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...

The discovery of superconductivity at 203 K in H3S 1 brought attention back to conventional superconductors whose properties can be described by the Bardeen-Cooper-Schrieffer (BCS) and the Migdal-Eliashberg theories. These theories predict that high, and even room temperature superconductivity (RTSC) is possible in metals possessing certain favorable parameters such as lattice vibrations at high frequencies. However, these general theories do not suffice to predict real superconductors. New superconducting materials can be predicted now with the aid of first principles calculations based on Density Functional Theory (DFT). In particular, the calculations suggested a new family of hydrides possessing a clathrate structure, where the host atom (Ca, Y, La) is at the center of the cage formed by hydrogen atoms 2-4 . For LaH10 and YH10 superconductivity, with critical temperatures Tc ranging between 240 and 320 K is predicted at megabar pressures 3-6 . Here, we report superconductivity with a record Tc  250 K within the Fm3m structure of LaH10 at a pressure P  170 GPa. We proved the existence of superconductivity at 250 K through the observation of zero-resistance, isotope effect, and the decrease of Tc under an external magnetic field, which suggests an upper critical magnetic field of 120 T at zerotemperature. The pressure dependence of the transition temperatures Tc (P) has a maximum of 250-252 K at the pressure of about 170 GPa. This leap, by  50 K, from the previous Tc record of 203 K 1 indicates the real possibility of achieving RTSC (that is at 273 K) in the near future at high pressures and the perspective of conventional superconductivity at ambient pressure.

A superconducting critical temperature above 200 K has recently been discovered in H2S (or D2S) under high hydrostatic pressure1, 2. These measurements were interpreted in terms of a decomposition of these materials into elemental sulfur and a hydrogen-rich hydride that is responsible for the superconductivity, although direct experimental evidence for this mechanism has so far been lacking. Here we report the crystal structure of the superconducting phase of hydrogen sulfide (and deuterium sulfide) in the normal and superconducting states obtained by means of synchrotron X-ray diffraction measurements, combined with electrical resistance measurements at both room and low temperatures. We find that the superconducting phase is mostly in good agreement with theoretically predicted body-centered cubic (bcc) structure for H3S (Ref.3). The presence of elemental sulfur is also manifest in the X-ray diffraction patterns, thus proving the decomposition mechanism of H2S to H3S + S under pressure4–6.

The discovery of superconducting H3S with a critical temperature Tc∼200 K opened a door to room temperature superconductivity and stimulated further extensive studies of hydrogen-rich compounds stabilized by high pressure. Here, we report a comprehensive study of the yttrium-hydrogen system with the highest predicted Tcs among binary compounds and discuss the contradictions between different theoretical calculations and experimental data. We synthesized yttrium hydrides with the compositions of YH3, YH4, YH6 and YH9 in a diamond anvil cell and studied their crystal structures, electrical and magnetic transport properties, and isotopic effects. We found superconductivity in the Im-3m YH6 and P63/mmc YH9 phases with maximal Tcs of ∼220 K at 183 GPa and ∼243 K at 201 GPa, respectively. Fm-3m YH10 with the highest predicted Tc > 300 K was not observed in our experiments, and instead, YH9 was found to be the hydrogen-richest yttrium hydride in the studied pressure and temperature range up to record 410 GPa and 2250 K.

High-temperature superconductivity remains a focus of experimental and theoretical research. Hydrogen sulfide (H2S) has been reported to be superconducting at high pressures and with a high transition temperature. We report on the direct observation of the expulsion of the magnetic field in H2S compressed to 153 gigapascals. A thin (119)Sn film placed inside the H2S sample was used as a sensor of the magnetic field. The magnetic field on the (119)Sn sensor was monitored by nuclear resonance scattering of synchrotron radiation. Our results demonstrate that an external static magnetic field of about 0.7 tesla is expelled from the volume of (119)Sn foil as a result of the shielding by the H2S sample at temperatures between 4.7 K and approximately 140 K, revealing a superconducting state of H2S.

According to the theoretical predictions, insulating molecular hydrogen dissociates and transforms to an atomic metal at pressures P370-500 GPa 1-3 . In another scenario, the metallization first occurs in the 250-500 GPa pressure range in molecular hydrogen through overlapping of electronic bands [4][5][6][7] .The calculations are not accurate enough to predict which option is realized. Here we show that at a pressure of 360 GPa and temperatures <200 K the hydrogen starts to conduct, and that temperature dependence of the electrical conductivity is typical of a semimetal. The conductivity, measured up to 440 GPa, increases strongly with pressure. Raman spectra, measured up to 480 GPa, indicate that hydrogen remains a molecular solid at pressures up to 440 GPa, while at higher pressures the Raman signal vanishes, likely indicating further transformation to a good molecular metal or to an atomic state.Achieving a metallic state of hydrogen, predicted to occur at high pressure, is one of the most attractive goals in condense matter physics and remains a long-standing challenge both for theory and experiment. In 1935 Wigner and Huntington 1 proposed that any lattice built of hydrogen atoms (protons) should display metallic properties  similar to the alkali metals. However, a metallic state can be stabilized only at very high pressures 370-500 GPa 2,3,8 . Besides the ultimate simplicity, atomic metallic hydrogen is attractive because of the predicted very high critical temperature for superconductivity 9 . Recently, experimental evidence on the transformation of hydrogen to the atomic state at 495 GPa was reported 10 . This work was met with strong criticism 11 : in particular, the pressure is likely significantly (>100 GPa) overestimated, and the observed enhanced reflectance could be related to a transformation observed in earlier work at 360 GPa 12 . There is another possibility for transformation to a metallic state: the band gap of the crystalline molecular hydrogen can decrease with pressure and eventually close prior the dissociation of molecules and transformation to the atomic state. This path to metallization is considered in many recent theoretical estimates 4-7 . It also requires very high pressures of 250-500 GPa.The calculations and prediction of metallization rely on the knowledge of the structure, however, only the structure of phase I (Fig. 1) was determined as P63/mmc at P=5.4 GPa 13 . The structure of phase III (the subject of the present study) still remains unidentified 14,15 . Ab initio structural predictions suggest that C2/c structure is the most likely candidate for phase III at P>200 GPa 2 . This structure generally agrees with the Raman and infrared data available in the 150-300 GPa 16 range while the quantitative description of the infrared spectra is not satisfactory 17 . The DFT-based methods which are used in the crystal structure search are not suitable for calculations of the bandgap, where the gap is strongly underestimated. The GW (Green's function approximation) is better, and...

The discovery of superconductivity at 260 K in hydrogen-rich compounds like LaH 10 re-invigorated the quest for room temperature superconductivity. Here, we report the temperature dependence of the upper critical fields μ 0 H c2 ( T ) of superconducting H 3 S under a record-high combination of applied pressures up to 160 GPa and fields up to 65 T. We find that H c2 ( T ) displays a linear dependence on temperature over an extended range as found in multigap or in strongly-coupled superconductors, thus deviating from conventional Werthamer, Helfand, and Hohenberg (WHH) formalism. The best fit of H c2 ( T ) to the WHH formalism yields negligible values for the Maki parameter α and the spin–orbit scattering constant λ SO . However, H c2 ( T ) is well-described by a model based on strong coupling superconductivity with a coupling constant λ ~ 2. We conclude that H 3 S behaves as a strong-coupled orbital-limited superconductor over the entire range of temperatures and fields used for our measurements.

The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K has provided fresh impetus to the search for superconductors at ever higher temperatures. Although this systems displays all the hallmarks of superconductivity, the mechanism through which it arises remains to be determined. Here we provide a first optical spectroscopy study of this superconductor. Experimental results for the optical reflectivity of H 3 S, under hydrostatic pressure of 150 GPa, for several temperatures and over the range 60 to 600 meV of photon energies, are compared with theoretical calculations based on Eliashberg theory. Two significant features stand out: some remarkably strong infrared active phonons at around 160 meV, and a band with a depressed reflectance in the superconducting state in the region from 450 meV to 600 meV. In this energy range H3S becomes more reflecting with increasing temperature, a change that is traced to superconductivity originating from the electron-phonon interaction. The shape, magnitude, and energy dependence of this band at 150 K agrees with our calculations. This provides strong evidence of a conventional mechanism. However, the unusually strong optical phonon suggests a contribution of electronic degrees of freedom. Keywordssuperconductivity; H3S; optical data; the electron-boson spectral density * pascale.roy@synchrotron-soleil.fr.† timusk@mcmaster.ca. Author contributionsThis project has been initiated and supervised by T.T., M.I.E. and P.R. Samples have been synthesized and characterized by A.D. and M.I.E. Infrared measurements and data treatment were carried by B.L., F.C., J.B.B., P.R. and T.T. The calculations were performed by E.J.N. and J.P.C. All authors contributed to the writing of the paper. Competing financial interestsThe authors declare no competing financial interests. Furthermore, the superconducting phase has been found to be H 3 S by x-ray diffraction6. Calculations based on density functional theory (DFT) suggest that superconductivity in H 3 S is caused by the electron-phonon interaction, enhanced by a combination of the light mass of hydrogen and very strong coupling to high energy modes7-11. What is lacking is an experimental verification of this mechanism. A step in that direction would be the identification of the spectrum of bosons that couple to the charge carriers to form the glue that leads to superconducting pairing.The mechanism whereby conventional metals become superconductors is well established and involves the electron-phonon interaction12,13. The current-voltage characteristics of tunneling junctions12 and optical spectroscopy14-17 have yielded detailed information on the electron-phonon spectral density α 2 F(Ω) as a function of phonon energy ħΩ. These phonon spectra were further verified by neutron scattering18.It is an experimental challenge to extend these methods to the recently discovered hydrogen sulfide under pressure of 150 GPa for several reasons. The sample size ≈ 50 μm clearly excludes ...