  Searching for superconductivity with Tc near room temperature is of great
interest both for fundamental science & many potential applications. Here we
report the experimental discovery of superconductivity with maximum critical
temperature (Tc) above 210 K in calcium superhydrides, the new alkali earth
hydrides experimentally showing superconductivity above 200 K in addition to
sulfur hydride & rare-earth hydride system. The materials are synthesized at
the synergetic conditions of 160~190 GPa and ~2000 K using diamond anvil cell
combined with in-situ laser heating technique. The superconductivity was
studied through in-situ high pressure electric conductance measurements in an
applied magnetic field for the sample quenched from high temperature while
maintained at high pressures. The upper critical field Hc(0) was estimated to
be ~268 T while the GL coherent length is ~11 angstrom. The in-situ synchrotron
X-ray diffraction measurements suggest that the synthesized calcium hydrides
are primarily composed of CaH6 while there may also exist other calcium
hydrides with different hydrogen contents.
