  The discovery of superconductivity at 203 K in H3S 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. For LaH10 and YH10
superconductivity, with critical temperatures Tc ranging between 240 and 320 K
is predicted at megabar pressures. Here, we report superconductivity with a
record Tc ~ 250 K within the Fm-3m 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 zero-temperature. 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 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.
