The powder-in-tube process has been employed to fabricate tape-like conductors with a strong metallic sheath and based on the newly discovered ${\mathrm{MgB}}_{\mathrm{2}}$ superconducting phase. Long superconducting tapes have been prepared by packing reacted ${\mathrm{MgB}}_{\mathrm{2}}$ powders inside pure Ag, Cu, and Ni tubes which are then cold worked by drawing and rolling. Such tapes have shown transport properties as good as bulk ${\mathrm{MgB}}_{\mathrm{2}}$ samples sintered in high pressure and high temperature conditions. At 4.2 K, the highest critical current density of ${10}^5 {\mathrm{A/cm}}^{\mathrm{2}}$ has been achieved on nickel-sheathed single-filament conductors. A direct correlation between the sheath tensile strength and the critical current of the unsintered tape has been observed.

We investigated the effect of alloying on the upper critical field $H_{c2}$ for 12 $\mathrm{Mg}{\mathrm{B}}_2$ films, in which disorder was introduced by growth, carbon doping or He-ion irradiation, finding a significant $H_{c2}$ enhancement in C-alloyed films, and an anomalous upward curvature of $H_{c2}\left(T\right)$. Record high values of $H_{c2}{}^{\perp }\left(4.2\right)\approx 35\mathrm{T}$ and $H_{c2}{}^{\Vert }\left(4.2\right)\approx 51\mathrm{T}$ were observed perpendicular and parallel to the $ab$ plane, respectively. The temperature dependence of $H_{c2}\left(T\right)$ is described well by a theory of dirty two-gap superconductivity. Extrapolation of the experimental data to $T=0$ suggests that $H_{c2}{}^{\Vert }\left(0\right)$ may approach the paramagnetic limit of $\sim 70\mathrm{T}$.

Epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by pulsed laser ablation deposition on different substrates. High purity phase and fully epitaxial growth were obtained. By varying the film thickness, superconducting transition temperatures up to 21 K were observed, significantly larger than the bulk value 16.2 K. Structural analyses indicated that the c-axis is smaller than the bulk value but it is almost independent of the film thickness and the a-axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe–(Se,Te) bond length and angle, suggesting the possibility of further enhancement.

We report measurements up to 85 Tesla of the upper critical fields $H_{c2}$($T$) on Ba${}_{1-x}$K${}_x$As${}_2$Fe${}_2$ single crystals and FeSe${}_{1-x}$Te${}_x$ films tuned by doping and strain. We observed an $H_{c2}$ enhancement by more than 25 T at low temperatures for the optimally doped Ba${}_{1-x}$K${}_x$As${}_2$Fe${}_2$ as compared to the previous measurements and extraordinarily high slopes of $d{H}_{c2}$/$\mathit{dT}$ $=$ 250–500 T/K near $T_c$ in FeSe${}_{1-x}$Te${}_x$, indicating almost-complete suppression of orbital pair breaking. Theoretical analysis of $H_{c2}$($T$) suggests an inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov state below 10K for $H//ab$ in the optimally doped Ba${}_{1-x}$K${}_x$As${}_2$Fe${}_2$ and below 3K for $H//c$ and 9K for $H//ab$ in FeSe${}_{1-x}$Te${}_x$. The analysis also shows that $H_{c2}$ in a multiband Fe-based superconductor can be significantly enhanced by doping and strain by shrinking and expanding different pockets of the Fermi surface, which can be more effective than the conventional way of increasing $H_{c2}$ by nonmagnetic impurities.

The nuclear and magnetic structures of Fe1+yTe1$-${}xSex (0$\le${}x$\le${}0.20) compounds was analyzed between 2 and 300 K by means of Rietveld refinement of neutron powder diffraction data. Samples with x$\le${}0.075 undergo a tetragonal to monoclinic phase transition at low temperature, whose critical temperature decreases with increasing Se content; this structural transition is strictly coupled to a long-range antiferromagnetic ordering at the Fe site. Both the transition to a monoclinic phase and the long-range antiferromagnetism are suppressed for 0.10$\le${}x$\le${}0.20. The onset of the structural and of the magnetic transition remains coincident with the increase in Se substitution. The low-temperature monoclinic crystal structure has been revised. Superconductivity arises for x$\ge${}0.05, therefore a significant region where superconductivity and long-range antiferromagnetism coexist is present in the pseudobinary FeTe-FeSe phase diagram.