We investigated the superconducting and transport properties in FeTe$_{1-x}$Se$_{x}$ (0.1 $leq$ $x$ $leq$ 0.4) single crystals prepared by O$_2$-annealing. Sharp superconducting transition width observed in magnetization measurement and the small res
idual resistivity prove the high quality of the crystals. All the crystals manifest large, homogeneous, and isotropic critical current density emph{J}$_c$ with self-field value over 10$^5$ A/cm$^2$ at 2 K. The large and field-robust critical current densities prove that the superconductivity in FeTe$_{1-x}$Se$_{x}$ (0.1 $leq$ $x$ $leq$ 0.4) is in bulk nature. The values of anisotropy parameter close to $T_c$ for crystals with different Se doping levels all reside in the range of 2 - 3. Hall coefficients $R_H$ keeps positive and almost constant value at high temperatures, followed by a sudden decreases before reaching $T$$_c$, which indicates that the electron-type charge carriers become dominant at low temperatures. Furthermore, the characteristic temperature for the sudden decrease in $R_H$ gradually increases with Se doping.
Iron chalcogenide Fe(Te,Se) attracted much attention due to its simple structure, which is favorable for probing the superconducting mechanism. Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based sup
erconductors. By intercalating spacer layers, superconducting transition temperature has been raised over 40 K. On the other hand, the presence of excess Fe is almost unavoidable in Fe(Te,Se) single crystals, which hinders the appearance of bulk superconductivity and causes strong controversies over its fundamental properties. Here we report a systematical study of O$_2$-annealing dynamics in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ by controlling the amount of O$_2$, annealing temperature, and time. Bulk superconductivity can be gradually induced by increasing the amount of O$_2$ and annealing time at suitable temperatures. The optimally annealed crystals can be easily obtained by annealing with ~ 1.5% molar ratio of oxygen at 400 $^{circ}$C for more than 1 hour. Superconductivity was witnessed to evolve mainly from the edge of the crystal to the central part. After the optimal annealing, the complete removal of excess Fe was demonstrated via STM measurements. Some fundamental properties were recharacterized and compared with those of as-grown crystals to discuss the influence of excess Fe.
Single crystals of Ca1-xLaxFe2As2 for x ranging from 0 to 0.25 have been grown and characterized by structural, transport and magnetic measurements. Coexistence of two superconducting phases is observed, in which the low superconducting transition te
mperature (Tc) phase has Tc ~ 20 K, and the high Tc phase has Tc higher than 40 K. These data also delineate an x - T phase diagram in which the single magnetic/structural phase transition in undoped CaFe2As2 appears to split into two distinct phase transitions, both of which are suppressed with increasing La substitution. Superconductivity emerges when x is about 0.06 and coexists with the structural/magnetic transition until x is ~ 0.13. With increasing concentration of La, the structural/magnetic transition is totally suppressed, and Tc reaches its maximum value of about 45 K for 0.15 < x < 0.19. A domelike superconducting region is not observed in the phase diagram, however, because no obvious over-doping region can be found. Two superconducting phases coexist in the x - T phase diagram of Ca1-xLaxFe2As2. The formation of the two separate phases, as well as the origin of the high Tc in Ca1-xLaxFe2As2 is studied and discussed in detail.
We have systematically investigated and compared different methods to induce superconductivity in iron chalcogenide Fe1+yTe0.6Se0.4 including annealing in vacuum, N2, O2, I2 atmosphere, and immersing samples into acid and alcoholic beverages. Vacuum
and N2 annealing are proved to be ineffective to induce superconductivity in Fe1+yTe0.6Se0.4 single crystal. O2 and I2 annealing, acid and alcoholic beverages can induce superconductivity by oxidizing the excess Fe in the sample. Superconductivity in O2 annealed sample is in bulk nature, while I2, acid and alcoholic beverages can only induce superconductivity near the surface. By comparing different effects of O2, I2, acid and alcoholic beverages, we propose a scenario to explain how the superconductivity is induced in the non-superconducting as-grown Fe1+yTe0.6Se0.4.
A series of polycrystalline SmFeAs1-xOx bulks was prepared to systematically investigate the influence of sample density on flux pinning properties. Different sample densities were achieved by controlling the pelletizing pressure. The superconducting
volume fraction, the critical current densities Jcm and the flux pinning force densities Fp were estimated from the magnetization measurements. Experimental results manifest that: (1) the superconducting volume fraction decreases with the decreasing of sample density. (2) The Jcm values have the similar trend except for the sample with very high density may due to different connectivity and pinning mechanism. Moreover, The Jcm(B) curve develops a peak effect at approximately the same field at which the high-density sample shows a kink. (3) The Fp(B) curve of the high-density sample shows a low-field peak and a high-field peak at several temperatures, which can be explained by improved intergranular current, while only one peak can be observed in Fp(B) of the low-density samples. Based on the scaling behaviour of flux pinning force densities, the main intragranular pinning is normal point pinning.
Temperature dependent resistivity of the iron-based superconductor NdFeAsO0.88F0.12 was measured under different applied fields and excitation currents. Arrhenius plot shows an anomalous tail effect, which contains obvious two resistivity dropping st
ages. The first is caused by the normal superconducting transition, and the second is supposed to be related to the weak-link between the grains. A model for the resistivity dropping related to the weak-link behavior is proposed, which is based on the Josephson junctions formed by the impurities in grain boundaries like FeAs, Sm2O3 and cracks together with the adjacent grains. These Josephson junctions can be easily broken by the applied fields and the excitations currents, leading to the anomalous resistivity tail in many polycrystalline iron-based superconductors. The calculated resistivity dropping agrees well with the experimental data, which manifests the correctness of the explanation of the obtained anomalous tail effect.
