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Register a new userSuperconductivity under high pressure in LaFeAsO
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Electrical resistivity measurements under high pressures up to 29 GPa were performed for oxypnictide compound LaFeAsO. We found a pressure-induced superconductivity in LaFeAsO. The maximum value of Tc is 21 K at ~12 GPa. The pressure dependence of the Tc is similar to those of LaFeAsO1-xFx series reported previously.
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The long-sought goal of room-temperature superconductivity has reportedly recently been realized in a carbonaceous sulfur hydride compound under high pressure, as reported by Snider et al. [1]. The evidence presented in that paper is stronger than in other similar recent reports of high temperature superconductivity in hydrides under high pressure [2-7], and has been received with universal acclaim [8-10]. Here we point out that features of the experimental data shown in Ref. [1] indicate that the phenomenon observed in that material is not superconductivity. This observation calls into question earlier similar claims of high temperature conventional superconductivity in hydrides under high pressure based on similar or weaker evidence [2-7].
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.
We investigated LaFeAsO single crystals by means of synchrotron Mossbauer spectroscopy under pressures up to 7.5 GPa and down to 13 K and provide a microscopic phase diagram. We found a continuous suppression of the magnetic hyperfine field with increasing pressure and it completely vanishes at $sim$ 7.5 GPa which is in contrast to the behavior in polycrystalline samples where the magnetic order vanishes at $sim$ 20 GPa. The different behavior of the polycrystalline samples might be due to As-vacancies. Our results are in qualitative agreement with density functional theory calculations where a reduction of the magnetic moment with increasing pressure was found. We found that among different samples at ambient pressure the magnetic phase transition temperature as well as the low-temperature magnetic hyperfine field decrease with increasing unit cell volume.
The discovery of high-temperature conventional superconductivity in H3S with a critical temperature of Tc=203 K was followed by the recent record of Tc ~250 K in the face-centered cubic (fcc) lanthanum hydride LaH10 compound. It was realized in a new class of hydrogen-dominated compounds having a clathrate-like crystal structure in which hydrogen atoms form a 3D framework and surround a host atom of rare earth elements. Yttrium hydrides are predicted to have even higher Tc exceeding room temperature. In this paper, we synthesized and refined the crystal structure of new hydrides: YH4, YH6, and YH9 at pressures up to 237 GPa finding that YH4 crystalizes in the I4/mmm lattice, YH6 in Im-3m lattice and YH9 in P63/mmc lattice in excellent agreement with the calculations. The observed very high-temperature superconductivity is comparable to that found in fcc-LaH10: the pressure dependence of Tc for YH9 also displays a dome like shape with the highest Tc of 243 K at 201 GPa. We also observed a Tc of 227 K at 237 GPa for the YH6 phase. However, the measured Tcs are notably lower by ~30 K than predicted. Evidence for superconductivity includes the observation of zero electrical resistance, a decrease of Tc under an external magnetic field and an isotope effect. The theoretically predicted fcc YH10 with the promising highest Tc>300 K was not stabilized in our experiments under pressures up to 237 GPa.
Various tin hydrides SnHx (x = 4, 8, 12, 14) have been theoretically predicted to be stable at high pressures and to show high-critical-temperature superconductivity with Tc ranging from about 70 to 100 K. However, experimental verifications for any of these phases are still lacking to date. Here, we report on the in-situ synthesis, electrical resistance, and synchrotron x-ray diffraction measurements of SnHx at ~ 200 GPa. The main phase of the obtained sample can be indexed with the monoclinic C2/m SnH12 via comparison with the theoretical structural modes. A sudden drop of resistance and the systematic downward shift under external magnetic fields signals the occurrence of superconductivity in SnHx at Tc = ~ 70 K with an upper critical field u0Hc2(0) = ~ 11.2 T, which is relatively low in comparison with other reported high-Tc superhydrides. Various characteristic superconducting parameters are estimated based on the BCS theory.
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