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.

In this paper, a study of the effect of Al substitution on the upper critical field, $B_{c\mathit{2}}$, in ${\mathrm{Al}}_x{\mathrm{Mg}}_{1-x}{\mathrm{B}}_2$ samples is presented. We find a straightforward correlation between $B_{c\mathit{2}}$ and the $\sigma$-band gap, ${\Delta }_{\sigma }$, evaluated by point-contact measurements. Up to $x=0.2,B_{c\mathit{2}}$ can be well described within a clean limit model and its decrease with $x$ is directly related to the suppression of ${\Delta }_{\sigma }$.

In this paper, we investigate the Ru-substituted LaFeAsO compound by studying the magnetotransport behavior and its relationship with the band structure in different regimes of temperature, magnetic field, and Ru content. In particular, we analyze the magnetoresistance of LaFe1$-${}xRuxAsO (0 $\le${} x $\le${} 0.6) samples with the support of ab initio calculations, and we find that in the whole series: (i) the transport is dominated by electron bands only; (ii) the magnetoresistance exhibits distinctive features related to the presence of Dirac cones; indeed, ab initio calculations confirm the presence of anisotropic Dirac cones in the band structure; (iii) the low temperature mobility is exceptionally high and reaches 18.6 m2/(Vs) in the Ru-free sample at T\,=\,2 K in the extreme limit of a single Landau level occupied in the Dirac cones; (iv) the mobility drops abruptly above 10\char21{}15 K; (v) the disorder has a very weak effect on the band mobilities and on the transport properties; and (vi) there exists a correlation between the temperature ranges of Dirac cones and SDW carrier condensation. These findings evidence the outstanding transport properties of Dirac cones in Fe-based pnictides parent compounds.

Herewith, we review the available experimental data of thermoelectric transport properties of iron-based superconductors and parent compounds. We discuss possible physical mechanisms into play in determining the Seebeck effect, from whence one can extract information about Fermi surface reconstruction and Lifshitz transitions, multiband character, coupling of charge carriers with spin excitations and its relevance in the unconventional superconducting pairing mechanism, nematicity, quantum critical fluctuations close to the optimal doping for superconductivity, correlation. Additional information is obtained from the analysis of the Nernst effect, whose enhancement in parent compounds must be related partially to multiband transport and low Fermi level, but mainly to the presence of Dirac cone bands at the Fermi level. In the superconducting compounds, large Nernst effect in the normal state is explained in terms of fluctuating precursors of the spin density wave state, while in the superconducting state it mirrors the usual vortex liquid dissipative regime. A comparison between the phenomenology of thermoelectric behavior of different families of iron-based superconductors and parent compounds allows to evidence the key differences and analogies, thus providing clues on the rich and complex physics of these fascinating unconventional superconductors.