Bulk and Single Crystal Growth Progress of Iron-Based Superconductors (FBS): 1111 and 1144

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

High gas pressure and high-temperature synthesis (HP-HTS) technique and its impact on iron-based superconductors

The high-pressure growth technique generally plays an important role in the improvement of the 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 the iron-based superconductors by using this unique HP-HTS technique. These results demonstrate the enhancement of the superconducting properties with the improved sample quality compared to the conventional synthesis process at ambient pressure.Comment: 12 pages, 8 figure

High-pressure synthesis and the enhancement of the superconducting properties of FeSe0.5Te0.5

A series of FeSe0.5Te0.5 bulk samples have been prepared through 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 Tatube. 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{\deg}C for 1 h) are favourable 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 >10^4 A/cm^2 at 0 T, which is improved over the entire magnetic field range and almost twice higher than the parent compound prepared through 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 also pinning properties of a superconductor

Comparison of Gd addition effect on the superconducting properties of FeSe0.5Te0.5 bulks under ambient and high-pressure conditions

We have prepared a series of (FeSe0.5Te0.5 + xGd) bulk samples, with x = 0, 0.03, 0.05, 0.07, 0.1 and 0.2, through the convenient solid-state reaction method at ambient pressure (CSP). High gas pressure and high-temperature synthesis methods (HP-HTS) are also applied to grow the parent compound (x = 0) and 5-wt% of Gd-added bulks. Structural, microstructural, transport and magnetic characterizations have been performed on these samples in order to draw the final conclusion. Our analysis results that the HP-HTS applied for the parent compound enhances the transition temperature (Tc) and the critical current density (Jc) with the improved sample density and intergrain connections. The lattice parameter c is increased with Gd additions, suggesting a small amount of Gd enters the tetragonal lattice of FeSe0.5Te0.5 and the Gd interstitial sites are along the c-axis. A systematic decrease of the onset transition temperature Tc is observed with Gd additions, however, the calculated Jc of these Gd-added samples is almost the same as that of the parent compound prepared by CSP. It specifies that there is no improvement of the grain connections or pinning properties due to these rare earth additions. However, Gd-added FeSe0.5Te0.5 bulks prepared by HP-HTS have revealed a slightly improved critical current density due to improved grain connections and sample density but have a lower transition temperature than that of the parent compounds.Comment: 33 pages, 9 figures, 3 table

Resurgence of superconductivity and the role of dxy hole band in FeSe1−x_{1−x}Tex_x

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$_{xy}$ orbital character. Interestingly, this nearly-flat d$_{xy}$ 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$_{xy}$ 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$_{xy}$ hole band and enhanced superconductivity is in contrast with FeSe$_{1−x}$S$_x$

EPR, FTIR, optical absorption and photoluminescence studies of Fe(2)O(3) and CeO(2) doped ZnO-Bi(2)O(3)-B(2)O(3) glasses

Glasses containing heavy metal oxide of the composition (wt.%) 23B(2)O(3)-5ZnO-72Bi(2)O(3)-xFe(2)O(3)/CeO(2) (0 <= x <= 0.0058 at.% in excess) were prepared by melt quenching technique. The glass formation was confirmed by XRD. FTIR spectra exhibit characteristic absorption bands for B(2)O(3) and Bi(2)O(3) for their various structural units. The EPR spectra exhibit two resonance signals at g approximate to 6.4 and g approximate to 4.2 for Fe(2)O(3) doped glasses. The resonance signals at g approximate to 4.2 and g approximate to 6.4 are attributed to Fe(3+) ions in rhombic and axial symmetry sites. respectively. The number of spins participating in resonance (N) and its paramagnetic susceptibility (chi) with composition has been evaluated. The effect of CeO(2) and Fe(2)O(3) on optical and structural properties of zinc bismuth borate glass was investigated. From EPR and optical studies it is observed that iron ions are present in trivalent state with distorted octahedral symmetry. The cerium is present in Ce(4+) state. Upon 400 nm excitation the emission at 548 and 652 nm are attributed to the Bi(2+) species. The emission at 804 nm upon 530 nm excitation suggests that Bi(+) ions are present in the sample. It is interesting to observe that the optical band gap energy (E(opt)) decreases with the increase of transition metal and rare-earth ion doping. (C) 2009 Elsevier B.V. All rights reserved

EPR, optical absorption and photoluminescence properties of MnO(2) doped 23B(2)O(3)-5ZnO-72Bi(2)O(3) glasses

Electron paramagnetic resonance (EPR), transmission electron microscopy (TEM), optical absorption and photoluminescence (PL) spectroscopic measurements are performed on Mn(2+) doped high bismuth containing zinc-bismuth-borate glasses. TEM images reveal homogeneously dispersed Bi(circle) nanoparticles (NPs) of spherical shape with size about 5 nm. EPR spectra exhibit predominant signals at g approximate to 2.0 and 4.3 with a sextet hyperfine structure. The resonance signal at g approximate to 2.0 is due to Mn(2+) ions in an environment close to octahedral symmetry, where as the resonance at g approximate to 4.3 is attributed to the rhombic surrounding of the Mn(2+) ions. The hyperfine splitting constant (A) indicates that Mn(2+) ions in these glasses are moderately covalent in nature. The zero-field splitting parameter D has been calculated from the allowed hyperfine lines. The optical absorption spectrum exhibits a single broad band centered at 518 nm (19,305 cm(-1)) is assigned to the (6)A(1g)(S) —> (4)T(1g)(G) transition of Mn(2+) ions. The visible and near infrared (NIR) luminescence bands at 548, 652 and 804 nm have been observed when excited at 400 and 530 nm, respectively. These luminescence centers are supposed to be caused by the lower valence state of bismuth, such as Bi(2+) and Bi(+) ions, generated during melting process. (C) 2010 Elsevier B.V. All rights reserved