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Decomposition of superconductors sometimes becomes crucial when studying essential physical properties of the superconductors. For example, the cuprate superconductor YBa2Cu3O7-d decomposes by long-time air exposure. In this study, we investigate the aging effects on superconducting properties of BiS2-based superconductors Bi4O4S3 and LaO0.5F0.5BiS2; both were first synthesized in 2012, using their polycrystalline samples synthesized several years ago. We find that 12-year-old Bi4O4S3 samples exhibit bulk superconductivity with a slight degradation of the superconducting transition temperature (Tc) of 0.2 K. For a high-pressure-synthesized LaO0.5F0.5BiS2 sample, clear decrease in Tc is observed, which suggests that high-pressure strain is reduced by aging. [ABSTRACT FROM AUTHOR]

Isotope effect on superconductive transition temperature (T c) is an essential indicator to examine whether the mechanism of superconductors is conventional. Unconventional isotope effect of BiS2-based superconductors has been previously reported in ambient-pressure tetragonal phase. However, to comprehensively ascertain the nature of superconductivity, the investigation of BiS2-based system in high-pressure structure is highly desirable. In this work, we carried out the first-principles calculations of phonon spectra and superconductivity in high-pressure monoclinic phase of LaO0.5F0.5BiS2 with 32S and 34S, and observed that the corresponding isotope coefficient is 0.13 ≤ α ≤ 0.20. This value is much greater than that of BiS2-based superconductors in ambient-pressure phase, but slightly smaller than that of conventional MgB2. Taking into account the calculated T c lower than experimental results, we finally conclude that the moderate phonon-mediated pairing plays a significant role in forming superconductivity of BiS2-based system in high-pressure phase, moreover, the cooperative multiple paring interactions should also be considered. [ABSTRACT FROM AUTHOR]

The slice-shaped LaO1-xBiS2 (x = 0.07) superconducting single crystal was grown by a high-temperature flux method. The X-ray diffraction analysis and symmetric M(H) curves along the H-axis indicate no any other impurities are involved in current LaO1-xBiS2 sample. The M(T) curves show the suppression of the Tconset and Tirr with increasing magnetic field applied parallel to the c-axis which is more noteworthy than that under magnetic field perpendicular to the c-axis. The Hc2(T) curves show upward curvature near Tc, which cannot be described by the one-gap model, and similar Hc2(T) curves are observed in REO1-xFxBiS2 and captured by the two-gap model, suggesting LaO1-xBiS2 may be a two-gap superconductor. The superconducting anisotropy parameter (2.8~5.58) of LaO1-xBiS2 (x = 0.07) is estimated by different methods. The structural distortion plays an important role in the smaller superconducting anisotropy parameter of LaO1-xBiS2. The critical current density in the self-field is about 5.9 × 103 Acm−2 and 6.7 × 103 Acm−2 for H//c and H⊥c, respectively. [ABSTRACT FROM AUTHOR]

We have investigated the structural, electrical, and magnetic properties of oxychalcogenide LaREO2Bi3Ag0.6Sn0.4S6 (RE = Pr and Nd) superconductors. The LaREO2Bi3Ag0.6Sn0.4S6 (RE = Pr and Nd) samples are composed of a single La2O2Bi3AgS6 phase without any impurity phase. The superconducting onset temperature of LaREO2Bi3Ag0.6Sn0.4S6 (RE = Pr and Nd) is ∼ 4.1 K, which is slightly higher than 2.8 K observed for La2O2Bi3Ag0.6Sn0.4S6. [ABSTRACT FROM AUTHOR]

Superconductivity with layered structure has attracted intensive research interests due to their exotic superconducting properties. In 2012, a new family of BiS2-based superconductor with their desired layered structure has been discovered. Recently, the isostructural superconductor with the substitution of Se for all S ions in BiS2-based compounds is recognized as the BiSe2-based superconductors. To date, enormous efforts were dedicated to explore the physical properties of this new family. Among them, synchrotron radiation-based X-ray absorption spectroscopy (XAS) contributed significantly to the understanding of local atomic structure and the electronic structure of BiS2/BiSe2-based systems. This article reviewed the progresses in the study of the new BiS2/BiSe2-based materials revealed by the unique technique XAS, including the determination of valence state, local atomic displacements, as well as the different structural distortion between BiS2-based and BiSe2-based systems. The superconductivity of BiS2- and BiSe2-based system is indeed intimately related to valence states and local atomic displacements. Further development of microscopic mechanism of superconductivity in BiS2/BiSe2-based materials requires synergetic endeavors integrating new experimental, numerical, and theoretical techniques. [ABSTRACT FROM AUTHOR]

To investigate the interlayer interaction in the recently synthesized high-entropy-alloy-type (HEA-type) REO 0.5 F 0.5 BiS 2 superconductors (RE: rare earth), we have systematically synthesized two sets of samples with close lattice parameters (close to those of PrO 0.5 F 0.5 BiS 2 or CeO 0.5 F 0.5 BiS 2) but having different mixing entropy (Δ S mix) for the RE site. The crystal structure was investigated using synchrotron X-ray diffraction and Rietveld refinement. For the examined samples having different Δ S mix , the increase in Δ S mix does not largely affect the bond lengths and the bond angle of the BiS 2 conducting layer but clearly suppresses the in-plane disorder at the in-plane S1 site, which is the parameter essential for the emergence of bulk superconductivity in the REO 0.5 F 0.5 BiS 2 system. Bulk nature of superconductivity is improved by the increase in Δ S mix for the present samples. The results of this work clearly show that the increase in mixing entropy at the blocking layer can positively affect the emergence of bulk superconductivity by modifying the local structure of the conducting layer. This is the evidence of the interaction between the high entropy states of the blocking layers and the physical properties of the conducting layers. • BiS 2 -based layered superconductors with different mixing entropy were synthesized. • Correlation among the mixing entropy at the RE site, local structure, and superconducting properties have been revealed. • High-entropy effects generated at the blocking layer can affect the local structure of conducting layer. [ABSTRACT FROM AUTHOR]

Antimony (Sb) substitution of less than 8% was examined on a single crystal of a layered superconductor NdO0.7F0.3BiS2. The superconducting transition temperature of the substituted samples decreased as Sb concentration increased. A lattice constant along the c-axis showed a large decrease compared with that along the a-axis. Since in-plane chemical pressure monotonically decreased as Sb concentration increased, the suppression of the superconductivity is attributed to the decrease in the in-plane chemical pressure.

Layered superconductors are attractive because some of them show high critical temperatures. While their crystal structures are similar, these compounds are composed of many elements. Compounds with many elements tend to be incongruent melting compounds, thus, their single crystals cannot be grown via the melt-solidification process. Hence, these single crystals have to be grown below the decomposition temperature, and then the flux method, a very powerful tool for the growth of these single crystals with incongruent melting compounds, is used. This review shows the flux method for single-crystal growth technique by self-flux, chloride-based flux, and HPHT (high-pressure and high-temperature) flux method for many-layered superconductors: high-Tc cuprate, Fe-based and BiS2-based compounds.

Structural and physical properties of layered chalcogenide superconductors are summarized. In particular, we review the remarkable properties of the Fe-chalcogenide superconductors, FeSe and FeTe-based materials. Furthermore, we introduce the recently discovered BiS2-based layered superconductors and discuss their prospects. [ABSTRACT FROM AUTHOR]