We investigated the superconducting transition and the pinning properties of undoped and Ag-doped FeSe 0.94 at magnetic fields up to 14 T. It was established that due to Ag addition the hexagonal phase formation in melted FeSe 0.94 samples is suppressed and the grain connectivity is strongly improved. The obtained superconducting zero-field transition becomes sharp (with a transition width below 1 K), T c and the upper critical field were found to increase, whereas the normal state resistivity significantly reduces becoming comparable with those of FeSe single crystals. In addition, a considerable magnetoresistance was observed due to Ag doping. The resistive transition of undoped and Ag-doped FeSe 0.94 is dominated by thermally activated flux flow. From the activation energy U vs H dependence, a crossover from single-vortex pinning to a collective creep pinning behavior was found with increasing the magnetic field.

The superconducting properties of mm-sized Fe1.02Se crystals grown by a flux method are investigated. The structural and morphological features are studied by x-ray diffraction (XRD) and by scanning electron microscopy-energy dispersive x-ray spectroscopy SEM-EDX analysis, which identified a co-growth of a dominant superconducting tetragonal phase, with the minority of a non-superconducting hexagonal phase. The ac magnetic response is analyzed using a combined method of the fundamental and the 3rd harmonic ac magnetic susceptibility as a function of the temperature at different ac magnetic field amplitudes and frequencies and with various superimposed dc fields. The variation of the ac magnetic field and frequency in different ranges especially affects the 3rd harmonic components, which are more sensitive to the changes in the flux dynamic regimes. This allows a fine observation of the evolution of the different linear and non-linear processes responsible for the ac magnetic response of the Fe1.02Se crystals. At low enough ac amplitudes and frequencies, and even in high imposed dc magnetic fields, the Fe1.02Se crystals show a typical critical state behavior, marking a high stability of the pinning, with very small influence of the vortex dynamical processes. With the change of ac field amplitude and frequency a gradual crossover is observed from the initial stable pinning state through the domination of the intermediate regimes as flux creep and finally to the complete dominance of flux flow. The ac magnetic response is also influenced by geometric edge barrier effects arising from the plate-like geometry of the Fe1.02Se crystals. The changes of the dominant irreversible (non-linear) mechanism from surface pinning to bulk pinning or to prevailing dynamical regimes is also identified by analyzing the behavior of the 3rd harmonic components.

We investigated the influence of different Ag additions (up to 10 wt %) on the superconducting properties of FeSe 0.94 . The structural investigations (XRD and SEM) indicated that Ag is present in three different forms. Ag at grain boundaries supports the excellent intergrain connections and reduces ΔT to values smaller than 1K at B=0 and ΔT ≤ 2.74 K at B=14 T. Ag insertion in the crystal lattice unit cell provides additional carriers and changes the electron hole imbalance in FeSe 0.94 . This results in an increase in the magnetoresistive effect (MR) and critical temperature (T c ). Reacted Ag forms a small amount (~1%) of Ag 2 Se impurity phase, which may increase the pinning energy in comparison with that of the undoped sample. The enhanced upper critical field (B c2 ) is also a result of the increased impurity scattering. Thus, unlike cuprates Ag addition enhances the T c , B c2 , pinning energy and MR making the properties of polycrystalline FeSe 0.94 similar to those of single crystals.

The tetragonal FeSe phase is an intensively investigated iron based superconductor. In this study we examined the influence of Ag addition on the superconducting properties of selenium deficient polycrystalline FeSe 0.94 . The samples were obtained by solid state reaction and melting methods. XRD analysis shows the presence of tetragonal phase and EDX analysis establishes inhomogeneous Ag distribution in the grains. The superconducting properties were investigated by fundamental and third harmonic AC magnetic susceptibility. The intergranular critical current determined from AC magnetic susceptibility in the Ag doped sample is several times higher than that in the undoped one, obtained by melting at approximately the same temperatures. Intragranular current is field independent up to almost 1000 Oe. Using the temperature dependence of third harmonic AC magnetic susceptibility at different DC magnetic fields, the irreversibility lines were obtained for all samples. It is found that Ag addition increased the irreversibility field in comparison with undoped melted and powder sintered samples. All results show that the Ag addition in selenium deficient (FeSe 0.94 ) samples leads to improvement of inter-and intra-granular properties: screening ability, pinning, activation energy and critical current and improves the irreversibility line.

We evaluate the effects of high pressure during annealing on the structural and superconducting properties of Ag-doped FeSe bulks. The results obtained in this work indicate that the annealing at high pressure increases the critical temperature, upper critical field and irreversibility field due to the improved uniformity and grain connectivity.

Abstract.The upper critical field H c2 (T) of sintered pellets of the recently discovered MgB 2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between T c ∼ 39 K and T = 3 K ∼ 0.1T c . The H c2 data from ac susceptibility are consistent with resistance data and represent the upper critical field of the major fraction of the investigated sample which increases up to H c2 (0) = 13 T at T = 0 corresponding to a coherence length of ξ o = 5.0 nm. A small fraction of the sample exhibits higher upper critical fields which were measured both resistively and by dc magnetization measurements. The temperature dependence of the upper critical field, H c2 (T), shows a positive curvature near T c and at intermediate temperatures indicating that MgB 2 is in the clean limit. The H c2 (T) dependence can be described within a broad temperature region 0. The recent discovery of superconductivity in MgB 2 [1] at temperatures as high as 40 K has stimulated considerable interest in this system. MgB 2 , which has a hexagonal AlB 2 structure, is a type IIsuperconductor. A significant boron isotope effect was observed [2] which is an indication for electronphonon mediated superconductivity in this compound. Magnetic parameters as the Ginsburg-Landau parameter κ = 26 [3] and the temperature dependence of the upper critical field H c2 (T) [3-6] were determined from transport and magnetization measurements [3][4][5][6][7]. So far, a complete H c2 (T) curve was reported for a MgB 2 wire sample showing a high residual resistivity ratio of about 25 [7]. In the present paper, the temperature dependence of the upper critical field of a sintered MgB 2 pellet was studied in magnetic fields up to 16 T in order to analyse the shape of H c2 (T) in the whole temperature range for a sample with a moderate residual resistivity ratio. Polycrystalline samples of MgB 2 were prepared by a conventional solid state reaction. A stoichiometric mixture of Mg and B was pressed into pellets. These pellets were wrapped in a Ta foil and sealed in a quartz vial. The samples were sintered at 950°C for two hours. Electrical resistance and the superconducting transition of a sample 5 mm in length with a cross-section of about 1 mm 2 (cut from the initially prepared pellet) were investigated in magnetic fields up to 16 T using the standard four probe method and current densities between 0.2 and 1 A/cm 2 . AC susceptibility and dc magnetization measurements were performed on other pieces from the same pellet in magnetic fields up to 9 T and 5 T, respectively. In Fig. 1a, the temperature dependence of the electrical resistance of the investigated sample is shown. The resistivity at 40 K and 300 K are about 6.4 µΩcm and 29 µΩcm, respectively, resulting in a residual resistance ratio (RRR) of approximately 4.5. The midpoint value of the normal-state resistivity of the superconducting transition at zero-magnetic field is 38.8 K. A similar T c value of T c =39,0 K was...

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We studied the effect of Sn substitution in a new mercury-based superconductor with nominal composition . At this stage of investigation the material is multiphasic with dominating superconducting phases up to x = 0.3. It was established that Sn stimulates the formation of the Hg-1212 phase and enhances the diamagnetic volume fraction but also the weak-link behaviour. Together with the SEM - EDS analysis, the shift of some of the XRD peaks, as well as the gradual decrease of with the dopant content x, suggests that the substitution is effective and some of the Sn is incorporated into the structure of the superconducting phases.