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Register a new userSuperconductivity at 44 K in K intercalated FeSe system with excess Fe
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We report here that a new superconducting phase with much higher Tc has been found in K intercalated FeSe compound with excess Fe. We successfully grew crystals by precisely controlling the starting amount of Fe. Besides the superconducting (SC) transition at ~30 K, we observed a sharp drop in resistivity and a kink in susceptibility at 44 K. By combining thermodynamic measurements with electron spin resonance (ESR), we demonstrate that this is a new SC transition. Structural analysis unambiguously reveals two phases coexisting in the crystals, which are responsible respectively for the SC transitions at 30 and 44 K. The structural experiments and first-principles calculations consistently indicate that the 44 K SC phase is close to a 122 structure, but with an unexpectedly large c-axis of 18.10 {AA}. We further find a novel monotonic dependence of the maximum Tc on the separation of neighbouring FeSe layers.
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Single-layer FeSe films grown on the SrTiO3 substrate (FeSe/STO) have attracted much attention because of their possible record-high superconducting critical temperature Tc and distinct electronic structures in iron-based superconductors. However, it has been under debate on how high its Tc can really reach due to the inconsistency of the results obtained from the transport, magnetic and spectroscopic measurements. Here we report spectroscopic evidence of superconductivity pairing at 83 K in single-layer FeSe/STO films. By preparing high-quality single-layer FeSe/STO films, we observe for the first time strong superconductivity-induced Bogoliubov back-bending bands that extend to rather high binding energy ~100 meV by high-resolution angle-resolved photoemission measurements. The Bogoliubov back-bending band provides a new definitive benchmark of superconductivity pairing that is directly observed up to 83 K in the single-layer FeSe/STO films. Moreover, we find that the superconductivity pairing state can be further divided into two temperature regions of 64-83 K and below 64 K. We propose the 64-83 K region may be attributed to superconductivity fluctuation while the region below 64 K corresponds to the realization of long-range superconducting phase coherence. These results indicate that either Tc as high as 83 K is achievable in iron-based superconductors, or there is a pseudogap formation from superconductivity fluctuation in single-layer FeSe/STO films.
We report superconductivity in as synthesized Nb2PdSe5, which is similar to recently discovered Nb2PdS5 compound having very high upper critical field, clearly above the Pauli paramagnetic limit [Sci. Rep. 3, 1446 (2013)]. A bulk polycrystalline Nb2PdSe5 sample is synthesized by solid state reaction route in phase pure structure. The structural characterization has been done by X ray diffraction, followed by Rietveld refinements, which revealed that Nb2PdSe5 sample is crystallized in monoclinic structure with in space group C2/m. Structural analysis revealed the formation of sharing of one dimensional PdSe2 chains. Electrical and magnetic measurements confirmed superconductivity in Nb2PdSe5 compound at 5.5K. Detailed magneto-resistance results, exhibited the value of upper critical field to be around 8.2Tesla. The estimated Hc2(0) is within Pauli Paramagnetic limit, which is unlike the Nb2PdS5.
Superconductivity in the cuprate superconductors and the Fe-based superconductors is realized by doping the parent compound with charge carriers, or by application of high pressure, to suppress the antiferromagnetic state. Such a rich phase diagram is important in understanding superconductivity mechanism and other physics in the Cu- and Fe-based high temperature superconductors. In this paper, we report a phase diagram in the single-layer FeSe films grown on SrTiO3 substrate by an annealing procedure to tune the charge carrier concentration over a wide range. A dramatic change of the band structure and Fermi surface is observed, with two distinct phases identified that are competing during the annealing process. Superconductivity with a record high transition temperature (Tc) at ~65 K is realized by optimizing the annealing process. The wide tunability of the system across different phases, and its high-Tc, make the single-layer FeSe film ideal not only to investigate the superconductivity physics and mechanism, but also to study novel quantum phenomena and for potential applications.
Exploration of new superconductors has always been one of the research directions in condensed matter physics. We report here a new layered heterostructure of [(Fe,Al)(OH)2][FeSe]1.2, which is synthesized by the hydrothermal ion-exchange technique. The structure is suggested by a combination of X-ray powder diffraction and the electron diffraction (ED). [(Fe,Al)(OH)2][FeSe]1.2 is composed of the alternating stacking of tetragonal FeSe layer and hexagonal (Fe,Al)(OH)2 layer. In [(Fe,Al)(OH)2][FeSe]1.2, there exists mismatch between the FeSe sub-layer and (Fe,Al)(OH)2 sub-layer, and the lattice of the layered heterostructure is quasi-commensurate. The as-synthesized [(Fe,Al)(OH)2][FeSe]1.2 is non-superconducting due to the Fe vacancies in the FeSe layer. The superconductivity with a Tc of 40 K can be achieved after a lithiation process, which is due to the elimination of the Fe vacancies in the FeSe layer. The Tc is nearly the same as that of (Li,Fe)OHFeSe although the structure of [(Fe,Al)(OH)2][FeSe]1.2 is quite different from that of (Li,Fe)OHFeSe. The new layered heterostructure of [(Fe,Al)(OH)2][FeSe]1.2 contains an iron selenium tetragonal lattice interleaved with a hexagonal metal hydroxide lattice. These results indicate that the superconductivity is very robust for FeSe-based superconductors. It opens a path for exploring super-conductivity in iron-base superconductors.
A huge enhancement of the superconducting transition temperature Tc was observed in tetragonal FeSe superconductor under high pressure. The onset temperature became as high as 27 K at 1.48 GPa and the pressure coefficient showed a huge value of 9.1 K/GPa. The upper critical field Hc2 was estimated to be ~ 72 T at 1.48 GPa. Because of the high Hc2, FeSe system may be a candidate for application as superconducting wire rods. Moreover, the investigation of superconductivity on simple structured FeSe may provide important clues to the mechanism of superconductivity in iron-based superconductors.
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