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Iron-chalcogenide superconductors continue to captivate researchers due to their diverse crystalline structures and intriguing superconducting properties, positioning them as both a valuable platform for theoretical investigations and promising candidates for practical applications. This review begins with a comprehensive overview of the fabrication techniques employed for various iron-chalcogenide superconductors, accompanied by a summary of their phase diagrams. Subsequently, it delves into the upper critical field, anisotropy, and critical current density. Furthermore, it discusses the successful fabrication of meters-long coated conductors and explores their applications in superconducting radio-frequency cavities and coils. Finally, several prospective avenues for future research are proposed.
RESUMEN
We have made the first observation of superconductivity in TlNi2Se2 at T(C)=3.7 K, and it appears to involve heavy electrons with an effective mass m*=(14-20)m(b), as inferred from the normal-state electronic specific heat and the upper critical field, H(C2)(T). We found that the zero-field electronic specific-heat data, C(es)(T) (0.5 K≤T<3.7 K) in the superconducting state can be fitted with a two-gap BCS model, indicating that TlNi2Se2 seems to be a multiband superconductor, which is consistent with the band calculation for the isostructural KNi2S2. It is also found that the electronic specific-heat coefficient in the mixed state γN(H) exhibits a H(1/2) behavior, which is considered as a common feature of the d-wave superconductors. TlNi2Se2, as a d-electron system with heavy electron superconductivity, may be a bridge between cuprate- or iron-based and conventional heavy-fermion superconductors.
RESUMEN
Increasing the thickness of a superconducting layer and simultaneously reducing the thickness effect in iron-based superconducting coated conductors are particularly essential for improving the critical current Ic. Here, for the first time, we have deposited high-performance FeSe0.5Te0.5 (FST) superconducting films up to 2 µm on LaMnO3-buffered metal tapes by pulsed laser deposition. An interface engineering strategy, alternating growth of a 10 nm-thick nonsuperconducting FST seed layer and a 400 nm-thick FST superconducting layer, was employed to guarantee the crystalline quality of the films with thicknesses of the order of micrometers, resulting in a highly biaxial texture with grain boundary misorientation angle less than the critical value θc â¼ 9°. Moreover, the thickness effect, that the critical current density (Jc) shows a clear dependence on thickness as in cuprates, is reduced by the interface engineering. Also, the maximum Jc was found for a 400 nm-thick film with 1.3 MA/cm2 in self-field at 4.2 K and 0.71 MA/cm2 (Hâ¥ab) and 0.50 MA/cm2 (Hâ¥c) at 9 T. Anisotropic Ginzburg-Landau scaling indicates that the major pinning centers vary from correlated to uncorrelated as the film thickness increases, while the thickness effect is most likely related to the weakening of flux pinning by the fluctuation of charge-carrier mean free path (δl) and strengthening of flux pinning caused by the variation of superconducting transition temperature (δTc) due to off-stoichiometry with thickness.
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High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the (Tl0.58Rb0.42)Fe1.72Se2 superconductor with a T(c) = 32 K. The Fermi surface topology consists of two electronlike Fermi surface sheets around the Γ point which is distinct from that in all other iron-based superconductors reported so far. The Fermi surface around the M point shows a nearly isotropic superconducting gap of â¼12 meV. The large Fermi surface near the Γ point also shows a nearly isotropic superconducting gap of â¼15 meV, while no superconducting gap opening is clearly observed for the inner tiny Fermi surface. Our observed new Fermi surface topology and its associated superconducting gap will provide key insights and constraints into the understanding of the superconductivity mechanism in iron-based superconductors.
RESUMEN
High-quality Co-doped BaFe2As2 thin films with thickness up to 2 µm were realized on flexible metal tapes with LaMnO3 as buffer layers fabricated by an ion beam-assisted deposition technique. Structural analysis indicates that increasing thickness does not compromise the film crystallinity, except for a small amount of impurities. Two types of thickness dependence of critical current density (J c) were found: one is almost thickness independent in the range of 0.6-1.5 µm and the other is highly thickness dependent. In addition, the maximum value for crucial current I c at 9 T and 4.2 K is about 55 A/12 mm-W for the 1.5-µm-thick film. Anisotropic Ginzburg-Landau scaling demonstrates that dominant pinning centers develop from correlated to uncorrelated with increasing film thickness. The further theoretical analysis shows that with film thickness increasing the pinning mechanism evolves progressively from a δl pinning to the δT c pinning mechanism.
RESUMEN
Copper sheath is the first choice for manufacturing high-T(c) superconducting wires and tapes because of its high electrical and thermal conductivities, low-cost and good mechanical properties. However, Cu can easily react with superconducting cores, such as BSCCO, MgB2 and pnictides, and therefore drastically decrease the transport J(c). Here, we report the fabrication of Cu-sheathed Sr(1-x)K(x)Fe2As2 tapes with superior J(c) performance using a simple hot pressing method that is capable of eliminating the lengthy high-temperature sintering. We obtained high-quality Sr(1-x)K(x)Fe2As2 tapes with processing at 800 °C for 30 minutes and measured high T(c) and sharp transition. By this rapid fabrication, Cu sheath does not give rise to apparent reaction layer, and only slightly diffuses into Sr-122 core. As a consequence, we achieved high transport J(c) of 3.1 × 10(4) A/cm(2) in 10 T and 2.7 × 10(4) A/cm(2) in 14 T at 4.2 K. The in-field J(c) performance is by far the highest reported for Cu-sheathed high-T(c) conductors. More importantly, Cu-sheathed Sr-122 tapes also showed a high J(e) value of 1.0 × 10(4) A/cm(2) in 10 T at 4.2 K, which has reached the widely accepted practical level for applications. These results demonstrate that Cu is a very promising sheath for the practical application of pnictide conductors.
RESUMEN
After our first discovery of multi-band superconductivity (SC) in the TlNi2Se2 crystal, we successfully grew a series of TlNi2Se(2-x)S(x) (0.0 ≤ x ≤ 2.0) single crystals. Measurements of resistivity, specific heat, and susceptibility were carried out on these crystals. Superconductivity with T(C) = 2.3 K was first observed in the TlNi2S2 crystal, which also appears to involve heavy electrons with an effective mass m* = 13-25 m(b), as inferred from the normal state electronic specific heat and the upper critical field, H(C2)(T). It was found that bulk SC and heavy-electron behavior is preserved in all the studied TlNi2Se(2-x)S(x) samples. In the mixed state, a novel change of the field dependence of the residual specific heat coefficient, γ(N)(H), occurs in TlNi2Se(2-x)S(x) with increasing S content. We also found that the T(C) value changes with the disorder degree induced by the partial substitution of S for Se, characterized by the residual resistivity ratio (RRR). Thus, the TlNi2Se(2-x)S(x) system provides a platform to study the effect of disorder on the multi-band SC.
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High-performance Sr0.6K0.4Fe2As2 (Sr-122) tapes have been successfully fabricated using hot pressing (HP) process. The effect of HP temperatures (850-925°C) on the c-axis texture, resistivity, Vickers micro-hardness, microstructure and critical current properties has been systematically studied. Taking advantage of high degree of c-axis texture, well grain connectivity and large concentration of strong-pinning defects, we are able to obtain an excellent Jc of 1.2 × 10(5)â A/cm(2) at 4.2â K and 10â T for Sr-122 tapes. More importantly, the field dependence of Jc turns out to be very weak, such that in 14â T the Jc still remains ~ 1.0 × 10(5)â A/cm(2). These Jc values are the highest ever reported so far for iron-pnictide wires and tapes, achieving the level desired for practical applications. Our results clearly strengthen the position of iron-pnictide conductors as a competitor to the conventional and MgB2 superconductors for high field applications.
RESUMEN
The excess Fe atoms which unavoidably exist in the Fe(Te, Se, S) crystal lattice result in a complicated antiferromagnetic ground state as well as the suppression of superconductivity. As a result, there are still discrepancies on their phase diagrams. In this paper, we report the synthesis of Fe1+δTe1-xSx (0 ≤ x ≤ 0.12) single crystals by a melting method. Superconductivity was greatly improved after air annealing by which we partially removed the excess Fe atoms. Based on the resistivity and susceptibility measurements, we concluded a phase diagram of the Fe1+δTe1-xSx (0 ≤ x ≤ 0.12) system with fewer excess iron atoms. We found a coexisting region (0.07 ≤ x ≤ 0.11) of antiferromagnetic order and bulk superconductivity. This phase diagram is similar to that of the K- or Co-doped BaFe2As2 system, as well as the Fe(Te, Se) system, implying a commonality of the iron-based superconductors.
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The results of a 57Fe Mössbauer spectroscopy study between 4.5 and 523.2 K and in external magnetic fields (up to 90 kOe) of semiconducting Tl0.53K0.47Fe1.64Se2 single crystals are reported. Evidence is provided for a possible phase separation into the magnetic majority and minority phases. It is demonstrated that the magnetic moments of the divalent Fe atoms located at the 16i site (space group I4/m) of the majority phase and of the minority phase are antiferromagnetically ordered, with the Néel temperature T(N) = 518.0(3.6) K. The magnetic moments at 5.0 K of 2.09(1) and 2.28(2) µ(B) in these two phases are tilted from the crystallographic c axis by 18(1)° and 32(2)°, respectively. The Debye temperature of Tl0.53K0.47Fe1.64Se2 is found to be 228(4) K.