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Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe layers (1111, 122,11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length, and unconventional pairing, On the other hand the Fe-based superconductors have metallic parent compounds, and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, the supposed order parameter symmetry is s wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviours and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest T c , huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates, while the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the T c of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families. 2 , to the ab-plane. 12 The temperature dependence is very different in the two directions, strongly departing from the WHH behaviour 16 mainly in the direction parallel to c. The anisotropy evaluated as γ = ab c ab c H H H ⊥ = 2 // 2 / γ, is also strongly temperature dependent, reminiscent of the two-gap behaviour seen in MgB 2 . 17,18 However, a different situation is observed in the 122 family. (Ba,K)Fe 2 As 2 single crystals exhibit nearly isotropic μ 0 H c2 with

We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe1−xCox)2As2, over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons, in excess of an order of magnitude in the low-doping, lowtemperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between AF and superconductivity (SC), while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin-fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing, but also in the normal transport.PACS numbers: 74.20. Rp, 74.25.Ha, 74.70.Dd The discovery in the last year of new iron-based superconductors [1] provided a tempting analogy with high-T c cuprates. Indeed, a simple comparison between phase diagrams reveals, particularly clearly for the BaFe 2 As 2 family[2, 3, 4], a couple interesting similarities with the cuprates: first and foremost, the parent compound is an antiferromagnet (AFM), and spin fluctuations appear important for carrier pairing. Second, the superconductivity (SC) appears with either hole or electron doping, at a finite doping level, and forms a dome-shaped region in the phase diagram, as in cuprates.A closer look, however, reveals equally striking differences: Indeed, unlike the cuprates, the parent compoundis in pnictides are metals that support quantum oscillations [5,6], and the Coulomb correlations appear to be weak [7]. Second, unlike cuprates, superconductivity can be induced without doping, by external or chemical pressure [8]. Finally, the superconducting dome is very asymmetric [9,10]. And, probably most importantly, electronic structure in cuprates is formed essentially by one band, while in the pnictiodes multiband effects are of primary importance.The doping dependence of the evolution of the multiband electronic structure and its relationship to AFM, spin fluctuations, and SC is the key to the physics of the high-T c ferropnictides. Systematic Hall coefficient and resistivity measurements are clearly well-suited to provide useful insight into these issues. In this Letter we select BaFe 2 As 2 for a systematic study of the Hall effect and resistivity. Through quantitative analysis of the experimental data, combined with theoretical calculations, we establish a unified view of the doping induced evolution of SC and AFM, as well as the ramifications for the pairing mechanism. The crystals were grown by self-flux method using FeAs as the flux; the details are described elswhere [11,12]. The main advantage of the 122 system [2, 3, 4] is that it allows fab...