  A superconductor is a material that can conduct electricity with no
resistance below its critical temperature (Tc). The highest Tc that has been
achieved in cuprates1 is 133 K at ambient pressure2 and 164 K at high
pressures3. As the nature of superconductivity in these materials has still not
been explained, the prospects for a higher Tc are not clear. In contrast, the
Bardeen-Cooper-Schrieffer (BCS) theory gives a guide for achieving high Tc and
does not put bounds on Tc, all that is needed is a favorable combination of
high frequency phonons, strong electron-phonon coupling, and a high density of
states. These conditions can be fulfilled for metallic hydrogen and covalent
compounds dominated by hydrogen4,5. Numerous calculations support this idea and
predict Tc of 50-235 K for many hydrides6 but only moderate Tc=17 K has been
observed experimentally7. Here we studied sulfur hydride8 where a Tc~80 K was
predicted9. 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 Tc with magnetic field. The pronounce
isotope shift of Tc in D2S 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 Tc=203K. The high Tc superconductivity most likely is due to H3S which is
formed from H2S under its decomposition under pressure. Even higher Tc, 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.
