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Register a new userIron superhydrides FeH5 and FeH6: stability, electronic properties and superconductivity
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Recently a big number of works devoted to search for new hydrides with record high-temperature superconductivity and at the same time the successful synthesis of potential high-TC superconducting FeH5 was reported. We present a systematic search for stable compounds in the Fe-H system using variable-composition version of the evolutionary algorithm USPEX. All known (FeH, FeH3, FeH5) and several new Fe3H5, Fe3H13 and FeH6 iron hydrides were found to be stable, resulting in a very complex phase diagram with rich structural relationships between phases. We calculate electronic properties of two potentially high-TC FeH5 and FeH6 phases in the pressure range from 150 to 300 GPa. Indeed, hydrogen-rich FeH5 and FeH6 phases were found to be superconducting within Bardeen-Cooper-Schrieffer theory, with TC values of up to 46 K.
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The discoveries of high-temperature superconductivity in H3S and LaH10 have excited the search for superconductivity in compressed hydrides. In contrast to rapidly expanding theoretical studies, high-pressure experiments on hydride superconductors are expensive and technically challenging. Here we experimentally discover superconductivity in two new phases,Fm-3m-CeH10 (SC-I phase) and P63/mmc-CeH9 (SC-II phase) at pressures that are much lower (<100 GPa) than those needed to stabilize other polyhydride superconductors. Superconductivity was evidenced by a sharp drop of the electrical resistance to zero, and by the decrease of the critical temperature in deuterated samples and in an external magnetic field. SC-I has Tc=115 K at 95 GPa, showing expected decrease on further compression due to decrease of the electron-phonon coupling (EPC) coefficient {lambda} (from 2.0 at 100 GPa to 0.8 at 200 GPa). SC-II has Tc = 57 K at 88 GPa, rapidly increasing to a maximum Tc ~100 K at 130 GPa, and then decreasing on further compression. This maximum of Tc is due to a maximum of {lambda} at the phase transition from P63/mmc-CeH9 into a symmetry-broken modification C2/c-CeH9. The pressure-temperature conditions of synthesis affect the actual hydrogen content, and the actual value of Tc. Anomalously low pressures of stability of cerium superhydrides make them appealing for studies of superhydrides and for designing new superhydrides with even lower pressures of stability.
The calcium superhydrides are synthesized at high pressure of 180 GPa and 1000 degree high temperatures. Superconductivity with Tc onset about 210 K is realized in thus obtained hydrogen rich calcium compounds at high pressure. The critical magnetic field Hc2(T) is estimated to around 166 T in the Ginzburg Landau model.
Since the discovery of high-temperature superconductivity in the thin-film FeSe/SrTiO$_3$ system, iron selenide and its derivates have been intensively scrutinized. Using ab initio density functional theory calculations we review the electronic structures that could be realized in iron-selenide if the structural parameters could be tuned at liberty. We calculate the momentum-dependence of the susceptibility and investigate the symmetry of electron pairing within the random phase approximation. Both the susceptibility and the symmetry of electron pairing depend on the structural parameters in a nontrivial way. These results are consistent with the known experimental behavior of binary iron chalcogenides and, at the same time, reveal two promising new ways of tuning superconducting transition temperatures in these materials. On the one hand by expanding the iron lattice of FeSe at constant iron-selenium distance and, on the other hand, by increasing the iron-selenium distance with unchanged iron lattice.
Angle resolved photoemission spectroscopy (ARPES) reveals the features of the electronic structure of quasi-two-dimensional crystals, which are crucial for the formation of spin and charge ordering and determine the mechanisms of electron-electron interaction, including the superconducting pairing. The newly discovered iron based superconductors (FeSC) promise interesting physics that stems, on one hand, from a coexistence of superconductivity and magnetism and, on the other hand, from complex multi-band electronic structure. In this review I want to give a simple introduction to the FeSC physics, and to advocate an opinion that all the complexity of FeSC properties is encapsulated in their electronic structure. For many compounds, this structure was determined in numerous ARPES experiments and agrees reasonably well with the results of band structure calculations. Nevertheless, the existing small differences may help to understand the mechanisms of the magnetic ordering and superconducting pairing in FeSC.
The layered iron superconductors are discussed using electronic structure calculations. The four families of compounds discovered so far, including Fe(Se,Te) have closely related electronic structures. The Fermi surface consists of disconnected hole and electron cylinders and additional hole sections that depend on the specific material. This places the materials in proximity to itinerant magnetism, both due to the high density of states and due to nesting. Comparison of density functional results and experiment provides strong evidence for itinerant spin fluctuations, which are discussed in relation to superconductivity. It is proposed that the intermediate phase between the structural transition and the SDW transition in the oxy-pnictides is a nematic phase.
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