By partially substituting the tri-valence element La with di-valence element Sr in LaOFeAs, we introduced holes into the system. For the first time, we successfully synthesized the hole-doped new superconductors (La1−xSrx)OFeAs. The maximum superconducting transition temperature at about 25 K was observed at a doping level of x = 0.13. It is evidenced by Hall effect measurements that the conduction in this type of material is dominated by hole-like charge carriers, rather than electron-like ones. Together with the data of the electron-doped system La(O1−xFx)FeAs, a generic phase diagram is depicted and is revealed to be similar to that of the cuprate superconductors.

The superconductor Sr4V2O6Fe2As2 with transition temperature at 37.2 K has been fabricated. It has a layered structure with the space group of p4/nmm, and with the lattice constants a = 3.9296Å and c = 15.6732Å. The observed large diamagnetization signal and zero-resistance demonstrated the bulk superconductivity. The broadening of resistive transition was measured under different magnetic fields leading to the discovery of a rather high upper critical field. The results also suggest a large vortex liquid region which reflects high anisotropy of the system. The Hall effect measurements revealed dominantly electron-like charge carriers in this material. The superconductivity in the present system may be induced by oxygen deficiency or the multiple valence states of vanadium.Since the discovery of superconductivity 1 at 26 K in oxy-arsenide LaF eAsO 1−x F x , tremendous attention has been paid to searching new superconductors in this family. Among the superconductors with several different structures, 2,3,4,5,6 the highest T c has been raised to 55-56 K 7,8,9,10,11 in doped oxy-iron-arsenides (F-doped LnFeAsO, the so-called 1111 phase, Ln=rare earth elements) or the fluoride derivative iron-arsenides (Lndoped AEFeAsF, AE=alkaline earth elements).12 The superconductivity can also be induced by applying a high pressure to the undoped parent samples.13,14 Although it remains unclear what governs the mechanism of superconductivity in the FeAs-based system, it turns out to be clear that the parent phase is accompanied by an antiferromagnetic (AF) order and the superconductivity can be induced by suppressing this magnetic order. A typical example was illustrated in the (Ba, Sr)F e 2 As 2 (so-called 122) system, the AF order is suppressed and superconductivity was induced by either doping K to the Ba or Sr sites, 2,15,16 or doping Co to the Fe sites. 17,18 On the other hand, superconductivity was also found in the parent phase of FeP-based system, such as LaFePO (T c = 2.75K)19 , or in LiFeAs. 3,4 Very recently superconductivity at about 17 K was found in another FeP based parent compound Sr 4 Sc 2 O 6 Fe 2 P 2 (so-called 42622).20 Due to the absence of the AF order in the superconductors mentioned above, one naturally questions whether the AF order is a prerequisite for the superconductivity in the iron-pnictide system. As far as we know, no superconductivity was detected in the parent phase of some FeAs-based compounds, including the 1111, 122 and the recently discovered 42622 and 32522 phases. 21,22,23,24,25 Although some trace of superconductivity was reported in the doped FeAs-based 42622 or 32522 compounds, the high-T c superconductivity was not supported by a clear large diamagnetization signal. 22,23 In this Letter, we report the discovery of superconductivity at about 37.2 K in the new compound Sr 4 V 2 O 6 Fe 2 As 2 . This work presents the unambiguous evidence for high temperature superconductivity in the FeAs-based 42622 system. The polycrystalline samples were synthesized by using a two-step solid state re...

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...