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The complex magnetic ordering of parent compounds of most unconventional superconductors is crucial for the understanding of high-temperature superconductivity (SC). Within this framework, we have performed temperature-dependent magnetotransport experiments on a single crystal of SmFeAsO, a parent compound of iron pnictide superconductors. We observe multiple features in the measured transport properties at temperatures below the antiferromagnetic (AFM) ordering of Sm, $T < {T_{{\text{NSm}}}}$ , which evolve with in-plane magnetic field, suggesting a rich variety of metamagnetic transitions never before observed in this compound. Considering that transport mainly involves Fe d orbitals at the Fermi level, these findings suggest that the features originate from magnetic transitions of the Fe moments sublattice, which in turn may be induced by magnetic transitions of the Sm moments sublattice via the interaction between Fe and Sm moments. We outline a possible scenario in which the Fe moments, strongly affected by the Sm ordering below T _NSm , reorder to an in-plane canted AFM structure, which is washed out by the application of an in-plane magnetic field up to 9 T. Our work shows that transport properties are a valuable tool for investigating magnetic ordering in iron pnictide parent compounds, where the interplay of magnetism and SC is believed to be the origin of high-temperature SC.

We report a systematic study of the fabrication and chemical and structural characterization of epitaxial Cr films and Pt/Cr bilayers on MgO substrates. Cr films of different thicknesses were grown by molecular beam epitaxy at 373 K and then post-annealed at different temperatures. These films are chemically stable, present a well-ordered tetragonal structure, with only a low degree of oxidation distributed throughout the films. The lattice parameter can be finely tuned by varying the film thickness and the post-annealing temperature, as the elastic energy is progressively relaxed by edge dislocations, so that the film can approach the structural and electrical properties of bulk chromium. Moreover, the chemical and structural properties of Cr are not affected by a Pt capping layer, which grows in a well-ordered Pt[1 1 0]||Cr[1 0 0] orientation, holding potential for applications in recently proposed Pt/Cr-based antiferromagnet spintronics and for the realization of epitaxial Pt films on MgO by using a thin buffer layer of chromium. [ABSTRACT FROM AUTHOR]

In this paper we explore the effects of 3.5 MeV proton irradiation on Fe(Se,Te) thin films grown on CaF2. In particular, we carry out an experimental investigation with different irradiation fluences up to 7.30 · 1016 cm−2 and different proton implantation depths, in order to clarify whether and to what extent the critical current is enhanced or suppressed, what are the effects of irradiation on the critical temperature, resistivity, and critical magnetic fields, and finally what is the role played by the substrate in this context. We find that the effect of irradiation on superconducting properties is generally small compared to the case of other iron-based superconductors. The irradiation effect is more evident on the critical current density Jc, while it is minor on the transition temperature Tc, normal state resistivity ρ, and on the upper critical field Hc2 up to the highest fluences explored in this work. In more detail, our analysis shows that when protons implant in the substrate far from the superconducting film, the critical current can be enhanced up to 50% of the pristine value at 7 T and 12 K; meanwhile, there is no appreciable effect on critical temperature and critical fields together with a slight decrease in resistivity. On the contrary, when the implantation layer is closer to the film–substrate interface, both critical current and temperature show a decrease accompanied by an enhancement of the resistivity and lattice strain. This result evidences that possible modifications induced by irradiation in the substrate may affect the superconducting properties of the film via lattice strain. The robustness of the Fe(Se,Te) system to irradiation-induced damage makes it a promising compound for the fabrication of magnets in high-energy accelerators. [ABSTRACT FROM AUTHOR]

In this work, we investigate the temperature dependence of the upper critical field, dHc2/dT, in an increasingly disordered NdFeAsO0.7F0.3 (NdFeAs(O,F)) single crystal that has been progressively irradiated up to a 5.25 × 1016 cm−2 total α-particle dose. For the H∣∣ab-plane, dHc2/dT does not vary remarkably with irradiation, while for the H∣∣c-axis it increases sharply after the first irradiation of 3.60 × 1015 cm−2 and then more gradually with further irradiation doses. Focusing on the H∣∣c-axis, we develop a phenomenological analysis of the Hc2 slope which allows us to inspect the crossover from the clean to the dirty regime. From the Hc2 slope normalized to the critical temperature and to its clean limit value, we extract the ratio of the coherence length ξBCS to the mean free path and we find that when Tc is reduced by a factor of four from its pristine value, ξBCS/ becomes as large as ∼7 and reaches values of ∼1.8 nm, indicating that NdFeAs(O,F) is well into the dirty regime. Our analysis of the Hc2 slope also allows us to compare the pair-breaking effectiveness of scattering in different superconductors, showing similarity between unconventional NdFeAs(O,F) and moderate-Tc phonon-mediated devices, such as MgB2 and A15 compounds, but much a stronger difference with YBa2Cu3O7-δ. This work thus shows that dHc2/dT is a reliable parameter, providing an alternative to residual resistivity, for investigating the pair-breaking mechanism induced by impurity scattering in superconductors. [ABSTRACT FROM AUTHOR]

In this work we investigate the effect of each different heat treatment stage in the fabrication of Bi2Sr2CaCu2O8+δ superconducting wires on intra-grain and inter-grain superconducting properties. We measure magnetic critical temperature Tc values and transport critical current density Jc at temperatures from 4 K to 40 K and in fields up to 7 T. From an analysis of the temperature dependence of the self-field critical current density Jc(T) that takes into account weak link behavior and the proximity effect, we study grain boundary (GB) transparency to supercurrents; we also establish a relationship between GB oxygenation in the different steps of the fabrication process and GB transparency to supercurrents. We find that GB oxygenation starts in the first crystallization stage, but it becomes complete in the plateau at 836 °C and in slow cooling stages and is further enhanced in the prolonged post-annealing step. Such oxygenation makes GBs more conductive, thus improving the inter-grain Jc value and temperature dependence. On the other hand, from inspection of the Tc values in the framework of the phase diagram dome, we find that grains are already oxygenated in the crystallization step up to the optimal doping, while successive slow cooling and post-annealing treatments further enhance the degree of overdoping, especially if carried out in oxygen atmosphere rather than in air. [ABSTRACT FROM AUTHOR]

A novel method to prepare bulk Fe(Se0.5Te0.5) samples is presented, based on a melting process and a subsequent annealing treatment. With respect to the standard sintering technique, it produces much more homogeneous and denser samples, characterized by large and well interconnected grains. The resulting samples exhibit optimal critical temperature values, sharp resistive and magnetic transitions, large magnetic hysteresis loops and high upper critical fields. Interestingly, the global critical current density is much enhanced as compared to the values reported in the literature for bulk samples of the same 11 family, reaching about 103 A cm[?]2 at zero field at 4.2 K, as assessed by magnetic, transport and magneto-optical techniques. Even more importantly, its field dependence turns out to be very weak, such that at u0H = 7 T it is suppressed only by a factor [?]2. [ABSTRACT FROM AUTHOR]

We present a systematic characterization of Mg1[?]xLixB2 polycrystalline samples with nominal Li content x up to 0.30. We explore the effectiveness of the substitution and investigate its influence on superconducting and normal state properties. The structural analyses by neutron and x-ray diffraction indicate that, despite the lattice parameters remaining unchanged within the experimental accuracy, around 30% of the nominal Li content actually enters the structure. Also the transition temperature is only weakly affected by Li substitution, but its relationship with the residual resistivity and magnetoresistivity data is compatible with a picture where p bands are filled with holes and/or affected by disorder, as predicted by theory. We also measured the magnetic penetration depth l by an inductance technique. Data fits based on the standard Bardeen-Cooper-Schrieffer two-band model yield the zero-temperature limit of both l and superconducting gaps: we find that l(0) increases weakly and quickly saturates with increasing x, whereas Dp(0) and Ds(0) decrease with x. This analysis suggests that lithium substitution induces disorder mainly in the p band. Point contact spectroscopy results on samples obtained from the same batch are in full agreement with magnetic penetration depth data on the undoped sample, but give superconducting gaps almost insensitive to Li substitution, showing at most a small increase in Dp(0) that may be related to an inhomogeneous distribution of Li content. [ABSTRACT FROM AUTHOR]