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A series of FeSe0.5Te0.5 bulk samples have been prepared using the high gas pressure and high-temperature synthesis (HP-HTS) method to optimize the growth conditions for the first time and investigated for their superconducting properties using structural, microstructure, transport, and magnetic measurements to reach the final conclusions. Ex situ and in situ processes are used to prepare bulk samples under a range of growth pressures using Ta-tube and without Ta-tube. The parent compound synthesized by convenient synthesis method at ambient pressure (CSP) exhibits a superconducting transition temperature of 14.8 K. Our data demonstrate that the prepared FeSe0.5Te0.5 sealed in a Ta-tube is of better quality than the samples without a Ta-tube, and the optimum growth conditions (500 MPa, 600 °C for 1 h) are favorable for the development of the tetragonal FeSe0.5Te0.5 phase. The optimum bulk FeSe0.5Te0.5 depicts a higher transition temperature of 17.3 K and a high critical current density of the order of >104 A/cm2 at 0 T, which is improved over the entire magnetic field range and almost twice higher than the parent compound prepared using CSP. Our studies confirm that the high-pressure synthesis method is a highly efficient way to improve the superconducting transition, grain connectivity, sample density, and pinning properties of a superconductor.

The discovery of iron-based superconductors (FBS) and their superconducting properties has generated huge research interest and provided a very rich physics high Tc family for fundamental and experimental studies. The 1111 (REFeAsO, RE = Rare earth) and 1144 (AEAFe4As4, AE = Ca, Eu; A = K, Rb) families are the two most important families of FBS, which offer the high Tc of 58 K and 36 K with doping and without doping, respectively. Furthermore, the crystal growth of these families is not an easy process, and a lot of efforts have been reported in this direction. However, the preparation of high-quality and suitable-sized samples is still challenging. In this short review, we will summarize the growth of materials with their superconducting properties, especially polycrystals and single crystals, for the 1111 and 1144 families, and make a short comparison between them to understand the developmental issues.

The high-pressure growth technique generally plays an important role in the improvement of sample quality and the enhancement of various physical and magnetic properties of materials. The high gas pressure technique provides a large sample space (10–15 cm) to grow various kinds of materials. In this paper, we introduce the high gas pressure and high-temperature synthesis (HP-HTS) technique that is present at our institute and is applied to the growth process of different kinds of superconducting materials, particularly iron-based superconductors. More details and the working principle of this HP-HTS technique are discussed. We have also demonstrated the current results based on iron-based superconductors by using this unique HP-HTS technique. These results demonstrate the enhancement of the superconducting properties with improved sample quality compared to the conventional synthesis process at ambient pressure.

We have investigated the effects of lead (Pb) additions ( x ) up to 40 wt% ( x  = 0–0.4) on the structure, electrical properties, and magnetic properties of FeSe 0.5 Te 0.5 superconductor. The samples were prepared by the solid-state reaction method and were characterized by various techniques. The parent compound ( x  = 0) showed the onset temperature T c onset of 15 K, and zero-resistance temperature, T c offset of 12 K. The addition of Pb enhances the metallic characteristics of FeSe 0.5 Te 0.5 , but both T c onset and T c offset are decreased to the lower temperature with the broadened transition width. The T c onset is nearly the same (10.3 K) at higher additions, such as x  = 0.3 and 0.4, but zero resistivity is not observed up to 7 K. Microstructural analysis and transport studies suggest that for x  > 0.05, Pb additions weakened the coupling between grains and suppressed the superconducting percolation, leading to a broad transition. More importantly, the inclusion of a relatively small amount of Pb ( x  = 0.05) increased the critical current density, J c , in the entire magnetic field, which might be attributed to better phase uniformity as well as good grain connectivity.

We report the optimization of the synthesis conditions for SmFeAsO0.80F0.20 (Sm1111) bulk superconductor using a cubic-anvil high-pressure (CA-HP) technique through both ex situ and in situ processes, under pressures up to 4 GPa and heating temperatures up to 1600 °C. A comprehensive characterization has been performed, including structural, microstructural, transport, and magnetic measurements. Our findings indicate that a modest growth pressure of approximately 0.5 GPa is sufficient for the formation of the Sm1111 phase via the ex situ process. In contrast, the in situ process requires higher synthesis pressure (4 GPa) and temperature (1400 °C for 1 h) to achieve the Sm1111 phase with enhanced superconducting properties. Notably, the optimized in situ process significantly reduces the reaction time needed for the formation of the Sm1111 phase compared to conventional synthesis process at ambient pressure (CSP), leading to an increase in the transition temperature by 3 K and improvements in the critical current density (Jc). Conversely, the optimized ex situ process maintains an onset transition temperature (Tc) of approximately 53 K, similar to that of CSP, while enhancing the Jc from ∼103 A cm−2 to ∼104 A cm−2 at 5 K. Despite these improvements, small amounts of impurity phases, as observed in CSP samples, persists in all Sm1111 samples prepared through either the in situ or ex situ CA-HP processes. These results suggest that the in situ process under optimized conditions (1400 °C, 4 GPa for 1 h) can effectively improve the superconducting properties of Sm1111, resulting an increased Tc to 56 K. Furthermore, a comprehensive comparison with high gas pressure techniques, spark plasma sintering, and CSP methods suggests that the formation of small amounts of impurity phases in Sm1111 cannot be completely eliminated by various pressure techniques, even under the applied pressures up to 4 GPa. These results provide valuable insights for both fundamental studies and applied research, contributing to the further advancement of iron-based superconductors (FBS).

Iron-based superconductors are under study for their potential for high-field applications due to their excellent superconducting properties such as low structural anisotropy, large upper critical fields and low field dependence of the critical current density. Between them, Fe(Se,Te) is simple to be synthesized and can be fabricated as a coated conductor through laser ablation on simple metallic templates. In order to make all the steps simple and fast, we have applied the spark plasma sintering technique to synthesize bulk Fe(Se,Te) to obtain quite dense polycrystals in a very short time. The resulting polycrystals are very well connected and show excellent superconducting properties, with a critical temperature onset of about 16 K. In addition, when used as targets for pulsed laser ablation, good thin films are obtained with a critical current density above 105 A cm−2 up to 16 T.

The enhanced performance of superconducting FeSe0.5Te0.5 materials with added micro-sized Pb and Sn particles is presented. A series of Pb- and Sn-added FeSe0.5Te0.5 (FeSe0.5Te0.5 + xPb + ySn; x = y = 0–0.1) bulks are fabricated by the solid-state reaction method and characterized through various measurements. A very small amount of Sn and Pb additions (x = y ≤ 0.02) enhance the transition temperature (Tconset) of pure FeSe0.5Te0.5 by ~1 K, sharpening the superconducting transition and improving the metallic nature in the normal state, whereas larger metal additions (x = y ≥ 0.03) reduce Tconset by broadening the superconducting transition. Microstructural analysis and transport studies suggest that at x = y > 0.02, Pb and Sn additions enhance the impurity phases, reduce the coupling between grains, and suppress the superconducting percolation, leading to a broad transition. FeSe0.5Te0.5 samples with 2 wt% of cometal additions show the best performance with their critical current density, Jc, and the pinning force, Fp, which might be attributable to providing effective flux pinning centres. Our study shows that the inclusion of a relatively small amount of Pb and Sn (x = y ≤ 0.02) works effectively for the enhancement of superconducting properties with an improvement of intergrain connections as well as better phase uniformity.

FeSe(11) family has a simple crystal structure belonging to iron-based superconductors (FBS) and has many stable phases including hexagonal and tetragonal structures, but only the tetragonal phase exhibits the superconductivity. In this study, we have investigated the effects of chemical pressure induced by As-doping at Se sites in the FeSe system by preparing a series of FeSe 1- x As x ( x  = 0.005, 0.01, 0.02, 0.05, 0.1 and 0.2) bulks. A broad characterization has been performed on these samples using structural, microstructural, transport and magnetic measurements. The obtained lattice parameters are increased by As-doping, which suggests the successful insertion of As at Se sites into the tetragonal lattice for low doping contents up to 5%, whereas the higher As substitution appears in the form of the FeAs impurity phase. The temperature dependence of the resistivity of all samples has similar behaviour and depicts the highest onset transition temperature of around 11.5 K, but the zero resistivity is not reached until the measured temperature of 7 K, which could be due to the presence of the impurity phases. Our study suggests that a dopant with a large ionic radius, i.e. arsenic, promotes the formation of the hexagonal phase of the 11 family and is effective for a small amount of doping level for the superconducting properties, whereas higher As-doping levels reduce the superconducting properties.

Iron-chalcogenide superconductors display rich phenomena caused by orbital-dependent band shifts and electronic correlations. Additionally, they are potential candidates for topological superconductivity due to the band inversion between the Fe d bands and the chalcogen p z band. Here we present a detailed study of the electronic structure of the nematic superconductors FeSe 1− x Te x (0 <  x  < 0.4) using angle-resolved photoemission spectroscopy to understand the role of orbital-dependent band shifts, electronic correlations and the chalcogen band. We assess the changes in the effective masses using a three-band low energy model, and the band renormalization via comparison with DFT band structure calculations. The effective masses decrease for all three-hole bands inside the nematic phase, followed by a strong increase for the band with d x y orbital character. Interestingly, this nearly-flat d x y band becomes more correlated as it shifts towards the Fermi level with increasing Te concentrations and as the second superconducting dome emerges. Our findings suggests that the d x y hole band, which is very sensitive to the chalcogen height, could be involved in promoting an additional pairing channel and increasing the density of states to stabilize the second superconducting dome in FeSe 1− x Te x . This simultaneous shift of the d x y hole band and enhanced superconductivity is in contrast with FeSe 1− x S x . The iron chalcogenide material FeSe1-xTex constitutes an important family of unconventional superconductors but its nematic phase was less explored due to a lack of single crystals. In this study, the authors provide a systematic study of the electronic structure for nematic FeSe1-xTex and observe that as the Te content increases a gradual shift and renormalization of the dxy orbital occurs, concomitant with the enhancement of superconductivity.

A series of FeSe Te bulk samples have been prepared using the high gas pressure and high-temperature synthesis (HP-HTS) method to optimize the growth conditions for the first time and investigated for their superconducting properties using structural, microstructure, transport, and magnetic measurements to reach the final conclusions. Ex situ and in situ processes are used to prepare bulk samples under a range of growth pressures using Ta-tube and without Ta-tube. The parent compound synthesized by convenient synthesis method at ambient pressure (CSP) exhibits a superconducting transition temperature of 14.8 K. Our data demonstrate that the prepared FeSe Te sealed in a Ta-tube is of better quality than the samples without a Ta-tube, and the optimum growth conditions (500 MPa, 600 °C for 1 h) are favorable for the development of the tetragonal FeSe Te phase. The optimum bulk FeSe Te depicts a higher transition temperature of 17.3 K and a high critical current density of the order of >10 A/cm at 0 T, which is improved over the entire magnetic field range and almost twice higher than the parent compound prepared using CSP. Our studies confirm that the high-pressure synthesis method is a highly efficient way to improve the superconducting transition, grain connectivity, sample density, and pinning properties of a superconductor.