  The phenomenon of high temperature superconductivity, approaching room
temperature, has been realized in a number of hydrogen-dominant alloy systems
under high pressure conditions1-12. A significant discovery in reaching room
temperature superconductivity is the photo-induced reaction of sulfur,
hydrogen, and carbon that initially forms of van der Waals solids at
sub-megabar pressures. Carbonaceous sulfur hydride has been demonstrated to be
tunable with respect to carbon content, leading to different superconducting
final states with different structural symmetries. A modulated AC
susceptibility technique adapted for a diamond anvil cell confirms a Tc of 260
kelvin at 133 GPa in carbonaceous sulfur hydride. Furthermore, direct
synchrotron infrared reflectivity measurements on the same sample under the
same conditions reveal a superconducting gap of ~85 meV at 100 K in close
agreement to the expected value from Bardeen-Cooper-Schrieffer (BCS)
theory13-18. Additionally, x-ray diffraction in tandem with AC magnetic
susceptibility measurements above and below the superconducting transition
temperature, and as a function of pressure at 107-133 GPa, reveal the Pnma
structure of the material is responsible for the close to room-temperature
superconductivity at these pressures.
