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تسجيل مستخدم جديدBaTh$_2$Fe$_4$As$_4$(N$_{0.7}$O$_{0.3}$)$_2$: An Iron-Based Superconductor Stabilized by Inter-Block-Layer Charge Transfer
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Recently, An electron-doped 12442-type iron-based superconductor BaTh$_2$Fe$_4$As$_4$(N$_{0.7}$O$_{0.3}$)$_2$ has been successfully synthesized with high-temperature solid-state reactions on basis of a structural design. The inter-block-layer charge transfer between the constituent units of BaFe$_2$As$_2$ and ThFeAsN$_{0.7}$O$_{0.3}$ was found to be essential to stabilize the target compound. Dominant electron-type conduction and bulk superconducting transition at ~22 K were demonstrated.
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$^{57}$Fe Mossbauer spectra at different temperatures between $sim 5$ K and $sim 300$ K were measured on an oriented mosaic of single crystals of CaKFe$_4$As$_4$ . The data indicate that CaKFe$_4$As$_4$ is a well formed compound with narrow spectral
lines, no traces of other, Fe - containing, secondary phases in the spectra and no static magnetic order. There is no discernible feature at the superconducting transition temperature in any of the hyperfine parameters. The temperature dependence of the quadrupole splitting approximately follows the empirical $T^{3/2}$ law. The hyperfine parameters of CaKFe$_4$As$_4$ are compared with those for KFe$_2$As$_2$ measured in this work, and the literature data for CaFe$_2$As$_2$, and were found to be in between those for these two, ordered, 122 compounds, in agreement with the gross view of CaKFe$_4$As$_4$ as a structural analog of KFe$_2$As$_2$ and CaFe$_2$As$_2$ that has alternating Ca - and K - layers in the structure.
In unconventional superconductors, it is generally believed that understanding the physical properties of the normal state is a pre-requisite for understanding the superconductivity mechanism. In conventional superconductors like niobium or lead, the
normal state is a Fermi liquid with a well-defined Fermi surface and well-defined quasipartcles along the Fermi surface. Superconductivity is realized in this case by the Fermi surface instability in the superconducting state and the formation and condensation of the electron pairs (Cooper pairing). The high temperature cuprate superconductors, on the other hand, represent another extreme case that superconductivity can be realized in the underdoped region where there is neither well-defined Fermi surface due to the pseudogap formation nor quasiparticles near the antinodal regions in the normal state. Here we report a novel scenario that superconductivity is realized in a system with well-defined Fermi surface but without quasiparticles along the Fermi surface in the normal state. High resolution laser-based angle-resolved photoemission measurements have been performed on an optimally-doped iron-based superconductor (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$. We find that, while sharp superconducting coherence peaks emerge in the superconducting state on the hole-like Fermi surface sheets, no quasiparticle peak is present in the normal state. Its electronic behaviours deviate strongly from a Fermi liquid system. The superconducting gap of such a system exhibits an unusual temperature dependence that it is nearly a constant in the superconducting state and abruptly closes at T$_c$. These observations have provided a new platform to study unconventional superconductivity in a non-Fermi liquid system.
We report an inelastic neutron scattering study on the spin resonance in the bilayer iron-based superconductor CaKFe$_4$As$_4$. In contrast to its quasi-two-dimensional electron structure, three strongly $L$-dependent modes of spin resonance are foun
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We use ultrafast optical spectroscopy to study the nonequilibrium quasiparticle relaxation dynamics of the iron-based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ with $T_c=33.5$ K. Our results reveal an evident pseudogap ($Delta_{PG}$ = 2.4 $pm$ 0.1 meV)
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