A superconductor is a material that can conduct electricity with no resistance below its critical temperature (T c ). The highest T c that has been achieved in cuprates 1 is 133 K at ambient pressure 2 and 164 K at high pressures 3 . As the nature of superconductivity in these materials has still not been explained, the prospects for a higher T c are not clear. In contrast, the BardeenCooper-Schrieffer (BCS) theory gives a guide for achieving high T c and does not put bounds on T c , all that is needed is a favorable combination of high frequency phonons, strong electronphonon coupling, and a high density of states. These conditions can be fulfilled for metallic hydrogen and covalent compounds dominated by hydrogen 4,5 . Numerous calculations support this idea and predict T c of 50-235 K for many hydrides 6 but only moderate T c =17 K has been observed experimentally 7 . Here we studied sulfur hydride 8 where a T c 80 K was predicted 9 . We found that it transforms to a metal at pressure 90 GPa. With cooling superconductivity was found deduced from a sharp drop of the resistivity to zero and a decrease of T c with magnetic field. The pronounce isotope shift of T c in D 2 S is evidence of an electron-phonon mechanism of superconductivity that is consistent with the BCS scenario. The superconductivity has been confirmed by magnetic susceptibility measurements with T c =203 K. The high T c superconductivity most likely is due to H 3 S which is formed from H 2 S under its decomposition under pressure. Even higher T c , room temperature superconductivity, can be expected in other hydrogen-based materials since hydrogen atoms provide the high frequency phonon modes as well as the strong electron-phonon coupling.A search for high, room temperature conventional superconductivity is promising as the BardeenCooperSchrieffer (BCS) theory in the Eliashberg formulation puts no apparent limits on T c .Materials with light elements are especially favorable as they provide high frequencies in the phonon spectrum. Indeed many superconductive materials have been found in this way, but only a moderately high T c =39 K has been found in this search in MgB 2 10 . N. Ashcroft 4 turned attention to hydrogen which has very high vibrational frequencies due to the light hydrogen atom, and provides a strong electron-phonon interaction. Further calculations showed that metallic hydrogen should be a superconductor with a very high critical temperature T c 100-240 K for molecular hydrogen, and T c = 300-350 K in the atomic phase at 500 GPa 11 . However superconductivity in pure hydrogen has not yet been found while the conductive and likely Similar to pure hydrogen, they have high Debye temperatures. Moreover, heavier elements might be beneficial as they contribute to the low frequencies that enhance electron phonon coupling.Importantly, lower pressures are required to metallize hydrides in comparison to pure hydrogen.Ashcroft's general idea was supported in numerous calculations 6,9 predicting high T c`s for many hydrides. So far onl...

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The recently discovered iron arsenide superconductors appear to display a universal set of characteristic features, including proximity to a magnetically ordered state and robustness of the superconductivity in the presence of disorder. Here we show that superconductivity in Fe 1+␦ Se, which can be considered the parent compound of the superconducting arsenide family, is destroyed by very small changes in stoichiometry. Further, we show that nonsuperconducting Fe 1+␦ Se is not magnetically ordered down to 5 K. These results suggest that robust superconductivity and immediate instability against an ordered magnetic state should not be considered as intrinsic characteristics of iron-based superconducting systems.

In this Letter we show that superconducting Fe(1.01)Se undergoes a structural transition at 90 K from a tetragonal to an orthorhombic phase but that nonsuperconducting Fe(1.03)Se does not. High resolution electron microscopy at low temperatures further reveals an unexpected additional modulation of the crystal structure of the superconducting phase that involves displacements of the Fe atoms, and that the nonsuperconducting composition shows a different, complex nanometer-scale structural modulation. Finally, we show that magnetism is not the driving force for the phase transition in the superconducting phase.

In this work a simple concept was used for a systematic search for new materials with high spin polarization. It is based on two semi-empirical models. Firstly, the Slater-Pauling rule was used for estimation of the magnetic moment. This model is well supported by electronic structure calculations. The second model was found particularly for Co2 based Heusler compounds when comparing their magnetic properties. It turned out that these compounds exhibit seemingly a linear dependence of the Curie temperature as function of the magnetic moment.Stimulated by these models, Co2FeSi was revisited. The compound was investigated in detail concerning its geometrical and magnetic structure by means of X-ray diffraction, X-ray absorption and Mößbauer spectroscopies as well as high and low temperature magnetometry. The measurements revealed that it is, currently, the material with the highest magnetic moment (6µB ) and Curietemperature (1100K) in the classes of Heusler compounds as well as half-metallic ferromagnets. The experimental findings are supported by detailed electronic structure calculations.