent resistivity, $$\rho${}(T)$, shows systematic doping-evolution, being close to ${T}^{2}$ in overdoped and revealing significant contribution of the $T$-linear component at optimum doping. The slope of the upper critical field, $d{H}_{c2}/dT$, scales linearly with ${T}_{c}$ for both $H$\parallel${}c$, ${H}_{c2,c}$, and $H$\parallel${}ab$, ${H}_{c2,ab}$. The anisotropy of the upper critical field $$\gamma${}$\equiv${}{H}_{c2,ab}/{H}_{c2,c}$ determined near zero-field ${T}_{c}$ increases from $$\sim${}$2 to 4\char21{}5 with increasing K doping level from optimal $x$\sim${}0.4$ to strongly overdoped $x=1$.